Insulin physiology and biochemical effects by Dr Shahjada Selim

PANCREATIC HORMONES: Insulin & Others- physiology and biochemical effects by Dr Shahjada Selim

1. PANCREATIC HORMONES:
Insulin & Others
Dr Shahjada Selim
Associate Professor
Department of Endocrinology
Bangabandhu Sheikh Mujib Medical University, Dhaka
Email: [email protected], [email protected]

2. Presentation Flow
•Functional Anatomy of Pancreas
•Hormones produced by the pancreas
•Insulin: Structure
•Insulin: Secretion
•Insulin: Action
•Insulin: Secretory Defects in T2DM

3. PANCREAS:
Pancreas isan
elongated hook
shapedorganthatlies
sideways inthe
abdomen,thehookon
therightsideand
turneddownwards.
Itliesbehind&below
thestomachfittedin
the C-shaped
concavityoftheloopof
duodenum (small
intestine).

4. Pancreas: Functional Anatomy
Pancreas is a mixed gland with
both exocrine & endocrine
functions.

5. FUNCTIONAL ANATOMY
OF PANCREAS
EXOCRINE TISSUES
•Larger part
•Grape-like clusters of
secretory cells forming
sacs called ACINI
•Which empty into the
pancreatic ducts that
eventually empty into
the duodenum
•It secrete Pancreatic
juice containing
trypsinogen,
chymotrypsinogen,
elastase,
carboxypeptidase,
pancreatic lipase,
nucleases and amylase

6. ENDOCRINE TISSUES
•Smaller part consists
of isolated islands of
endocrine tissues
called as Islets of
Langerhans
dispersed throughout.
•It forms only 1% of
the total pancreatic
mass.
•1-2 million in number
•0.3 mm in diameter
These secrete pancreatic
hormones.

7. Islets of Langerhans
A or αcells (25%)
secrete Glucagon
B or βcells (60%)
secrete Insulin and
Amylin
D or δcells (10%)
secrete Somatostatin
F or PP cells are very
few and secrete
Pancreatic Polypeptide
Islets of
Langerhans

8. Pancreatic Hormones
•Insulin (also
Proinsulin & C-
peptide)
•Glucagon
•Amylin
•Somatostatin
•Pancreatic
peptide

9. INSULIN

10. Insulin: Structure
•InsulinisaPeptide
hormone.
•Itconsistsof2amino
acid(51AA)chains
thatare joined
togetherbydisulphide
linkages.Ifthe2AA
chainsaresplitapart
thenthefunctional
activityofinsulinis
lost.
•Ithasamolecular
weightof5808.

11. Synthesis of Insulin
Preproinsulin (RER)
(11,500)
↓
Proinsulin (Golgi
Apparatus)
(9,000)
↓cleavage
Insulin + C-peptide
(stored in secretory
vesicles)
↓
Secreted into blood
↓
After use degraded by
the enzyme called as
Insulinase in the liver &
to a lesser extent in the
kidneys & muscles

12. Insulin: Metabolism
•Plasma half-life: 4-6 minutes
•Cleared from the circulation in 10-15
minutes
•However, the C-peptide takes about
30 Minutes to be degraded.
Advantage?

13. Insulin Secretion
Insulinreleaseisnot
continuousevenafter
amealbutoscillates
withaperiodof3-6
minutes:spurtsof
insulinreleasemore
than800pmol/ltoless
than100pmol/l.
Thisoscillationis
importanttoconsider
when administering
insulin-stimulating
medication as
oscillationisthetarget
&notaconstanthigh
concentration.

14. Theidealizeddiagramshowsthefluctuationofbloodsugar(red)andthesugar-
loweringhormoneinsulin(blue)inhumansduringthecourseofadaycontainingthree
meals.Inaddition,theeffectofasugar-richversusastarch-richmealishighlighted.

15. INSULIN RECEPTOR
Theinsulinreceptor(IR)isatransmembrane
receptorthatisactivatedbyinsulin,IGF-I,
IGF-IIandbelongstothelargeclassof
tyrosinekinasereceptors.
Inotherwords,itfunctionsasanenzymethat
transfersphosphategroupsfromATPto
tyrosineresiduesonintracellulartarget
proteins.

16. INSULIN RECEPTOR
It isrformed by 4
glycoprotein subunits:
•2 alpha subunits
(present outside the
cell membrane)
•2 beta subunits
(penetrate through
the memb. Into the
cell cytoplasm)
The alpha and beta
subunits are linked by
disulfide bonds.
It is an enzyme-linked
receptor.

