Insulin decreases phosphoenolpyruvate carboxykinase (GTP) mRNA activity by a receptor-mediated process

Melatonin stimulates transcription of the rat phosphoenolpyruvate carboxykinase gene in hepatic cells

Melatonin plays physiological roles in various critical processes, including circadian rhythms, oxidative stress defenses, anti-inflammation responses, and immunity; however, the current understanding of the role of melatonin in hepatic glucose metabolism is limited. In this study, we examined whether melatonin affects gene expression of the key gluconeogenic enzyme, phosphoenolpyruvate carboxykinase (PEPCK). We found that melatonin treatment increased PEPCK mRNA levels in rat highly differentiated hepatoma (H4IIE) cells and primary cultured hepatocytes. In addition, we found that melatonin induction was synergistically enhanced by dexamethasone, whereas it was dominantly inhibited by insulin. We also report that the effect of melatonin was blocked by inhibitors of mitogen-activated protein kinase/extracellular signal-regulated protein kinase (MAPK/ERK), RNA polymerase II, and protein synthesis. Furthermore, the phosphorylated (active) forms of ERK1 and ERK2 (ERK1/2) increased 15 min after melatonin treatment. We performed luciferase reporter assays to show that melatonin specifically stimulated promoter activity of the PEPCK gene. Additional reporter analysis using 5'-deleted constructs revealed that the regulatory regions responsive to melatonin mapped to two nucleotide regions, one between -467 and -398 nucleotides and the other between -128 and +69 nucleotides, of the rat PEPCK gene. Thus, we conclude that melatonin induces PEPCK gene expression via the ERK1/2 pathway at the transcriptional level, and that induction requires de novo protein synthesis.

Phenobarbital reduces blood glucose and gluconeogenesis through down-regulation of phosphoenolpyruvate carboxykinase (GTP) gene expression in rats

The regulatory mechanism of phosphoenolpyruvate carboykinase (GTP) (EC 4.1.1.32) (PEPCK) gene expression and gluconeogenesis by phenobarbital (PB), which is known to induce drug-metabolizing enzymes, was investigated. Higher level of PEPCK mRNA was observed in spherical rat primary hepatocytes on EHS-gel than monolayer hepatocytes on TIC (type I collagen). We found that PB directly suppressed PEPCK gene expression in spherical hepatocytes on EHS-gel, but not in those on TIC. PB strongly suppressed cAMP-dependent induction of PEPCK gene expression. Tyrosine aminotransferase (TAT), another gluconeogenic enzyme, was induced by cAMP, but not suppressed by PB. Chronic administration of PB reduced hepatic PEPCK mRNA in streptozotocin-induced diabetic and nondiabetic rats, and PB reduced blood glucose level in diabetic rats. Increased TAT mRNA in diabetic rats was not suppressed by PB. These results indicated that PB-dependent reduction is specific to PEPCK. From pyrvate challenge test, PB suppressed the increased gluconeogenesis in diabetic rats. PEPCK gene promoter activity was suppressed by PB in HepG2 cells. In conclusion, we found that spherical hepatocytes cultured on EHS-gel are capable to respond to PB to suppress PEPCK gene expression. Moreover, our results indicate that hypoglycemic action of PB result from transcriptional repression of PEPCK gene and subsequent suppression of gluconeogenesis.

Modulation of insulin-stimulated degradation of human insulin receptor substrate-1 by Serine 312 phosphorylation

Ser/Thr phosphorylation of insulin receptor substrate-1 (IRS-1) is a negative regulator of insulin signaling. One potential mechanism for this is that Ser/Thr phosphorylation decreases the ability of IRS-1 to be tyrosine-phosphorylated by the insulin receptor. An additional mechanism for modulating insulin signaling is via the down-regulation of IRS-1 protein levels. Insulin-induced degradation of IRS-1 has been well documented, both in cells as well as in patients with diabetes. Ser/Thr phosphorylation of IRS-1 correlates with IRS-1 degradation, yet the details of how this occurs are still unknown. In the present study we have examined the potential role of different signaling cascades in the insulin-induced degradation of IRS-1. First, we found that inhibitors of the phosphatidylinositol 3-kinase and mammalian target of rapamycin block the degradation. Second, knockout cells lacking one of the key effectors of this cascade, the phosphoinositide-dependent kinase-1, were found to be deficient in the insulin-stimulated degradation of IRS-1. Conversely, overexpression of this enzyme potentiated insulin-stimulated IRS-1 degradation. Third, concurrent with the decrease in IRS-1 degradation, the inhibitors of the phosphatidylinositol 3-kinase and mammalian target of rapamycin also blocked the insulin-stimulated increase in Ser(312) phosphorylation. Most important, an IRS-1 mutant in which Ser(312) was changed to alanine was found to be resistant to insulin-stimulated IRS-1 degradation. Finally, an inhibitor of c-Jun N-terminal kinase, SP600125, at 10 microm did not block IRS-1 degradation and IRS-1 Ser(312) phosphorylation yet completely blocked insulin-stimulated c-Jun phosphorylation. Further, insulin-stimulated c-Jun phosphorylation was not blocked by inhibitors of the phosphatidylinositol 3-kinase and mammalian target of rapamycin, indicating that c-Jun N-terminal kinase is unlikely to be the kinase phosphorylating IRS-1 Ser(312) in response to insulin. In summary, our results indicate that the insulin-stimulated degradation of IRS-1 via the phosphatidylinositol 3-kinase pathway is in part dependent upon the Ser(312) phosphorylation of IRS-1.

