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Table 1

The evidence of salivary ow rate in NIDDM from clinical studies.

Model (all human) Age Method Major ndings Interpretation Ref.

Obese

with

NIDDM Female: 11 Male: 9

Non-obese with NIDDM Female: 10 Male: 10

Healthy subject Female: 12 Male: 10

38–66 years

Fasting

and

unstimulated

salivalevels 5 min or 5 mL

No medication details of patients’

No

change

in

salivary ow rate in

all groupsNIDDM

had

no

effect

on

thesalivary ow rate

Aydin(2007)

NIDDMa with hypoglycemic,anti-hypertensive and anti-cholesterol medication Female: 12 Male: 33

Healthy subjects Female: 23

Male:

13

20–65 years

Each patient attended an out-patient diabetes educationalprogram

Fasting blood glucose (FBS) Unstimulated whole saliva and

citric acid-stimulated parotidsaliva

NIDDM: Fasting blood sugar(FBS) "

No difference in unstimulatedand stimulated salivary owrate between all groups

NIDDM without xerogenicmedication had no inuence onsalivary output

Salivary ow rate was notchanged following the change of FBS

The glycemic control programdid

not

affect

the

salivary ow

rate

Dodds andDodds(1997)

Men

Study

A Impaired glucose tolerance

(IGT): 10

NIDDM:

10

Control:

12 Study B NIDDM patients Insulin treatment:15 Anti-diabetic drugs: 9

Control:12

59–77 years

Citric acid-stimulated parotidsaliva collection at 0, 15, 30, 45,

60

and

120

min

(interval

0.5–

1 min) Patients in study A were not on

any medication OGTT

was

used

to

determineNIDDM and IGT in study A

HbA1C was used to determinetreatment, either insulin or anti-diabetic drug treatment, for thepatients in study B

In study A, there was no changein salivary ow rate.

In

study

B,

there

was

no

changein salivary ow rate betweeninsulin-treated NIDDM patientsand

medication-treated

NIDDMpatients

NIDDM had no effect on stim-ulated salivary ow rates

(BorgAndersson

et

al.,

1998)

NIDDM:

45 Female: 13 HbA1C: 9.2 2.2 Male: 32

HbA1C: 8.2 1.9 Control: 86 Female: 45

Male:

32

59–79years

The

medication

use

in

NIDDMand control subjects wasrecorded

Unstimulated saliva was col-lected at 1 h after meal for 5 min

Paraf n wax-stimulated whole

saliva was collected for 5 min Deep

breathing

test

(expiratoryto inspiratory (E/I) ratio), E/I 1.10 indicates parasym-

pathetic

neuropathy

Orthostatic

test

(systolicblood pressure change)

SBP decreased more than30 mmHg: indicates sympa-thetic neuropathy

Parasympathetic

and

sympa-thetic neuropathies were sig-nicantly higher in NIDDMpatients

No difference in unstimulatedand stimulated salivary ow

rate among groups An

increase

in

the

number

of drugs used daily resulted in adecrease in both resting andstimulated

ow rate in thecontrol

subjects,

but

not

inNIDDM patients

The

resting

and

stimulated

sa-liva secretions were not differ-ent between the NIDDMpatients and the control subjects

NIDDM did not seem to affect

salivary ow

Meurmanet al.,(1998)

Non-NIDDM: 38

Female:

20 Male: 18

NIDDM:35 Female: 17

Male:

18

43–45

13years

Unstimulated whole saliva for10

min Flow rate (mL/min) Salivary resistin (ng/mL)

No difference in salivary owrate between

groupsNIDDM had no effect on thesalivary

ow rateYin et al.(2012)

Non-NIDDM Female: 14

Male:

9 Well-controlled NIDDM Female: 3 Male: 8

Poor-controlled NIDDM (HbA1C> 9%) Female: 10

Male:

8

54–90 years

OGTT, HbA1c Unstimulated whole saliva col-

lection Unstimulated parotid saliva

collection using Carlson-Crit-tenden cup

Stimulated parotid saliva col-lection

All current medication use wererecorded

No differences in salivary owrate between well-controlledNIDDM

and

non-NIDDM. Poor-controlled DM: # salivary ow rate Taking xerostomia medication

in well-controlled and poor-controlled NIDDM: # salivaryow rate

Poorly-controlled DM could beassociated with salivary dys-function

Xerogenic medication acceler-ated a decrease in salivary owrate in NIDDM

Chavezet al.,(2000)

62 J. Ittichaicharoen et al. / Archives of Oral Biology 64 (2016) 61–71


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salivary glands through the muscarinic acetylcholine receptors,particularly

M1

and

M3

receptors

causes

uid

secretion

(Naka-mura

et

al.,

2004).On

the

other

hand,

the

sympathetic

stimulationof salivary glands via b-adrenergic receptors leads to the release of salivary proteins (Proctor & Carpenter, 2007). It has beendemonstrated

that

levels

of

alpha-amylase

in

saliva

representsthe

sympathetic

activity

of

salivary

glands

(Enberg,

Alho,Loimaranta, & Lenander-Lumikari, 2001). The activation of parasympathetic nerves results in a high salivary ow rate withlow

salivary

proteins.

However,

sympathetic

activation

leads

tosecretion

of

a

high

level

of

salivary

proteins

without

the

reductionin salivary ow rate (Carpenter, 2013). Therefore, salivary ow rate

is

the

direct

result

of

parasympathetic

function.Non-insulin

dependent

diabetes

mellitus

(NIDDM)

(also

knownas

Type

2

Diabetes

Mellitus)

is

a

chronic

metabolic

disorder,characterized by insulin resistance, hyperglycemia, as well asLangerhans islets beta-cells dysfunction (DeFronzo, 2004a, 2004b).NIDDM

causes

complications

in

several

organs,

including

salivaryglands

(Al-Rawi,

2011;

Desai

&

Mathews,

2014).

