Kinetic model of the human hepatocyte glycolysis, gluconeogenesis and glycogen metabolism

Transcription

1 Kinetic model of the human hepatocyte glycolysis, gluconeogenesis and glycogen metabolism Matthias König Charite Berlin Group meeting Berlin [11/01/10]

2 Outline Introduction Glucose homeostasis & role of liver Regulatory mechanisms Diabetes Model hypotheses Kinetic Model Overview model Insulin and glucagon dose-response curves Signal function Interconvertible enzymes Simulations & Results

3 Glucose homeostasis Plasma glucose controlled in small range despite large changes in glucose supply (meals) and glucose usage (muscle activity) ~ 5mM (max < 9mM: postprandial, min > 3mM: prolonged fasting / intensive muscle activity ) Other metabolites like glycerol, free fatty acids or ketone bodies much larger fluctuations

4 Glucose homeostasis Plasma glucose controlled in small range despite large changes in glucose supply (meals) and glucose usage (muscle activity) ~ 5mM (max < 9mM: postprandial, min > 3mM: prolonged fasting / intensive muscle activity ) Other metabolites like glycerol, free fatty acids or ketone bodies much larger fluctuations I. Avoid hypoglycaemic conditions Glucose supply for tissues and cells (brain, erythrocytes) I. Avoid hyperglycaemic conditions Toxicity of glucose (protein modifications, glucose induced oxidative effects) Glycogen has buffer role (storage of glucose)

5 Central role of liver in homeostasis glucose production under hypoglycaemic conditions (hepatic glucose production HGP) glycogenolysis and gluconeogenesis liver central organ of gluconeogenesis (kidney only under prolonged fasting of importance) glucose uptake under hyperglycaemic conditions (hepatic glucose utilization HGU) glycogen storage and glycolysis bulk of glucose is postprandial removed in extra-hepatic tissue (especially muscle)

6

7 Gluconeogenesis Synthesis of glucose from precursors Most important precursor lactate (glutamine, alanine and glycerol also important) Lactate important in Cori cycle

8 Regulation mechanisms Two different modes depending on blood glucose (and hormones) The switch between HGU and HGP is regulated by different mechanisms on different time scales Fast allosteric regulation Regulation by interconvertible enzymes in the range of minutes (hormonal signals) Slow regulation by changes in gene expression of key enzymes Regulation mainly by reciprocal regulation of enzymes characteristic for HGP and HGU HGU (GK, PFK1, PK, GS) HGP (G6PASE, FBP1, PC, PEPCK)

9 Allosteric regulations Fast regulation of key enzymes Especially important are fru26bp and glucose fru26bp potent allosteric activator of PFK1 and competitive inhibitor of FBP1 (control of flux trough glycolysis and gluconeogenesis) glucose has positive feedback mechanism on GK by decreasing binding of GKRP to GK; glucose potent inhibitor of GP [p]

10 Regulation by insulin and glucagon Insulin and glucagon are the main hormones which control the glucose homeostasis in the liver Insulin and glucagon depend on plasma glucose concentrations Insulin and glucagon are counterregulatory hormones

11 Insulin and glucagon Glucagon inhibits HGU (glycogen synthesis and glycolysis) and activates HGP (gluconeogenesis and glycogenolysis) Insulin reverse effects (activates HGU and inactivates HGP) During fasting the glucagon level is increased and insulin level decreased Postprandial the insulin level is increased and the glucagon level decreased

12 Interconvertible enzymes I Insulin and glucagon activate signal cascades which change the phosphorylation state of interconvertible enzymes of HGP and HGU short term changes in [camp] PKA phosphorylates most of the interconvertible enzymes of the HGP and HGU changed phosphorylation state of key enzymes phosphorylated [p] and dephosphorylated [dp] form of the enzymes have different kinetics changed fluxes

13 Interconvertible enzymes II

14 Diabetes Diabetes most frequent disease of glucose homeostasis In diabetes type II the insulin and glucagon response is defective blood glucose level increased insulin level decreased glucagon level increased

15 Diabetes Diabetes most frequent disease of glucose homeostasis In diabetes type II the insulin and glucagon response is defective blood glucose level increased insulin level decreased glucagon level increased Postprandial hyperglycaemia is due to reduced glucose clearance by muscle or adipose tissue and increased HGP. Increased gluconeogenesis instead of increased glycogenolysis is the dominant process

16 Hypotheses I. Hepatocyte switches between HGP and HGU depending on blood glucose (insulin and glucagon). only the fast regulations via interconvertible enzymes and allosteric effectors necessary no changes in gene expression necessary II. The glycogen store is emptied during HGP and filled during HGU. By the use of the glycogen store the task of glucose homeostasis can be better fulfilled III. Defective insulin and glucagon signals lead to an disturbed switch between HGU and HGP. The hormonal signals in diabetes should lead to diabetic effects (increased HGP).