Insulin signal transduction pathways and insulin-induced gene expression

Insulin regulates metabolic activity, gene transcription, and cell growth by modulating the activity of several intracellular signaling pathways. Insulin activation of one mitogen-activated protein kinase cascade, the MEK/ERK kinase cascade, is well described. However, the effect of insulin on the parallel p38 pathway is less well understood. The present work examines the effect of inhibiting the p38 signaling pathway by use of specific inhibitors, either alone or in combination with insulin, on the activation of ERK1/2 and on the regulation of gene transcription in rat hepatoma cells. Activation of ERK1/2 was induced by insulin and was dependent on the activation of MEK1, the kinase upstream of ERK in this pathway. Treatment of cells with p38 inhibitors also induced ERK1/2 activation/phosphorylation. The addition of p38 inhibitors followed by insulin addition resulted in a greater than additive activation of ERK1/2. The two genes studied, c-Fos and Pip92, are immediate-early genes that are dependent on the ERK1/2 pathway for insulin-regulated induction because the insulin effect was inhibited by pretreatment with a MEK1 inhibitor. The addition of p38 inhibitors induced transcription of both genes in a dose-dependent manner, and insulin stimulation of both genes was enhanced by prior treatment with p38 inhibitors. The ability of the p38 inhibitors to induce ERK1/2 and gene transcription, both alone and in combination with insulin, was abolished by prior inhibition of MEK1. These data suggest possible cross-talk between the p38 and ERK1/2 signaling pathways and a potential role of p38 in insulin signaling.

Purification of a RNA-binding protein from rat liver. Identification as ferritin L chain and determination of the RNA/protein binding characteristics

In cultured rat hepatocytes the degradation of phosphoenolpyruvate carboxykinase mRNA might be regulated by protein(s), which by binding to the mRNA alter its stability. The 3'-untranslated region of phosphoenolpyruvate carboxykinase mRNA as a potential target was used to select RNA-binding protein(s) from rat liver by the use of gel retardation assays. A cytosolic protein was isolated, which bound to the phosphoenolpyruvate carboxykinase mRNA 3'-untranslated region and other in vitro synthesized RNAs. The protein was purified to homogeneity; it had an apparent molecular mass of 400 kDa and consisted of identical subunits with an apparent size of 24.5 kDa. Sequence analysis of a tryptic peptide from the 24.5-kDa protein revealed its identity with rat ferritin light chain. Binding of ferritin to RNA was abolished after phosphorylation with cAMP-dependent protein kinase and was augmented after dephosphorylation with alkaline phosphatase. Weak binding was observed in extracts from okadaic acid-treated cultured hepatocytes compared with untreated cells. Preincubation of ferritin with an anti-phosphoserine or an anti-phosphothreonine antibody attenuated binding to RNA, while an anti-phosphotyrosine antibody generated a supershift indicating that phosphoserine and phosphothreonine but not phosphotyrosine residues were in close proximity to the RNA-binding region. Ferritin is the iron storage protein in the liver. Binding of ferritin to RNA was diminished in the presence of increasing iron concentrations, whereas the iron chelator desferal was without effect. It is concluded that ferritin might function as RNA-binding protein and that it may have important functions in the general regulation of cellular RNA metabolism.