Xerostomia

is

oneof

the

most

common

complaints

in

NIDDM

patients

(Albert

et

al.,2012; Ali & Kunzel, 2011; Chomkhakhai, Thanakun, Khovidhunkit,Khovidhunkit,

&

Thaweboon,

2009; Collin,

Niskanen,

et

al.,

2000;Collin,

Sorsa

et

al.,

2000).

Several

clinical

studies

demonstratedthat

NIDDM

subjects

had

a

reduction

in

salivary ow rate (Chavez,

Taylor, Borrell, & Ship, 2000; Izumi et al., 2015; Lin, Sun, Kao, & Lee,2002).

However,

some

studies

showed

contradictive

ndings

(Aydin,

2007;

Borg

Andersson,

Birkhed,

Berntorp,

Lindgarde,

&Matsson,

1998; Hartman

et

al.,

2015; Meurman

et

al.,

1998; Yin,Gao, Yang, Xu, & Li, 2012). Currently, it is known that obesity is arisk

factor

of

developing

insulin

resistance

and

NIDDM

(Arner

&Rydén,

2015; Esser,

Legrand-Poels,

Piette,

Scheen,

&

Paquot,

2014;Guo,

2014; Hardy,

Czech,

&

Corvera,

2012;

Leahy,

2005;Vollenweider,

von

Eckardstein,

&

Widmann,

2015; Wellen

&Hotamisligil,

2005).

Excess

free

fatty

acids

(FFAs)

from

an

obesecondition

has

been

shown

to

impair

insulin

signaling

through

bothcell

autonomous

mechanisms

and

inammatory process (Hardy,Czech,

&

Corvera,

2012). It

has

been

shown

that

obesity–insulinresistance

could

cause

the

salivary

gland

dysfunction

(Mozaffariet

al.,

2011;

Zalewska

et

al.,

2014).Despite

these

previous

reports,

the

relationships

betweenNIDDM, obesity–insulin resistance, and salivary gland function are

still not clearly understood. In this review, the current evidence of the

available

reports

regarding

the

relationship

between

NIDDM,obesity–insulin

resistance

and

the

alterations

of

the

salivary

glandsare comprehensively summarized. Controversial reports as well asthe mechanistic insights regarding the roles of NIDDM and obesityon

salivary

gland

function

are

also

presented

and

discussed.

1.1. Search strategy and selection criteria for this review

Relevant

publications

in

the

PubMed

database

published

beforeAugust,

2015

were

identied

by

using

search

terms

“NIDDM

ortype 2 diabetes and salivary gland” and “obesity–insulin resistance

and

salivary

gland”. Only

articles

published

in

English

werereviewed.

1.2. Salivary ow rate in NIDDM from clinical studies

NIDDM

has

been

shown

to

lead

to

the

development

of

systemicinammation

and

that

this

inammation

subsequently

initiatesmicro-angiopathy,

macro-angiopathy

and

neuropathy

in

severalvital organs (Garcia et al., 2010; Lee et al., 2012; Rolo & Palmeira,2006).

The

effects

of

NIDDM

on

the

salivary

gland

function

havebeen

widely

studied;

however

these

ndings

are

still

controversial.The

salivary

gland

function

has

been

evaluated

by

several

methods,including the measurement of salivary ow rate, alpha-amylaseactivity,

salivary

cortisol

levels,

salivary

adipokine

levels

and

other

salivary

in

ammatory

biomarkers

(Desai

&

Mathews,

2014).However,

the

most

common

indicator

for

salivary

gland

functionis to measure salivary ow rate.

In

NIDMM

subjects,

it

has

been

shown

that

the

salivary

owrate

in

both

adults

and

children

was

not

signicantly

different

fromthe

normal

subject

group

(Aydin,

2007;

Borg

Andersson

et

al.,1998;

Chavez,

Borrell,

Taylor,

&

Ship,

2001; Dodds,

&

Dodds,

1997;Hartman

et

al.,

2015;

Meurman

et

al.,

1998;

Yin

et

al.,

2012).

Thesestudies

measured

the

salivary ow rate by performing either

unstimulated

saliva

collection

(Aydin,

2007;

Borg

Andersson

et

al.,1998;

Chavez

et

al.,

2001;

Dodds,

&

Dodds,

1997;

Hartman

et

al.,2015;

Yin

et

al.,

2012)

or

stimulated

saliva

collection

(BorgAndersson

et

al.,

1998;

Dodds,

&

Dodds,

1997;

Meurman

et

al.,1998).

Since

whole

unstimulated

salivary ow rate indicates the

function of the submandibular and sublingual glands which

Table 1 (Continued)

Model (all human) Age Method Major ndings Interpretation Ref.

Healthy:

24

Female:

13 Male: 11

NIDDM:

24

Female:

13 Male: 11

58.8–70.8years

Citric-acid

stimulated

salivaryow rate Parotid gland (PS)

Submandibular

and

sublin-gual

gland

(SS) Total protein Parotid gland (PS)

Submandibular

and

sublin-gual gland (SS)

Salivary ow rate (PS)

Parotid

gland:

no

change Submandibular and sublin-

gual

gland:

#

Diabetic

patients

showed

a

re-duction

in

salivary

gland

pro-duction and secretion in specicsalivary

glands

Izumi

et

al.(2015)

Group 1: NIDDM with xerosto-mia

Female:

13 Male: 23

Group 2: NIDDM without xer-ostomia Female: 13 Male: 23

Control

Group:

no

NIDDM Female: 1 Male:23

56 years Known xerogenic drugs werestopped

1

week

before

study.