17. ContainsmultipleenzymegroupscalledasInsulin-
receptorsubstrates(IRS).DifferenttypesofIRS
areexpressedindifferenttissues(IRS1-3).
Bindingofinsulintothealphasubunitscausesthe
betasubunitstophosphorylatethemselves
(autophosphorylation),thusactivatingthecatalytic
activityofthereceptor.
Theactivatedreceptorthenphosphorylatesa
numberofintracellularproteins,whichinturnalters
theiractivity,therebygeneratingabiological
response.

18. Severalintracellularproteinshavebeen
identifiedasphosphorylationsubstratesfor
theinsulinreceptor,thebest-studiedofwhich
isinsulinreceptorsubstrate1orIRS-1.
WhenIRS-1isactivatedbyphosphorylation,
alotofthingshappen.Amongotherthings,
IRS-1servesasatypeofdockingcenterfor
recruitmentandactivationofotherenzymes
thatultimatelymediateinsulin'seffects.

19. INSULIN RECEPTOR
Wheninsulinlevelfalls,especiallyinthosewith
highinsulinsensitivity,bodycellsbeginonlyto
haveaccesstolipidsthatdonotrequire
transportacrossthemembrane.
So,inthisway,insulinisthekeyregulatoroffat
metabolismaswell.Biochemically,theinsulin
receptorisencodedbyasinglegeneINSR,
fromwhichalternatesplicingduring
transcriptionresultsineitherIR-AorIR-B
isoform.

20. INSULIN RECEPTOR
Downstreampost-translationaleventsofeither
isoformresultintheformationofa
proteolyticallycleavedαandβsubunit,which
uponcombinationareultimatelycapableof
homoorhetero-dimerisationtoproducethe
≈320kDadisulfide-linkedtransmembrane
insulinreceptor.

21. Insulin receptor activation
Insulin attaches to alpha subunit of receptor
↓
The beta subunit of the receptor becomes
autophosphorylated
↓
Tyrosine kinase is activated
↓
IRS is Phosphorylated.
↓
Action exerted e.g. glucose uptake by the cells is
enhanced.

23. MECHANISM OF INSULIN
SECRETION:

24. Mechanism of Insulin secretion
Glucose binds to GLUT-2 in the beta cells of Islets of Langerhans and enters the
cell.
↓
Glucokinase phophorylates Glucose to G-6-phosphate
↓
G-6-phosphate is oxidized to form ATP
↓
ATP inhibits the ATP-sensitive K channels of the cell
↓
Closure of the K channel leads to depolarization of the cell membrane
↓
Voltage-gated Ca channels are opened
↓
Ca influx takes place
↓
Stimulates fusion of insulin-containing vesicles with cell membrane
↓
Exocytosis of Insulin to the ECF

25. Insulinincreasesglucosetransportinto
most,butnotall,insulin-sensitivecells
byactivationofitsreceptor(GLUT).
RoleofGLUCOSETRANSPORTERS (GLUT).
•FourteendifferentGlucosetransportershavebeen
characterized,GLUT1-14intheorderofdiscovery.
•TheGLUTallaccomplishpassivediffusionof
glucoseacrossthecellmembrane.Onceinsidethe
cell,glucoseisimmediatelyphosphorylatedto
Glucose-6-Phosphatewhichcannotleavethecell
throughthebi-directionalGLUTproteinandis
“trapped”insidethecell.
•EachGLUThasbeenevolvedforadifferenttask&
adifferenttissue.

26. GLUT Distribution
GLUT-1,2,3&5areNOTaffectedbyinsulin:
GLUT-1:transportsglucoseacrossbloodbrainbarrier
GLUT-2:fromkidney&intestinalcellsintotheblood
stream.Glucoseentersthekidney&intestinalcells
bySGLT(Na&Glucotransporters)
GLUT-3:neurons
GLUT-4:inallmajorcellsusingGlucose(asmuscle
andadiposetissues)
Therefore,inallthesetissuestheglucoseentryis
insulinindependent.

27. •OnlyGLUT-4isinsulin-dependent&
occursinthemuscles&adipocytes.
•ThesecellsmaintainapoolofGLUT-
4moleculesinvesiclesintheircell
cytoplasm.
GLUT Distribution

28. GLUT-4 MOA
Insulin attaches to the
receptor
↓
Insulin receptor is activated
↓
Vesicles containing GLUT-
4 move rapidly to the cell
membrane
↓
Vesicles fuse with the cell
membrane, inserting the
transporter in the
membrane
↓
When insulin action
ceases, the transporter-
containing patches of
membrane are
endocytosed
↓
Vesicles are reformed
ready for the next action

29. ROLE OF GLUT-4
In the absence of Insulin, Glucose
cannot enter the cell
Insulin signals the cell to insert GLUT 4
transporters into the membrane, allowing
glucose to enter the cell.