The inhibition of phosphoenolpyruvate carboxykinase following in vivo chronic phenobarbital treatment in the rat is due to a post-translational event

Chronic treatment of rats with phenobarbital has been reported to decrease gluconeogenesis in rat hepatocytes by a 50% inhibition of phosphoenolpyruvate (P-pyruvate) carboxykinase activity [Argaud, D., Halimi, S., Catelloni, F. & Leverve, X. (1991) Biochem. J. 280, 663-669]. Contrary to the current knowledge of P-pyruvate carboxykinase regulation, we failed to find a diminution of either P-pyruvate carboxykinase protein (by using a polyclonal antibody) or P-pyruvate carboxykinase mRNA, in the liver of rats treated with phenobarbital for 2 weeks. Kinetic studies of P-pyruvate carboxykinase activity, measured by either carboxylation of P-pyruvate or decarboxylation of oxaloacetate, revealed a decrease in both V(max) and Km after phenobarbital treatment, whereas the nutritional state affected only the V(max), as expected. Assessment of P-pyruvate carboxykinase specificity was confirmed by the full inhibition of the enzyme with its specific inhibitor 3-mercaptopicolinate in the micromolar range. P-Pyruvate carboxykinase, purified either by ammonium sulfate fractionation or by immunoprecipitation, exhibited a similar decrease in affinity after phenobarbital treatment. Although the molecular mass does not appear to be altered, the pH sensitivity to 3-mercaptopicolinate inhibition and the enzyme recovery after immunoprecipitation both seemed to be affected. This leads us to propose that the effect of chronic phenobarbital treatment on P-pyruvate carboxykinase activity is not the result of transcriptional regulation but is exerted at the post-translational level.

Transcriptional regulation of liver phosphoenolpyruvate carboxykinase by biotin in diabetic rats

Rat liver phosphoenolpyruvate carboxykinase (PEPCK) activity was followed over a time period of 5 h following administration of biotin to streptozotocin-induced diabetic rats. In parallel with the decrease in enzyme activity liver PEPCK mRNA decreased by 85% at 3 h after injection of biotin to diabetic rats. There was no significant change in the accumulation of kidney PEPCK mRNA. Parallel studies with insulin indicated that biotin had a regulatory effect similar to that of insulin on liver PEPCK mRNA. The administration of biotin did not change the insulin status of the diabetic rat indicating that biotin did not act via insulin. The transcriptional activity of the hepatic PEPCK gene, as measured by nuclear run-on assay, was decreased by 57% within 30 min of biotin administration. The results suggest that biotin regulates hepatic, but not renal, PEPCK mRNA concentration at the transcriptional level in diabetic rats.

Identification of a sequence in the PEPCK gene that mediates a negative effect of insulin on transcription

Phosphoenolpyruvate carboxykinase (PEPCK) governs the rate-limiting step in gluconeogenesis. Glucocorticoids and adenosine 3',5'-monophosphate (cAMP) increase PEPCK gene transcription and gluconeogenesis, whereas insulin has the opposite effect. Insulin is dominant, since it prevents cAMP and glucocorticoid-stimulated transcription. Glucocorticoid and cAMP response elements have been located in the PEPCK gene and now a 15-base pair insulin-responsive sequence (IRS) is described. Evidence for a binding activity that recognizes this sequence is presented.

Regulation of gene expression by insulin in adipose cells: opposite effects on adipsin and glycerophosphate dehydrogenase genes

Insulin is known to play the role of a positive effector both in vitro on the adipose conversion process and in vivo on the fatty acid synthesis and esterification processes in adipose tissue. The effects of insulin on the expression of two genes activated during adipose conversion, glycerol-3-phosphate dehydrogenase (GPDH) and adipsin genes, have been investigated in 3T3 F442A adipose cells. Within a physiological range of concentrations, insulin exerts opposite effects on the levels of GPDH (EC50 approximately 0.2 nM) and adipsin (EC50 approximately 1 nM) mRNAs. Its negative effect on the abundance of adipsin mRNA involves primarily a rapid inhibition of the transcriptional rate (less than 2 h). Its positive effect on the abundance of GPDH mRNA is due to a stimulation of the transcriptional rate accompanied by a delayed stabilization of GPDH mRNA. In addition, insulin exerts a specific effect on the length of the poly(A) tract of the adipsin mRNA. These results show that a single mechanism for the regulation of adipose-related genes by insulin can be excluded but rather suggest a complex phenomenon in which various levels of regulation take place.