Fasting

salivary

uptake

(UR)—which represents the function of salivary production

Fasting salivary excretion ratio(ER), —which represents thefunction of salivary excretion

measured

by

scintigraphy

Group 1 had signicantly lowerUR

values

and

ER

values

at

the1st

and

15th

minute,

whencompared with controls andNIDDM patients without xeros-tomia

Impaired salivary productionand

excretion

were

found

inNIDDM

patients

with

xeros-tomic symptoms

Lin et al.(2002)

a All NIDDM patients without xerogenic medication.

J. Ittichaicharoen et al. / Archives of Oral Biology 64 (2016) 61–71 63


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Table 2

Evidence of salivary secretions in NIDDM from clinical studies.

Model Age Method Major ndings Interpretation Ref.

Obese

with

NIDDM

(OD) Female: 11 Male: 9

Non-obese subjects withNIDDM (NOD) Female: 10

Male: 10

Healthy subjects Female: 12

Male:

10

38–66 years

No

medication Plasma and saliva ghrelin levels

were measured Plasma glucose levels were

measured Total salivary protein was mea-

sured Amylase activity was measured

OD

and

ND

subjects Salivary glucose: " Total protein: " a-amylase activity: "

Glucose

was

released

into

salivaof T2DM subjects

Sympathetic stimulation wasincreased regarding to the in-crease of a-amylase activity

Parasympathetic stimulationdid not change

Aydin(2007)

NIDDMa with hypoglycemic,anti-hypertensive and anti-cholesterol medication Female: 12 Male: 33

Healthy subjects

Female:

23

Male:

13

20–65 years

Each patient attended an out-patient diabetes educationalprogram.

Fasting blood glucose (FBG) Unstimulated whole saliva and

citric acid-stimulated parotidsaliva

Parotid

salivary

protein,

includ-ing proline-rich proteins (PRPs),histatins, statherin and amylase

Amylase activity compared be-tween

DM and control DM with glycemic control and

DM without glycemic control

NIDDM after glycemic controlprogram Amylase activity: #

No differences in salivary pro-teins among all groups

Amylase activity showed a pos-itive correlation with bloodglucose level

The salivary proteins were notchanged along with the glyce-mic condition

Dodds &Dodds(1997)

Men

Study A Impaired glucose tolerance

(IGT): 10 NIDDM: 10 Control: 12

Study B

NIDDM

patients

Insulin

treatment:15 Anti-diabetic drugs: 9

Control:12

59–77 years

Citric acid-stimulated parotidsaliva collection at 0, 15, 30, 45,60 and 120 min (interval 0.5–1 min)

Patients in study A were not onany medication

OGTT was used to determineNIDDM and IGT in study A

HbA1C

was

used

to

determinetreatment,

either

insulin

or

anti-diabetic drug treatment, for thepatients in study B

Study A : IGT and NIDDM: salivary glu-

cose " compared with control The salivary glucose between

IGT and NIDDM were notsignicantly different

Study B: Both diabetic groups: salivary

glucose

"

compared

withcontrol

The salivary glucose levelsbetween IGT and NIDDM werenot

signicantly different Both study A and study B

Blood glucose: NIDDM > IGT >Normal

A positive correlation be-tween salivary glucose andblood

glucose

concentrationwas found

Salivary glucose concentrationwas varied according to bloodglucose concentration

BorgAnderssonet al.(1998)

Healthy: 24 Female: 13 Male: 11

NIDDM: 24 Female: 13

Male:

11

58.8–70.8years

Citric-acid stimulated salivaryow rate Parotid gland (PS) Submandibular and sublin-

gual gland (SS)

Total

protein Parotid

gland

(PS) Submandibular and sublin-

gual gland (SS)

FBG of diabetic patient wassignicantly higher than control

Parotid gland Total protein: no change

Submandibular and sublingualgland

Total

protein:

#

NIDDM patients showed a re-duction in salivary gland pro-duction and secretion

Izumi et al.(2015)

NIDDM: 45 Female: 13 Male: 32

Control:

86 Female: 45 Male: 32

59–79years

The history of medication use inNIDDM and control subject wasrecord

Unstimulated

saliva

was

col-lected at 1 h after meal for 5 min

Paraf n wax-stimulated wholesaliva was collected for 5 min

Biochemical

analyses: Lysozyme Amylase Total protein, albumin, Ig (G,

M and A) Urea

Salivary composition showed nodifference between groups.

There were no statistically sig-nicant differences in the con-centrations of salivary secretionbetween well-glycemic con-trolled patients and poor- gly-cemic

controlled

patients

(usingthe median of HbA1c values toclassify)

The resting and stimulated sa-liva secretions were not differ-ent between the NIDDMpatients

and

the

control

subjects

Stimulated salivary secretioncan reect the quality of para-sympathetic

and

sympatheticactivity

Meurmanet al.(1998)



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maintain

oral

moisture

during

resting

period

(Falcao,

da

Mota,Pires, & Bezerra, 2013), and that the salivary ow rate during theresting period is commonly regulated by the parasympatheticnervous

system

(Carpenter,

2013;

Proctor

&

Carpenter,

2007),those

ndings suggested that the parasympathetic activity insalivary glands of NIDDM patients has not been affected. Thissuggestion has been supported by the study of Meurman andcolleagues

who

demonstrated

that

no

signicant

difference

insalivary

ow rate or parasympathetic activity (measured by the

expiratory

to

inspiratory

(E/I)

ratio)

was

seen

between

NIDDMand control groups (Meurman et al., 1998). In addition, the salivaryow

rate

of

the

NIDDM

subjects

had

no

correlation

with

fastingplasma

glucose

levels

(Dodds

&

Dodds,

1997;

Meurman

et

al.,1998). Although the salivary ow rate did not change in NIDDMpatients, the amylase activity, which represents sympatheticactivity

in

salivary

glands,

signicantly

increased

(Aydin,

2007;Dodds

&

Dodds,

1997).

