Mudasir bashir

M.V.Sc.scholar

DIV--.VSR

Roll no.4856

Incretins Incretins are hormones having

insulinotropic activity at usual physiological concentrations.

These are secreted from gut of animal after ingestion of nutrition[glucose or fat]

INCRETIN EFFECT:-insulin secretory response of incretins..

It accounts for atleast 50% of the total insulin secreted after oral glucose.

Historical aspect

The concept of incretins is atleast a century old.

In 1902 bayliss and starling published”mechanism of pancreatic secretion”.---- chemical stimulus and not nervous

system controls pancreatic secretion.• The chemical is secretin—inducing exocrine

secretion.• They also considered the possibility of

chemical excitent for internal secretion.

Moore 1906 gave Intestinal extract----first

attempt for diabetes treatment on incretin

based.

In 1932,La Berre used the word ‘incretin’ to an

extract from upper gut that produces hypoglycemia

but doesn’t induce exocrine secretion.but he didn’t

prove that incretins existed.

Insulin existence ------ radioimmunoassay.

Between 1964 to 1967 three groups{elrick et al,1964;mcintyre et al,1964;perley and kippins,1967}------ inc.insulin secretion on oral intake of glucose than i.v route.

In 1971,john c.brown isolated and deduced structure of a peptide isolated from intestinal mucosa---GIP.

insulinotropic properties---glucose dependent insulinotropic peptide[Gip]

GIP------first incretin to be isolated.

Another peptide in 1985 was found having potent insulinotropic activity{schemidt et al}—glucagon like peptide{GLP-1}

GIP and GLP-1--------together account for full incretin effect.

Only GLP-1 and not GIP could increase insulin secretion and normalise blood glucose in type2DM patients when given at supraphysiological levels.

Like GIP,GLP-1 also shows glucose dependence for its insulin secretory response.

Synthesis, secretion and degradation of incretins

1. Glucose dependent insulinotropic peptide{GIP}:-

Synthesis:-gipgene---153amino acid precursor----GIP-----a member of a family of structurally related harmones—secretin,glucagon, and VIP

GIP is synthesised in and released in response to nutrients from enteroendocrine cells[k cells] .

K cells are primarily present in duodenum and jejunum.

Expression of gip gene in gut is regulated by nutrients.

glucose and lipid administration increases GIP mRNA levels in git.

Transcription factors like GATA-4,isl-1,PDX-1 and Pax2 are essential in cell specific expression of GIP within the k cells.

Rat salivary glands and lens epithelial cells also expresses gip gene.

Secretion:- dose dependent basis of nutrients. varies among species—in humans lipid causes

more secretion than carbohydrates.in pig and rodents secretion occurs more on carbohydrate intake.

GIP secretion coupling pathways are poorly understood.presumably there is increased intracellular calcium conc.

Some k cells also contain sweet receptors[GPCRS] and their activation by sugars and sweeteners lead to GIP secretion.

Degradation of GIP

1 2 3 42

GIP

Des-HA-GIP (inactive)

Enzymatic cleavage of GLP-1 by DPP-4 inactivates GLP-1

1 2 3 42

DPP-4

Two possible solutions to utilize GLP-1 action therapeutically:

1) Long-acting DPP-4-resistant GLP-1 analogues / incretin mimetics2) DPP-4 inhibitors / incretin enhancers

Mentlein et al. Eur J Biochem. 1993; Gallwitz et al. Eur J Biochem. 1994

GLUCAGON-LIKE PEPTIDE-1:- synthesis:-

proglucagon gene. Also encodes glucagon and GLP-2. GLP-1 and GLP-2 are produced in

enteroendocrine cells[l cells],scattered in small bowel and ascending colon.

Post transcriptional processing --- pc1/3 ,pc2 and furin which varies with tissues.

GUE and CRE----proglucagon gene expression.

Secretion:- Primary physiological stimuli for secretion of

GLP-1 are fat and carbohydrates. Although GLP-1 also found in taste buds and

brain but majority of GLP-1 seen in peripheral blood is from l cells.

Secretion of GLP-1 is also regulated by neural and endocrine signals.

Also GPCRs on l cells ----- free fatty acids and lipids------ increase in intracellular calcium and activation of mapk.

Degradation of GLP-1

Enzymatic cleavage of GLP-1 by DPP-4 inactivates GLP-1

DPP-4

7 8 9

Des-HA-GLP-1 (inactive)

7 8 9

Two possible solutions to utilize GLP-1 action therapeutically:

1) Long-acting DPP-4-resistant GLP-1 analogues / incretin mimetics2) DPP-4 inhibitors / incretin enhancers

Secretion,effect and degredation

Incretin effect--GIP + GLP-1Incretin effect--GIP + GLP-1

Time (min)

Insu

lin (

mU

/l)

60 120 180

80

60

40

20

0

Insu

lin (

mU

/l)Incretin effect

Time (min)0 60 120 180

Control subjects

People with Type 2 diabetes

Oral glucose loadIntravenous glucose infusion

Nauck et al. Diabetologia. 1986

Incretin effect GIP:- almost lost in Type2DM

Loss of insulinotropic effect ----chronic

desensitization of GIPRs or reduction in expressions of GIPRs on pancreatic cells.