Regulation of the expression of the phosphoenolpyruvate carboxykinase gene in cultured rat hepatocytes by glucagon and insulin

The induction of phosphoenolpyruvate carboxykinase (PEPCK) by glucagon was studied in primary rat hepatocyte cultures by determining the time course of the sequential events, increases in the enzyme's mRNA abundance, synthesis rate, amount and activity, and by investigating the antagonistic action of insulin on the induction by glucagon. 1. The mRNA of PEPCK was induced maximally 2-3 h after addition of 10 nM glucagon, as detected by Northern-blot analysis after hybridization with a biotinylated antisense RNA of PEPCK. 2. The synthesis rate of PEPCK increased maximally 2-3 h after application of glucagon as revealed by pansorbin-linked immunoprecipitation of [35S]methionine-labelled PEPCK. 3. The enzyme amount and activity was maximally induced 4 h after glucagon application. 4. The mRNA of PEPCK was half-maximally induced by 0.1 nM and maximally by 1 nM and 10 nM glucagon. The half-maximal induction by 0.1 nM glucagon was antagonized almost totally, and the maximal induction by 1 nM glucagon partially, while the maximal induction by 10 nM glucagon remained unaffected by 10 nM insulin. The results show that in cultured rat hepatocytes physiological concentrations of glucagon stimulated the induction of PEPCK by an increase in mRNA, that the glucagon-dependent increase in mRNA and enzyme-synthesis rate occurred in parallel and preceded the increase of enzyme amount and activity by 1-1.5 h, and that physiological levels of insulin antagonized the induction by glucagon in the physiological concentration range, with glucagon being the dominant hormone.

Selective transcription of an insulin-regulated gene in nuclear extracts of rat hepatoma cells

We have previously shown that when H4 hepatoma cells are pretreated with insulin, plant lectins, phorbol esters, or insulin mediator, the steady state concentration of gene 33 mRNA is markedly increased. The increase in gene 33 mRNA concentration with insulin is due to an increase in the transcription rate of this gene. In the present report we demonstrate that nuclear extracts prepared from H4 hepatoma cells pretreated with insulin exhibit enhanced transcription of gene 33 RNA from a DNA template containing the cap site and 1500 bp upstream of the 33 gene. This is a stable effect of insulin on the nuclear RNA polymerase II system since it is observed in frozen and thawed nuclear extracts as well as fresh nuclear extracts.

Insulin-mimetic actions of wheat germ agglutinin and concanavalin A on specific mRNA levels

Insulin has pleiotropic effects on sensitive cells, including the regulation of specific mRNA accumulation initiated by the binding of insulin to its plasma membrane receptor. Lectins, such as wheat germ agglutinin (WGA) and concanavalin A (Con A), are known to be insulin mimetic. It is thought that WGA and Con A interact with the insulin receptor or associated membrane glycoproteins which, when activated, lead to insulin-mimetic responses. We attempted to determine whether WGA and Con A could induce the accumulation of a specific messenger RNA (p33-mRNA). Insulin treatment of H4IIE (H4) hepatoma cells increased the concentration of p33-mRNA within 30 min after addition, with a maximum effect of 10- to 15-fold. WGA and Con A also exhibited time- and dose-dependent stimulatory effects on p33-mRNA accumulation with maximal effects of 30- to 40-fold. The effect of insulin was maximal by 1 h and plateaued thereafter, whereas lectins had maximal effects at 2 h after addition to cell cultures. Insulin, WGA, and Con A did not significantly alter the stability (half-life) of p33-mRNA. The addition of RNA synthesis inhibitors blocked the ability of insulin, WGA, and Con A to induce the amount of p33-mRNA. These data suggest that lectins, as well as insulin, induce the synthesis of p33-mRNA in acutely treated H4 hepatoma cells.

Modification of membrane physical properties, biological response and insulin binding in Friend cells by low serum concentration

The effect of low serum concentration on plasma membrane fluidity and lipid composition, differentiation and insulin binding was investigated in three Friend erythroleukemia clones. Both FLC (clones No. 745) and F(+) (Ostertag F4N) Friend erythroleukemia cells can be induced to differentiate and to produce hemoglobin when exposed to DMSO. Clone R(3) (Ostertag F4-D5-1) is a DMSO-resistant clone when grown under normal conditions (15% serum) but could undergo differentiation with accumulation of protoporphyrin IX upon induction with DMSO when grown in low serum concentration (2.5% serum). Electron spin resonance measurements of the order parameters (S) and S(T parallel) demonstrate that R(3) has a more fluid plasma membrane than the FLC and F(+). The order parameters of the outer hyperfine splittings S(T parallel) at 37 degrees C are 0.60 +/- 0.009, 0.62 +/- 0.008 and 0.64 +/- 0.009 for R(3), F(+) and FLC, respectively. We have used the insulin receptors as a model for a polypeptide hormone receptor associated with the plasma membrane of the Friend clones. Insulin binding studies demonstrated that the receptor of R(3) had a decreased affinity for insulin manifest as a 10-fold increase in the amount of insulin required to compete for half of the tracer binding (18 nM for R(3) vs. 2 nM for FLC and F(+)). Computer-fit Scatchard plot analysis by the negative cooperativity model reveal that R(3) possessed a similar number of sites/cell (about 70,000) as the FLC or F(+) cells, with similar high and low affinities. Growing the DMSO-resistant clone R(3) in low serum concentration caused a decrease in receptor number by 35%, and an increase in receptor affinity to that seen with the differentiable clones. Thus, the abnormal properties of the plasma membrane and insulin receptor of the DMSO-resistant clone in our earlier report (Simon et al. (1984) Biochim. Biophys. Acta 803, 39-47) were partially reversed by growing the cells in a low serum concentration, restoring the cellular response to the differentiation agent.