These

ndings

indicate

that

the

diabeticcondition alters the salivary gland activity, particularly sympa-thetic activity, without changes in salivary ow rate. Interestingly,these

patients

did

not

have

any

record

of

using

xerogenicmedications.

Despite those reports, inconsistent ndings exist. Severalclinical studies demonstrated the reduction of salivary ow rate

in

diabetes

(Chavez

et

al.,

2000;

Izumi

et

al.,

2015;

Lin

et

al.,

2002).

For

example,

the

study

of

Chavez

and

colleagues

demonstratedthat the poorly glycemic controlled NIDDM patients (HbA1C >9%)had lower unstimulated salivary ow rate, compared with well-controlled

NIDDM

patients

and

the

control

group.

In

addition,unstimulated

salivary ow rate was signicantly reduced in poor-

controlled NIDDM patients and well-controlled NIDDM patientswho were taking xerogenic medications, when compared with thenon-NIDDM

subjects

(Chavez

et

al.,

2000).

Moreover,

Izumi

et

al.(2015)

showed

that

the

stimulated

salivary ow rate and total

proteins

produced

by

the

parotid

gland

showed

no

rate

of

changebetween diabetic patients and healthy control subjects, whilst thestimulated

salivary

ow

rate

and

total

proteins

collected

fromsubmandibular

and

sublingual

glands

of

diabetic

patients

showeda signicant decrease, when compared with that of healthy controlsubjects. Furthermore, Lin and colleagues demonstrated thatimpaired

salivary

function

was

found

in

NIDDM

patients

withxerostomia

by

the

reduction

of

salivary

secretory

rate

andexcretory rate (Lin et al., 2002). The reduction of salivary owrate in NIDDM patients from these previous studies (Chavez et al.,2000;

Izumi

et

al.,

2015;

Lin

et

al.,

2002) may

be

associated

withthe

severity

of

NIDDM

condition,

the

xerogenic

medication,

andthe specic salivary gland where saliva was collected from. Thecomprehensive summary of reports on salivary ow rate in NIDDM

patients

is

shown

in

Table

1.

Table 2 (Continued)

Model Age Method Major ndings Interpretation Ref.

Control Female: 30 Male: 30

NIDDM Female: 30 Male: 30

Adult

Fasted

for

2

h,

unstimulatedsaliva collection,

colorimetric assay for Salivary exoglycosidases: N -

acetyl-b-glucosaminidase(HEX) activity, showing im-

paired

epithelial

membranestructure b-D-glucuronidase activity

(GLU),

showing

degradationof

proteoglycans,

represent-ing salivary dysfunction

NIDDM HEX A: activity and output " HEX B: activity and output " GLU: activity and output "

There

were

functional

changesin the salivary gland of DMpatients

There was a degeneration of thesalivary glands of NIDDMpatients

There

was

local

in

ammation

inthe salivary glands of DMpatients

Zalewskaet al.(2013)

Non-NIDDM: 38 Female: 20 Male: 18

NIDDM:35

Female:

17

Male:

18

43–45 13years

Unstimulated whole saliva for10 min Flow rate (mL/min) resistin (ng/mL)

Unstimulated

whole

saliva

Salivary resistin: " Salivary and serum resistin were

not affected by eating There was a positive correlation

between

salivary

and

serumresistin

Salivary resistin can reectNIDDM condition

Yin et al.(2012)

Human parotid glands

NIDDM:

5 Control:11

42–68 years

Immunocytochemistry:immunogold labeling human

amylase

and

visualization

withtransmission electron microscope(TEM).

NIDDM showed only the in-crease of secretory granules, but

the

deposition

of

anti

a-amy-lase- labelled gold particle wasnot different when comparedwith

the

control

No

signicant difference in thenumber of lysosomes or lipiddroplets

Changes in salivary gland mor-phology were found in NIDDM

NIDDM

showed

only

the

in-crease of secretory granules, butthe expression of a-amylasewas

not

different

when

com-pared

with

the

control

Piras et al.(2010)

NIDDM

patients:

Controlled NIDDM: 27

Uncontrolled

NIDDM:

26

Healthy

group:

40

33–84 years

Unstimulated

whole

saliva Chemicals: glucose, protein and

Uric acid (UA) assays Salivary

enzymatic

antioxi-dants:

catalase

(CAT),

superox-ide dismutase (SOD)

Salivary non-enzymatic param-eters: antioxidant activity(AOA), glutathione (GSH), UA,glucose and total protein

NIDDM: high UA, GSH and total pro-

tein

low

AOA

and

CAT

Strong

positive

association

be-tween salivary glucose and blood

glucose salivary glucose and GSH salivary glucose and UA

Saliva

can

be

used

for

thediagnosis and the managementof DM

Mussaviraet al.(2015)

a All NIDDM patients without xerogenic medication.



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Table 3

The evidence of salivary ow rate and salivary secretions in obesity–insulin resistance from clinical studies.