30. Actions of Insulin

31. Theprincipalfunctionofinsulinisto
maintainbloodglucosehomeostasis.

32. Blood sugar pool:

33. Insulin is the only hormone
capable of lowering the
blood glucose level.

34. Insulin
•Insulin is an anabolichormone.
•Insulin is the hormone of the fed/absorptivestate.
•Insulin has important effects on:
-Carbohydrates
-Fats
-Proteins
•It lowersblood glucose levels of:
-Glucose
-fatty acids
-amino acids
•It is a hormone associated with energy abundance.

35. Lack of effect of Insulin on Glucose
uptake by the brain
•Brainusesonlyglucoseasitsenergysource,
therefore,itisimportantthatbloodglucose
levelsbemaintainedaboveacriticallevel.
•BrainispermeabletoGlucose&canuseit
evenwithouttheintermediationofInsulin.
•Whenbloodglucoselevelsfalltoolow(1.2-
2.8mmol/dL),symptomsofhypoglycemic
shockdevelop[characterizedbyprogressive
nervousirritabilitythatleadstofainting,
seizures&evencoma].

38. Glucose entry into the LIVER
Glucose does not depend on GLUT-4 for
entry into the LIVER.

39. 1.InsulinstimulatesGlucoseuptakebythe
cells(mostlythruGLUT-4).
2.InsulinstimulatesGlycogenesisinboththe
Liver&theSkeletalmuscles.
3.ItinhibitsGlycogenolysis.
4.ItinhibitsGluconeogenesis.
5.Itpromotesliveruptake,use&storageof
Glucose
Insulin Action on Carbohydrate Metabolism

40. Throughouttheday,themusclesuseFattyAcids
asanenergysourceEXCEPT:
-Duringmoderate&heavyexercisewhenthey
uptakeGlucosethroughInsulin-independent
pathway.
-POST-MEAL:Insulinfacilitateslargeamounts
ofglucoseintothecells.
Underrestingconditions,thecellsaredefendanton
GLUT-4forglucoseuptake
But,
Withmoderateorsevereexercise,specialGLUT-4
vesicles(presentonlyinmuscles)moveintocell
membraneinresponsetoexerciseonly&donot
requireInsulin
ThatiswhyEXERCISELOWERSBLOODSUGAR!
Insulin Action on Carbohydrate Metabolism

41. Liver does not use GLUT-4 for Glucose uptake. Instead it uptakes the
glucose through the following mechanism:
Insulin activates two very important enzymes:
-Hexokinase
-Glucokinase
↓
Glucose enters the liver along its conc. Gradient. Both these enzymes
phosphorylate the glucose present inside the liver cells to Glucose
6-phosphate.
↓
Glucose-6-phosphate cannot cross the hepatic membrane and is
trapped inside it.
↓
Thus, ratio of intracellular free Glucose to extracellular free glucose is
decreased.
↓
Glucose continues to diffuses into the liver cells along the continuous
concentration gradient
Insulin Action on Carbohydrate Metabolism

42. Effect on
GLYCOGENOLYSIS
Insulin inhibits
Glycogenolysis by inactivating
Liver phosphorylase:
GLYCOGEN
GLUCOSE
Liver phosphorylase
Net effect is Increased
storage of Glycogen in the
liver.
Effect on
GLYCOGENESIS
Insulin enhances
Glycogenesis by increasing
the activity of Glycogen
synthase &
phosphofruktokinase
enzyme.
GLUCOSE
GLYCOGEN
Net effect is increased
synthesis of Glycogen in
the liver.
Insulin Action on Carbohydrate Metabolism

43. Action on CHO metabolism.
Liver (cont. )
Effect on FATTY ACID SYNTHESIS:
•Insulin promotes conversion of
excess glucose into fatty acids
–When the Glycogen content
exceeds 5-6% of the liver mass
(about 100gms), then the excess
glucose entering the liver cells is
converted into Fatty acids.
↓
Triglycerides
↓
Taken to the adipocytes and
stored there.
Effect on Gluconeogenesis
•Insulininhibits
Gluconeogenesis
byalteringthe
quantity&activity
ofLiverenzymes
requiredforthe
reaction.

44. Glucose is released from the Liver b/w meals:
Glucose is released from the liver between meals by following pathways:
1.Decreasing blood glucose
↓
Insulin secretion decreased
↓
All actions exerted by the pancreas are reversed!
2.Activity of enzyme Liver Phosphorylaseis enhanced
↓
Glycogen is split into Glucose phosphate
3.Enzyme Glucose phosphatase is activated
↓
Phosphate radical is split away from the Glucose
↓
Glucose-6-Phosphate Glucose.
↓
Free Glucose diffuses back into the blood.