17 Outline Introduction Glucose homeostasis & role of liver Regulation mechanisms Diabetes Model hypotheses Kinetic Model Overview model Hormone dose-response curves Signal function Interconvertible enzymes Simulations & Results

18 Overview model Glycolysis, gluconeogenesis and glycogen metabolism allosteric regulation hormonal regulation (insulin and glucagon) Detailed kinetic model 36 fluxes, 49 compounds 3 compartments (blood, cytosol, mitochondrium) detailed kinetics for all reactions [model view]

19 Dose-response curves Sigmoid insulin and glucagon dose-response curves Altered response curves in diabetes type II

20 Signal function Signal function defines the phosporylation state of the interconvertible enzymes depending on insulin and glucagon ( [ins], [glu] ) Simplifies the signal cascade between the hormones and the change in the phosphorylation state completely dephosphorylated completely phosphorylated monoton decreasing in insulin monoton increasing in glucagon

21 Signal function phosporylation state depending on insulin and glucagon

22 Signal function

23 Interconvertible enzymes Different kinetics for phosphorylated [p] and dephophorylated [dp] form. Enzyme kinetics is combination of [p] and [dp] form depending on the phosphorylation state

24 GS

25 Outline Introduction Kinetic Model Overview model Insulin and glucagon dose-response curves Signal function Interconvertible enzymes Simulations & Results I.Switch between HGU and HGP based on interconvertible enzymes and allosteric regulation II. Role of the glycogen storage for glucose homeostasis III. Diabetes

26 HGP concentrations [3mM] glucose

27 HGP fluxes [3mM] glucose

28

29 Summary HGP Biphasic First phase of glycogen depletion Second phase with empty glycogen stores Nearly all fluxes and concentrations change between the two phases Glycogen can be used for HGP During glycogen depletion an increased HGP can be found The system is able to perform HGP with empty glycogen stores, the glucose is fully produced by gluconeogenesis Gluconeogenesis is increased if the glycogen store is depleted

30 HGU concentrations [8mM] glucose

31 HGU fluxes [8mM] glucose

32

33 Summary HGU Biphasic First phase of glycogen storage Second phase with full glycogen stores Nearly all fluxes and concentrations change between the two phases Glycogen storage can improve the HGU During glycogen storage an increased HGU is found The system is able to perform HGU with full glycogen stores, the glucose is completely used in the glycolysis Glycolysis is increased if the glycogen store is filled

34 Switch between HGU and HGP

35 Important fluxes and concentrations

36 Summary switch depending on glucose The system switches between HGP and HGU depending on the blood glucose concentration Switch at ~5-6.5 mm depending on the glycogen content Glycogen stored above ~5mM blood glucose, the glycogen store is depleted below 5 mm

37 Glucose-lactate dependency

38 Summary glucose - lactate dependency Lactate nearly no effect on the glycogen storage Switch point changed a little velocity of glycogen depletion depends on lactate concentration (HGP) Lactate has only effect on HGP under hypoglycaemic condition low concentrations of lactate limiting factor for the gluconeogenesis (no HGP possible without lactate) increase in lactate (>1mM) does not increase the HGU

39 Diabetes type II

40 Diabetes type II

41 Summary diabetes type II Glycogen storage is nearly unaffected by changed insulin and glucagon dose-response curves Diabetic effects can be found with the diabetes type II response curves switch between HGP and HGU is changed to increased glucose concentrations (~9-10 mm) rncreased HGP (mainly due to increased gluconeogenesis) reduced HGU Insulin and glucagon restoration reduce the diabetic effects glucagon is more effective than insulin

42 Summary The hepatocyte switches between HGP and HGU depending on blood glucose, insulin and glucagon. only the fast regulations via interconvertible enzymes and allosteric effectors are necessary no changes in gene expression are necessary The glycogen store is emptied during HGP and filled during HGU. By the use of the glycogen store the task of glucose homeostasis can be better fulfilled If the insulin and glucagon signals are defective also the switch between HGU and HGP is disturbed. In the case of diabetes type II results the insulin and glucagon defect in an increased HGP. glucagon more effective than insulin

43 Summary II First detailed kinetic model of hepatic glycolysis, gluconeogenesis and glycogen metabolism with the incorporation of the interconvertible enzymes and hormones Many experimental effects can be shown Nearly no glycolysis during glycogen storage in HGU Effect of gluconeogenic precursors (lactate) rate limiting in small concentrations, no further increase in gluconeogenesis in high concentrations Diabetic results Higher HGP and higher plasma glucose concentration Increased gluconeogenesis not increased glycogenolysis main contributor to HGP Optimization of states by changes in gene expression (not shown)

44 Hermann-Georg Holzhütter Sascha Bulik

45 Gene expression

46

47 Regulation by insulin and glucagon Insulin and glucagon are the main hormones which control the glucose homeostasis in the liver Insulin and glucagon concentrations depend on the plasma glucose concentrations