Model Age Method Major ndings Interp

Human

Obese (n=369) Healthy (n =60)

50 years old Screening tests for Cushing ’s syndrome: Dexamethasone suppression test (DST) Measurements of 24-h urine creatinine

and cortisol excretion (UFC) Measurement of bedtime salivary cortisol Analyzed all metabolic parameters

Salivary cortisol tended to rise as BMIincreased and correlated with an increase inwaist circumference

Obesecortiso

Children (n= 213 from total of 8319)Body Mass Index (BMI) z-score was used toclassied the subjects into

Normal Underweight Overweight Obese

11years

3 mL Unstimulated whole saliva collection Flow rate (mL/h) Total protein (mg/dL)

No differences in salivary ow rate amonggroups

No differences in total salivary proteinconcentration among groups

BMItota

Children (n= 8319)Body Mass Index (BMI) z-score was used toclassied the subjects into

Normal Underweight Overweight Obese

11years

3 mL Unstimulated whole saliva collection Insulin C-reactive protein (CRP) Adiponectin Leptin

Obese: Insulin " CRP " Adiponectin " Leptin "

Obesynd

Children (n= 744 from total of 8319)

Obese healthy group (OH): 186 Obese with high insulin (OI): 186 Obese with high CRP (OC): 186 Non-Obese with high CRP (NC): 186 Non-Obese with high insulin (NI):186 Non-obese healthy group (NH): 186

11years

Sjögren’s syndrome related cytokines IL-4 IL-10 L12p70 IL-17A IFN-g

NIDDM related cytokines Insulin Ghrelin Myeloperoxidase (MPO) Vascular endothelial growth factor

(VEGF)

OC: " CRP and IL-6, leptin NC: " CRP, IL-6, IL-10, IL-1b, MM-9 and

resistin OH: " IL-10 and adiponectin

Man Thos

or a

Human

Normal weight: 30 Underweight: 2 Overweight: 12 Obese: 21

10.60.2years Salivary ow rate Salivary glucose: glucose oxidase method

using uorescent emission Plasma glucose analysis

Blood pressure and fasting plasma glucoseshowed no difference among groups

Salivary ow rate and salivary glucoseshowed no difference among groups

Salivary ow rate did not correlate withfasting plasma glucose levels

There wasa signicant associationbetweenplasma and salivary glucose levels

Obe Saliv

marhigh


7/11

1.3. Salivary secretion in NIDDM from clinical studies

The

alterations

of

salivary

secretion

in

NIDDM

have beendemonstrated. It has been shown that increased amylase activitywas found in salivary glands of NIDDM patients (Aydin, 2007;Dodds

&

Dodds,

1997).

In

a

report

by

Piras

and

colleagues,

theyfound

no

signicant

difference

in

amylase

expression

of

salivarygland isolated from NIDDM patients, when compared with that of the control group (Piras, Hand, Mednieks, & Piludu, 2010).However,

amylase

activity

was

not

investigated

in

that

study.NIDDM

patients

also

showed

higher

levels

of

salivary

and

serumresistin (a peptide hormone) and pro-inammatory cytokineswhich are produced by adipocytes and macrophages (Yin et al.,2012),

than

non-NIDDM

patients.

Moreover,

it

has

been

shown

thatthere

was

an

increase

in

salivary

pro-inammatory

cytokines,including interleukins-1b, -6 and -8 (IL-1b, -6 and -8), tumornecrosis factor-a (TNF-a) and matrix metalloproteinases (MMP)-8

and

-9

in

NIDDM

patients

(Collin,

Niskanen,

et

al.,

2000;

Collin,Sorsa

et

al.,

2000;

Rathnayake

et

al.,

2013).

The

positive

correlationof glucose levels between blood and saliva in impaired glucosetolerance subjects and NIDDM subjects suggested the potentialrole

of

plasma

glucose

on

salivary

gland

function

(Aydin,

2007;Borg

Andersson

et

al.,

1998;

Garcia

et

al.,

2010;

Yin

et

al.,

2012).

In

addition, Zalewska et al. (2013) showed that the salivary glands of NIDDM patients exhibited greater changes in function andmorphology

than

those

of

control,

by

increasing

salivary

N -acetyl-b-glucosaminidase,

representing

impaired

epithelial

mem-brane structure, and salivary b-D-glucuronidase activity, repre-senting the degradation of proteoglycans). All of these ndingssuggest

that

increased

sympathetic

activity,

increased

inamma-tion,

impaired

insulin

signaling,

and

the

degradation

of

extracel-lular matrix occurs in salivary glands of NIDDM subjects. Thecomprehensive summary of reports on salivary secretions inNIDDM

patients

is

shown

in

Table

2.All of these clinical studies indicate that NIDDM can lead to

alterations in the salivary gland with or without a reduction insalivary

ow

rate.

The

factors

inuencing

the

salivary

ow

rate

in

NIDDM could be dependent on the severity of the diabeticcondition, the use of xerogenic medication and the speciccollected salivary gland.

Salivary

ow

rate

and

salivary

secretions

in

obesity–insulinresistance

or

pre-diabetic

condition

from

clinical

studies

Evidence demonstrated that obesity–insulin resistance doesnot have an effect on salivary ow rate (Borg Andersson et al.,1998;Hartman

et

al.,

2015).

Impaired

secretion

of

saliva

is

indicatedusing

the

unstimulated

salivary

ow

rate 0.2 mL/min in men and

0.18 mL/min in women (Chavez et al., 2000). In subjects withimpaired glucose tolerance (IGT or obesity–insulin resistance), ithas

been

shown

that

the

unstimulated

salivary

ow

rates

were

notsignicantly

different

from

the

control

group

(Borg

Anderssonet al., 1998). Goodson and colleagues in 2014 evaluated twenty

biomarkers

in

fasting

saliva

samples

taken

from

11-year

oldchildren

who

had

been

designated

as

underweight,

normal

healthyweight,

overweight

and

obese

subjects.

The

study

found

thatsalivary C-reactive protein (CRP), insulin and leptin levels of obesesubjects signicantly increased, when compared with those of normal

healthy

weight

subjects.