Desensitization ----receptor internalization,down regulation and uncoupling from G-proteins.

Hyperglycaemia triggers the association of GIPR and ubiquitin ligase complexes--------reduction of GIPR protein levels and down-regulation of GIP action.

GLP-1: effects in humans

After food ingestion…After food ingestion…

GLP-1 is secreted fromL-cells of the jejunum

and ileum

That in turn…

• Stimulates glucose- dependent insulin secretion

• Suppresses glucagonsecretion

• Slows gastric emptying

• Leads to a reduction offood intake

• Improves insulin sensitivity

Long-term effectsin animal models:

• Increase of β-cell mass and improved β-cell function

Pleotropic effect of incretin in pancreas EFFECT ON ß-CELLS:- GIP+GIPR--- increases insulin secretion GIP also replenishes insulin in ß-cells-----increasing

insulin gene transcription and biosynthesis.

EFFECT ON GLUCAGON SECRETION:- GIP stimulates glucagon secretion via GIPR present

on pancreatic alpha cells but under certain conditions.

GLUCAGON-LIKE PEPTIDE EFFECTS ON ß-CELLS:-

GLP-1 stimulates insulin secretion

via GLP-1R present on ß-cells.

GLP-1 replenishes ß-cell insulin stores and prevents

exhaustion of ß-cell reserves via increased insulin

mRNA stability,gene transcription and biosynthesis. Effect on glucagon secretion:- Inhibits glucagon secretion in euglycemic and

hyperglycemic condition-somatostatin secretion

CENTRAL AND PERIPHERAL NERVOUS SYSTEM EFFECTS ON FOOD INTAKE AND GLUCOSE HOMEOSTASIS:-

Reciprocal systems “gut-brain axis”----regulate short and long term energy homeostasis.

GLP-1------key regulator of food intake,appetite and body weight.

GLP -1R agonist reduces short term food intake when injected peripherally or into CNS.

Prolonged use of GLP-1 agonists reduces both food intake as well as body weight.

GASTROINTESTINAL TRACT:-GLP-1 is potent inhibitor of several GIT functions like gastric acid secretion,gastric emptying and motility----decreasing nutrient absorbtion----decreasing blood glucose levels.

GLP-1 is important mediator of “ILEAL BRAKE EFFECT”---endocrine inhibition of upper GIT functions activated by presence of nutreints in the ileum.

Slowing of gastric emptying may be via neural mechanism.

Muscle,adipose tissue and liver:-

Insulin-like effect in major

extrapancreatic tissues. This helps in glucose homeostasis. GLP-1 treatment mediated through

inositol phosphogylcan generation.

Incretin therapy in type2DM

B-cell function and glucagon in T2DM β-Cell mass in Type 2 diabetes

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

ND IFG T2DM ND T2DM

b-C

ell

volu

me

(%)

-50%-50%

-63%-63%

ND=non-diabetic; IFG=impaired fasting glucose; T2DM=Type 2 diabetes mellitus

Obese Lean

Excessive hepatic glucose production in Excessive hepatic glucose production in Type 2 diabetesType 2 diabetes

Hepatic glucose output

Insulin; IR

GlucagonFasting & Fasting & postprandial postprandial hyperglycaemia hyperglycaemia

plasma glucose concentration.

IR=insulin resistance

Insulin and glucagon dynamics in response to meals in normal subjects and Type 2 diabetes

20.018.316.615.013.36.14.4

14013012011010090

1209060300

Glucose (mmol/l)

Insulin (mU/l)

Glucagon (ng/l)

Normal subjects

Type 2 diabetes

Meal

Delayed/depressedinsulin response

Nonsuppressed glucagon

Action of glucagon

Glycogen Glucose

Low blood glucose and Low blood glucose and insulin promotes glucagon insulin promotes glucagon release from release from --cells of pancreascells of pancreasGlucagon stimulatesGlucagon stimulatesbreakdown of glycogenbreakdown of glycogen

Raises blood glucose

INCRETIN SECRETION IN TYPE 2 DM T2DM is characterised by a severely impaired or

absent GIP insulinotropic effect that most likely results in worsening insulin secretion.

Note:-T2dm doesn’t result primarily from deficient incretin secretion.

Insulinotropic effect of GLP-1 is intact in T2DM and is used for its treatment.

GLP-1 based therapy for 3months in T2DM patients results in restoration of both first phase insulin secretion as well as late-phase secretion.

Postprandial GLP-1 levels are decreased in people with IGT and Type 2 diabetes

2001

20

15

10

5

00 60 120 180 240

Time (min)

GL

P-1

(pm

ol/l)

* * * **

**

*

Meal

NGT

T2DM

IGT

*P<0.05 T2DM vs NGT

Toft-Nielsen et al. J Clin Endocrinol Metab.