45. Insulin Action On Protein
Metabolism

46. Effect on Protein Metabolism
•Insulinenhancesuptakeofmanyamino
acidbythecells(esp.valine,leucine,
tyrocine).ItisinterestingtonotethatInsulin
causesuptakeofapp.6aminoacidswhile
GHstilmulatestheuptakeofanother6AA,
thusbothtargetdifferentAA.So,theyare
synergisticinaction.
•ItincreasesthetranscriptionofDNA(more
RNAformed).

47. Effect on Protein Metabolism
•ItincreasesthetranslationofmRNAonthe
ribosomesformingnewproteins.
•Insulininhibitsproteincatabolism&
decreasestherateofAAreleasefromthe
cells.
•Intheliver,itdecreasestherateof
gluconeogenesis&thusconservesamino
acidsforproteinsynthesis.

48. Effect on Protein catabolism
•Insulinpromotesproteinsynthesis
&inhibitsitsCatabolism.
•Itisthusessentialforgrowth.
•Insulin&Growthhormonethus
actsynergisticallytopromote
growth.

49. INSULIN ACTION ON FAT METABOLISM

50. Action on Fat metabolism
InsulinPromotes
1.SynthesisofFattyacidsandTriglycerides
2.TransportofFattyacidsintotheadiposetissues.
3.StorageofFatintheadiposetissue.
•InsulinincreasesGlycogenesisbutwhen5-6%of
livermassisGlycogen,thenadditionalGlucose
enteringtheliverisconvertedtoFat.
•Fattyacidssynthesizedbylivercellsareusedby
themforTGsynthesis.

51. InsulinPromotes
•Itincreasesthetransportofglucoseintoadipose
tissuecellsbymeansofGLUT-4recruitment.
Glucoseservesasaprecursorfortheformationof
fattyacidsandglycerol,whicharetherawmaterials
fortriglyceridesynthesis.
•Insulinpromotesthestorageoffatsintheadipose
tissuebyinhibitingtheenzymehormone-sensitive
lipase,whichdegradestheTG.

52. Summary of Insulin Actions
•InsulinpromotesuptakeofGlucose&amino
acidsbydifferentcellsofthebody.Indoing
soitlowersthebloodglucoselevelspost
meal.
•InsulinincreasesGlycogenesis,Lipogenesis
&proteinformation.

53. Summary of Insulin Actions
•InsulininhibitsGlycolysis,Gluconeogensis,
Lipolysis&proteinbreakdown.
•Severaltissuesdonotdependoninsulinfor
theirglucoseuptake—namely:thebrain,
workingmuscles,andliver.
•Severaltissuesdonotdependoninsulinfor
theirglucoseuptake—namely:thebrain,
workingmuscles,andliver.

54. Factors affecting Insulin secretion
Stimulating factors
•Glucose
•Amino acids
•Free fatty acids
•Gastrointestinal hormones:
gastrin, secretin, GIP (glucose-
dependantinsulinotropic
peptide)
•Parasympathetic stimulation:
Ach
•Sulfonylurea drugs
Hormones releasedfromthe
digestivetractthat“notify”the
pancreaticcelloftheimpendingrise
inbloodnutrients(primarilyblood
glucose)aretermedincretins.They
leadtoanincreaseininsulin
secretion.
Inhibiting Factors
•Decreased blood
Glucose
•Fasting
•Somatostatin
•Sympathetic
stimulation (as seen
in fear & flight):
epinephrine
•Leptin

56. J Clin Invest DOI: 10.1172/JCI24269
Aschematicpresentationofglucose-
inducedbiphasicinsulinsecretionby
isletβcells.Stimulationbyahigh
concentrationofglucoseelicitsarapid
increaseininsulinsecretionwithin1–2
minutes(firstphase).Within10
minutes,inthepresenceofthesame
stimulatoryconcentrationofglucose,
thesecretionrateisdecreased,andit
graduallybeginstoincreaseagain
thereafter(secondphase).Second-
phaseinsulinreleaseismore
prominentinratandhumanthanin
mouseβcells.
First-phaseinsulinreleaseisgeneratedbyfusionofβgranulesalreadyin
theRRPandtheplasmamembrane,whichiscalledtriggering.Forthe
secondphase,granulestranslocatefromtheRPtotheRRP,whichyields
expansionand/orreplenishmentoftheRRP.
ThefirstandsecondphasesofinsulinsecretionareduetoK
ATPchannel–
dependentand–independentglucosesignaling,respectively.