However,

salivary

adiponectinlevel

in

obese

children

signicantly

showed

a

decrease,

whencompared with levels in normal healthy weight subjects (Goodsonet al., 2014). Alterations in those biomarkers in plasma could be theindicators

of

the

development

of

the

metabolic

syndrome

(MetS)(Oh

et

al.,

2014). In

addition,

Goodson's

study

showed

that

neithertotal salivary protein level nor salivary ow rate were signicantlydifferent between body weight groups. The results of Goodson’sstudy

suggested

that

obesity

did

not

affect

salivary

protein

levels

or

salivary

ow

rate

in

this

population,

but

the

alteration

of

salivary

H u m a n

M e t a b o l i c s y n d r o m e

F e m a l e ( 5 )

M a l e ( 7 )

H e a l t h y

F e m a l e ( 1 4 )

M a l e ( 2 0 )

2 0 – 7 0 y e a r s

B l o o d p r e s s u r e

W a i s t c i r c u m f e r e n c e

F a s t i n g 1 2 h t h e n c o l l e c t i o n o f :

B l o o d g l u c o s e

T o t a l c h o l e s t e r o l

T r i g l y c e r i d e s

H D L , L D L , i n s u l i n

H O M A - I R

U n s t i m u l a t e d s a l i v a c o l l e c t i o n ( 1 1 – 1 2 p m )

S a l i v a r y c o r t i s o l

M i d n i g h t s a l i v a r y c o r t i s o l c o n c e n t r a t i o n s

w e r e s i g n i c a n t l y a n d p o s i t i v e l y c o r r e l a t e d

w i t h a b d o m i n a l c i r c u m f e r e n

c e , f a s t i n g b l o o d

g l u c o s e a n d H O M A - I R

T h e r e w a s a p o s i t i v e c o r r e l a t i o n b e t w e e n

m i d n i g h t s a l i v a r y c o r t i s o l a n d

t h e m e t a b o l i c

s y n d r o m e ( M e t S )

J a n g , L e e ,

K i m ,

K i m

a n d S o n g ,

( 2 0 1 2 )



8/11

Table 4

The evidence of salivary gland alterations in the NIDDM and obesity–insulin resistance from in vivo studies.

Model Age Experimentalperiod

Methods Major ndings Interpretation Ref.

Female mice (each groupn = 6)

Normal C57BL/6

BALB/c

Non-obese diabetic mice(NOD mice):

NOD/Lt

NOD-scid NOD.B10.H

Decorin and biglycan main-tain

ECM

structure

in

thesalivary gland and can bedestroyed by MMP

8–20 weeks

– Stimulated saliva collectionfor 10 min: stimulated byeither pilocarpine (0.5 mg/100 g) or isoproterenol(0.2 mg/100 g)

Protein analysis in saliva

and salivary glands bydetecting MMP, proteogly-cans

and

TGFb1 Detection

of

salivary

dec-orin and biglycan

TGFb1 " in NOD mice bothin saliva and lysated sali-vary gland

MMP inhibitor can reducethe expression of TGFb1

Decorin and biglycan were

degraded in the saliva of diabetic mice

The salivary glands in dia-betic mice were destroyedvia increased proteolyticactivities inside the salivaryglands

Yamachikaet al.(2000)

Male

obese

Zucker

rats

Lean (LZR) (n = 7-9) Obese (OZR) OZR- fed Cr 5 mg/kg (n = 7–

9)

OZR- fed Cr 10 mg/kg (n = 7–9)

(Cr: Chromium picolinate forglycemic control)

6

weeks

6Months

afterchromiumadministration

Fasting

plasma

glucose

Insulin

sensitivity

byQUICKI

Western blot: NF-kB,phospho-NF-kB, VCAM-1 and ICAM-1 in salivary

glands

OZR:

"

Plasma

insulin

levels " QUICKI " Lipid droplet " p-NF-kB/NF-kB ratio " ICAM-1

OZR-fed Cr 10: showed abenign expansion of thesecretory acinar units

Histological features of salivary

glands:

OZR

similarto

LZR

Obesity

led

to

an

increase

inthe

inammation of thesalivary glands withoutsignicant morphologicalchanges

Mozaffariet

al.

(2011)

Male Wistar rats

Normal diet (C): n = 9 High fat diet (HFD): n = 9

5Weeks of dietaryprogram

Blood analysis: insulin,glucose and fatty acidmethyl esters

Antioxidant prole of pa-rotid gland (PG) and sub-mandibular

glands

(SG)

Specic activity of sali-vary peroxidase

Superoxide dismutase 2(SOD2)

CAT Uric acid (UA) Total antioxidant salivary

status (TAS)

HFD-fed rats developed theinsulin resistant condition

HFD: # SG peroxidase activity # PG peroxidase activity #

total

PG

peroxidase

Obesity–insulin resistanceled to the decrease of anti-oxidants in salivary glands

Parotid and submandibularglands responded differ-ently

to

the

insulin

resistantconditions

The parotid glands seemedto be more affected fol-lowing

the

insulin

resistantcondition

The main source of anti-oxidants was in the parotidglands

Zalewskaet al.(2014)

Lacrimal gland (LG) andsubmandibular gland (SG)of male Wistar rats

Young: 2 months old Old: 20 months old

Insulin tolerance test afterinjection of 100 mL of 10mM insulin

Tissue collection: 3 min af-ter injection of 100 mL of 10 mM insulin into the in-ferior vena cava

Immunoblotting: IR, Shcand

STAT-1

Old rats: euglycemia andhyperinsulinemia

Old rats LG: # p-IR

Old rats SG: # p-IR # p-STAT-1

Aging induced a reductionin tyrosine phosphorylationof insulin receptors in theLG and SG, suggesting in-sulin resistance in the exo-crine glands of aging rats

Rocha et al.(2003)

Female

mice

NOD: n = 9 BALB/c: n = 9

24

weeks

24

weeks

Pilocarpine-stimulated

sal-ivary ow rate for 10 min Evaluation of submandibu-

lar salivary gland inam-mation by the number of mononuclear cells in H&E-stained

slides

and

immu-nohistochemistry analysis(CD4+ T-cell).