T2DM16

12

8

4

0 2200 0200 0600 1000 1400 1600

Breakfast Lunch Snack

Clock time (hours)

Glu

cose

(m

mol

/l)

T2DM + GLP-1

Normalisation of diurnal plasma glucose Normalisation of diurnal plasma glucose concentrations by continuous IV GLP-1 infusionconcentrations by continuous IV GLP-1 infusion(1.2 pmol/kg/min)(1.2 pmol/kg/min)

Rachman et al. Diabetologia. 1997

GLP-1 preserves human islet morphology and GLP-1 preserves human islet morphology and function in cultured islets in vitrofunction in cultured islets in vitro

Day 1

Day 3

Day 5

Control + GLP-1

Farilla et al. Endocrinology. 2003

Effect of GLP-1 on β-cell mass in Zucker diabetic fatty rats

Control GLP-1treated

β-C

ell m

ass

(mg)

16

12

8

4

0

β-Cell mass

Pro

lifer

atin

g β

-cel

ls (

%)

2.5

2.0

1.5

1.0

0.5

0Control GLP-1

treated

30

20

10

0Control GLP-1

treated

β-Cell apoptosisβ-Cell proliferation

Apo

ptot

ic β

-cel

ls (

%)

Farilla et al. Endocrinology. 2002

GLP-1 enhancement

GLP-1 secretion is impaired in Type 2 diabetes

Natural GLP-1 has extremely short half-life

Add GLP-1 analogues with longer half-life:

• exenatide

• liraglutide

Injectables

Block DPP-4, the enzyme that degrades GLP-1:• sitagliptin• vildagliptin

Oral agents

EXENATIDE:-[Byetta] It is a synthetic EX-4[39 amino acid] produced in

the salivary glands of Gila monster lizard[Haloderma suspectum].

It is not substrate for DPP4 ----GLY in place of ALA.

It lacks target bonds for neutral endopeptidaseNET. Its sec. and tertiary str.------prevents NET thus has long biological activity. No specific receptors are available for its action in

Gila monster. It has greater affinity for GLP-1R than GLP-1 in

GLP-1R expressing cells.

It is given subcutaneously [bid]. It has biological activity of 8hr. It has 5000 fold greater potency than GLP-1 in

reducing blood glucose. It delays gastric emptying both non-diabetic as

well as diabetic.. It can best be used with sulfonylureas. Its side effects are---nausea,vomiting and rarely

hypoglycemia. Now Exenatide LAR can be used ----once a

week. It has beneficial effect in osteopenia.

Exenatide and first-phase insulin response in Type 2 diabetes

Type 2 diabetes,placebo Type2 diabetes,exenatide

Healthy subjects, placebo

Exenatide vs healthy

Time (min)

Insu

lin s

ecr

etio

n

(pm

ol•

kg-1•m

in-1)

Fehse et al. J Clin Endocrinol Metab. 2005

SITAGLIPTIN:-[januvia] it is the sole DPP4 inhibitor in use for T2DM. It is used as non-peptide oral based therapy for

T2DM. Its selective inhibitor of DPP4 and donot interact

with closely related proteases. Its biological half life is 8-14hr and its bioavailability

is 87% in absence or presence of food. Appx 80% is excreted unchanged and rest is

metabolised by CYP3A4 and CYP2C8 in liver. Adverse effects-----

headaches,nasopharyngites,contact dermatites and arthralgia.

It also has beneficail effect in osteopenia.—thyroid secretion.

The main drawback of using DPP4 inhibitors ---------------decrease incretin secretion------------inhibition of glucose lowering effect.

Metformin and DPP4 have better effect in reducing blood glucose levels.----metformin increases GLP-1 secretion.

Dosage-100mg daily.in increased creatinin dose should be reduced to half[ <50mg creatinine clearence].

Sitagliptin improves fasting glucose level in monotherapy vs placebo

Pla

sma g

luco

se (

mm

ol/l)

Time (weeks)

0 6 123 18 24

Placebo Sitagliptin 100 mg

9.0

9.5

8.5

10.0

10.5

8.0

Charbonnel et al. Diabetes Care. 2006; Rosenstock et al. Clin Ther. 2006

Sitagliptin added to metformin [janumet]improves 24-hour glucose profile in Type 2 diabetes

Glu

cose

(m

mol

/l)

8:00 Day 1

13:00 19:00 0:00Day 2

7:30

6

7

8

9

10

13

11

12

Dose 17:30

Dose 218:30

Breakfast Lunch Dinner

Placebo + metformin

Sitagliptin 50 mg bd + metformin

Time

Therapies under development. VIDAGLIPTIN:- DPP4 inhibitor. Achieves plasma peak conc. Within 1-2hr. Half life is shorter than sitagliptin[by 1-2hr]. It is metabolised in liver. Can be given in kidney patients at normal dose. Not approved yet by FDA but by european

medicines agency.

LIRAGLUTIDE:- Potent GLP-1 analog with substitution of lys with

arg and attachment of C-16 free fatty acid to lys. It increases it half life as it gets bind with plasma

albumin. Also injected S.C and half life is 11-13hr. Side effects:-nausea,vomiting,and pancreatites

Why to go for incretin therapy??? Lowers blood glucose levels. Prevent deterioration of ß-cells. Leads to weight loss. Decreasing release of glucagon. Delaying gastric emptying. Restoring first-phase and second

phase insulin secretion. Decreasing food intake.

hp