Salivary cytokine levels

Serum

cytokines

levels

24-week-old

NOD

mice Salivary ow rate # IL-4 " Granulocyte–macro-

phage colony-stimulat-ing factor

(GM-CSF)

"

TNF-a " IL-5 # IFN-g: not detectable

The

longer

duration

of

in-sulin-resistant conditionsled to a lower salivary owrate and an increase insalivary gland inamma-tion

Jonsson,Delaleu,Brokstad,BerggreenandSkarstein,(2006)



9/11

biomarkers, including CRP, insulin, leptin and adiponectin, insteadof

plasma

biomarkers,

may

be

useful

indicators

of

the

occurrenceof

MetS

in

children

(Goodson

et

al.,

2014).

In

addition,

Hartman

andcolleagues demonstrated that salivary glucose levels may be usefulfor the screening of high fasting plasma glucose levels in children(Hartman

et

al.,

2015). The

recent

study

also

showed

thatsignicant

alterations

in

salivary

adipocytokines

were

observedin overweight and obese children, when compared with normal-weight children (Shi et al., 2015). All of these ndings suggestedthat

salivary

biomarkers

in

obese

subjects

could

be

useful

tools

topredict

the

development

or

progression

of

MetS,

particularly

inchildren.

An increase in salivary cytokines, including IFN-g, TNF-a, IL-1,IL-4,

IL-10,

IL-12,

and

IL-17,

have

been

shown

to

be

associated

withoral

dryness

in

primary

and

secondary

Sjögren's

syndrome(Furuzawa-Carballeda et al., 2014; Kang, Lee, Hyon, Yun, & Song,2011; von Bültzingslöwen et al., 2007). These ndings suggestedthat

an

increase

in

these

particular

salivary

cytokines

could

be

apredictor

of

oral

dryness.Despite the unaltered salivary ow rate in obesity–insulin

resistance, both studies showed an increase in saliva glucose levels(Borg

Andersson

et

al.,

1998;

Hartman

et

al.,

2015). These

ndingssuggest

that

impaired

insulin

signaling

might

occur

in

the

salivary

glands of obesity–insulin resistance.In obese subjects, it has been demonstrated that saliva cortisol

levels

were

signicantly

increased,

compared

to

normal

subjects(Abraham,

Rubino,

Sinaii,

Ramsey,

&

Nieman,

2013; Jang,

Lee,

Kim,Kim, & Song, 2012). Several studies showed that there was anincreased level of salivary pro-inammatory cytokines in insulin-resistant

obese

patients,

including

tumor

necrosis

factor-alpha

(TNF-a), interleukin-6 (IL-6), interferon gamma (IFN-g), macro-phage

inammatory

protein-1

beta

(MIP-1b)

(Desai

&

Mathews,2014)

as

well

as

an

increased

level

of

oxidative

stress

(Al-Rawi,2011). These ndings suggest that obesity–insulin resistancesubjects developed impaired insulin sensitivity in the salivarygland,

increased

level

of

salivary

oxidative

stress,

and

increasedlevel

of

inammation,

possibly

leading

to

salivary

gland

dysfunc-tion. The comprehensive summary of reports on salivary glanddysfunction in obesity–insulin resistance is shown in Table 3.

1.4. Possible mechanisms of salivary gland dysfunction in NIDDM and

obesity–insulin resistance: lesson learned from in vitro and in vivostudies

Although

alterations

in

salivary

gland

activity

with

or

without

achange in salivary ow rate in NIDDM and obesity–insulinresistance have been reported in clinical studies, the underlyingmechanisms

responsible

for

these

alterations

have

been

docu-mented

from

basic

studies.

Findings

from

in

vivo

studies

supportedthe clinical ndings from NIDDM and obesity–insulin resistancefor the occurrence of salivary gland dysfunction. NIDDM rats havebeen

shown

to

develop

the

degradation

of

salivary

glands

throughan

increase

in

salivary

decorin

and

biglycan

levels

(Yamachika,

Brayer, Oxford, Peck, & Humphreys-Beher, 2000). Decorin andbiglycan are parts of the extracellular matrix structure in thesalivary

gland,

and

degradation

can

be

caused

by

matrix

metal-loproteinases

(MMPs)

(Yamachika

et

al.,

1998). An

increase

insalivary decorin and biglycan represents the degradation of salivary glands (Yamachika et al., 2000). In addition, Masagoand

colleagues

demonstrated

the

elevation

of

proapoptotic

Bax

Fig.

1.

The

possible

mechanisms

of

salivary

gland

dysfunction

in

NIDDM

and

obesity–

insulin

resistance.



10/11

and caspase 3 activation in glandular parenchyma of non-obesediabetic

mice

(Masago

et

al.,

2001).

Thus,

the

degradation

of salivary

gland

in

NIDDM

and

the

obese-insulin

resistant

conditioncould be due to the role of apoptosis in glandular parenchyma. Ithas been shown that increased inammation correlates with TGF-b1

expression

(Imai,

Hiramatsu,

Fukushima,

Pierschbacher,

&Okada,

1997).

In

addition,

decorin

and

biglycan

are

known

as

thereservoirs of TGF-b1 (Cs-Szabo, Roughley, Plaas, & Glant, 1995).These ndings suggest that salivary gland inammation as well asthe

degradation

of

the

salivary

gland

develops

in

NIDDM.

Theexcessive

inammation

and

degradation

of

salivary

gland

inNIDDM could be responsible for salivary gland dysfunction,resulting in the decrease of salivary ow rate. However, it ispossible

that

less

severe

salivary

gland

dysfunction

might

not

alterthe

salivary

ow

rate

in

NIDDM,

as

observed

in

several

clinicalstudies (Aydin, 2007; Borg Andersson et al., 1998; Chavez et al.,2001; Dodds, & Dodds, 1997; Hartman et al., 2015; Meurman et al.,1998;

Yin

et

al.,

2012).Increased

free

fatty

acid

levels

in

the

plasma

are

commonlyfound in obesity–insulin resistance (DeFronzo, 2004a, 2004b). Inhuman parotid/submandibular gland epithelial cell lines, theapplication

of

saturated

fatty

acids

(SFAs)

on

them

can

lead

toincreased

IL-6

secretion,

cell

apoptosis

and

the

degradation

of

salivary gland cells (Shikama et al., 2013). These ndings suggestedthat high levels of free fatty acid could cause salivary glandsdamage.

In

addition,

several

in

vivo

studies

support

these

ndings.Zalewska

et

al.

(2014)

found

that

there

was

a

reduction

in

anti-oxidants levels, by measuring peroxidase activity, in both thesubmandibular and parotid glands in insulin-resistant obese ratsinduced

by

high-fat

diet,

when

compared

with

the

control

group.Interestingly,

the

peroxidase

enzyme

activity

in

the

submandibulargland was still higher than that in the parotid gland of the high-fatdiet-fed rats. These ndings suggest that obesity–insulin resis-tance

could

lead

to

increased

oxidative

stress

in

the

parotid

glands,rather the submandibular glands. In obese Zucker rats, an increasein inammation, demonstrated by increased NFkB phosphoryla-tion

and

ICAM-1

levels,

was

demonstrated

in

their

salivary

glands

(Mozaffari et al., 2011). However, no histological changes in thesalivary glands of these obese Zucker rats were found. A possibleexplanation for this could be that the level of inammation in thesalivary

glands

of

obese

Zucker

rats

might

not

be

high

enough

tocause

morphological

changes.

Rocha

and

colleagues

also

demon-strated that aged rats (20 months old) developed insulinresistance, characterized by hyperinsulinemia with euglycemia,and

that

the

reduction

of

insulin

receptor

phosphorylation

insalivary

glands

also

developed

in

these

aged

rats

(Rocha,

Carvalho,Saad, & Velloso, 2003).

All of these ndings suggest that obesity–insulin resistanceleads

to

increased

oxidative

stress,

inammation

and

decreasedinsulin

sensitivity

in

salivary

gland,

possibly

leading

to

furtherdegradation of salivary glands. The comprehensive summary of

reports

on

salivary

gland

dysfunction

in

obesity–

insulin

resistanceand

NIDDM

from

basic

research

is

shown

in

Table

4.

2. Conclusion

Although

the

alterations

of

salivary ow

rate

in

NIDDM

havebeen

demonstrated

previously

(Chavez

et

al., 2000; Izumi

et

al.,2015; Lin et al ., 2002), several studies showed contradictoryndings (Aydin, 2007; Borg Andersson et al., 1998; Chavez et al.,2001;

Dodds,

&

Dodds,

1997;

Hartman

et

al., 2015; Meurmanet

al., 1998;

Yin et

al.,

2012). These

inconsistent

ndings

could

bedue to the difference in the severity of the diabetic condition, theuse of xerogenic medications and the different methods for salivacollection

in

those

studies. Changes

in

salivary gland

activity

with

or

without an

alteration

in

salivary

ow

rate

can

be

found

in

NIDDM and obesity–insulin resistance. The underlying mecha-nisms

of

salivary gland

dysfunction

could

be

due

to

hyperglyce-mia,

hyperinsulinemia

and dyslipidemia

following

NIDDMand obesity–insulin resistance, resulting in increased oxidativestress, inammation, increased sympathetic activity, and im-paired

insulin

signaling

in

the

salivary gland.

High

levels

of oxidative

stress,

inammation

and

insulin

resistance

in

thesalivary gland can lead to the degradation of the salivary gland. Inaddition, this degradation in NIDDM and obese patients can causea

reduction

in

salivary ow

rate.

These

potential mechanismsresponsible

for salivary

gland dysfunction

in

NIDDM

and

obesity–insulin resistance are summarized in Fig. 1. Since the develop-ment of salivary gland dysfunction could happen as early as thedevelopment

of

obesity–insulin resistance (i.e. before

the

occur-rence

of

NIDDM),

future

studies

are

needed

and

a

suggested

focusis on the protection of salivary gland function in obesity–insulinresistance.

Conict

of

interest

The authors declare that there is no conict of interest.

Author

contribution

JI co-designed the study, analyzed the data and wrote themanuscript.

NC

co-designed

the

study,

and

wrote

the

manuscript.SCC

co-designed

the

study,

analyzed

the

data

and

wrote

themanuscript. All authors have read and approved the nal article.

Acknowledgments

This work was supported by grants from the Thailand ResearchFund TRF-BRG5780016 (SC), a CMU 50th Anniversary grant byChiang

Mai

University

(JI

and

SC),

National

Research

Council

of Thailand (SC), a NSTDA Research Chair Grant from the NationalScience and Technology Development Agency Thailand (NC), andthe

Chiang

Mai

University

Excellent

Center

Award

(NC).

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