Insulin enhances transcription of the tyrosine aminotransferase gene in rat liver

The mechanism of insulin-mediated induction of tyrosine aminotransferase in rat liver was investigated using a cloned cDNA probe. The level of aminotransferase mRNA increases about fourfold following administration of the hormone. This induced mRNA accumulation does not require de novo protein synthesis. Nuclear runoff transcription assays in isolated liver nuclei demonstrate that insulin has a rapid and time-dependent stimulatory effect on aminotransferase gene transcription. The magnitude of enhanced transcription can fully account for the increase in the mRNA. We conclude that the induction of tyrosine aminotransferase in rat liver by insulin is primarily a consequence of a selective increase in the rate of transcription of the aminotransferase gene.

The effects of insulin and concanavalin A on the accumulation of a specific mRNA in rat hepatoma cells

One of insulin's effects is to stimulate specific mRNA synthesis. Treatment of H4IIE hepatoma cells with 0.01-1.0 nM insulin results in a maximum 10-15 fold increase in the accumulation of a specific mRNA (p33-mRNA) as measured with a cloned cDNA. Concanavalin A, a lectin known to mimic many of insulin's effects, also stimulates the accumulation of p33-mRNA. The effects of both insulin and Con A were blocked by the addition of two RNA synthesis inhibitors, actinomycin D or 5,6 dichloro-1-beta-D-ribofuranosyl-benzimidazole. We therefore suggest that insulin and concanavalin A act to stimulate p33-mRNA synthesis.

Effects of insulin and IGF-I on RNA synthesis in isolated soleus muscle

The effects of insulin and insulin-like growth factor I (IGF-I) on RNA synthesis and the effect of IGF-I on polypeptide chain initiation have been studied in the isolated mouse soleus muscle. Both peptides after a 3 h incubation enhanced net incorporation of the labelled uridine into RNA to a similar extent (40% increase over basal). The maximally effective concentrations were 66 nM and 100 nM for insulin and IGF-I respectively. Actinomycin D prevented the peptides' effect on RNA synthesis without modifying their effect on protein synthesis. Furthermore IGF-I increased the rate of initiation of polypeptide chains. It is suggested that in muscle, IGF-I and insulin stimulate protein synthesis by a dual mechanism: a rapid effect on the rate of polypeptide chain initiation; a slower effect on RNA synthesis.

Primary activation of cytosolic phosphoenolpyruvate carboxykinase gene in fetal rat liver and the biogenesis of its mRNA

The primary appearance of phosphoenolpyruvate (P-pyruvate) carboxykinase RNA transcripts in fetal liver was induced by a number of different stimulii . This may occur as rapidly as an hour after injection in utero of N6,O2'-dibutyryl-adenosine 3',5'-monophosphate (Bt2cAMP) to fetuses, suggesting that all stimulii predominantly affect activation of the P-pyruvate carboxykinase gene. Bt2cAMP treatment induces the appearance of the enzyme RNA transcripts, predominantly of the mature type in the cytoplasm. However, insulin deficiency by streptozotocin treatment causes the appearance of large-size as well as mature mRNA in the nucleus, in addition to the appearance of P-pyruvate carboxykinase mRNA in the cytoplasm. Insulin treatment of such diabetic fetuses, prior to causing the disappearance of P-pyruvate carboxykinase mRNA, reduces nuclear transcripts but increases the abundance of mature cytoplasmic enzyme mRNA. Bt2cAMP treatment of insulin-deficient fetuses causes an additive effect, increasing the abundance of not only the mature but the large P-pyruvate carboxykinase RNA transcripts as well. The results are best interpreted as insulin acting both to inhibit transcription of and accelerate post-transcriptional processes affecting P-pyruvate carboxykinase RNA.

Hormonal regulation of translatable mRNA of glucose-6-phosphate dehydrogenase in primary cultures of adult rat hepatocytes

The quantity of translatable mRNA of glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate: NADP+ 1-oxidoreductase, EC 1.1.1.49) in primary cultures of adult rat hepatocytes subjected to different hormonal conditions was determined with a reticulocyte-lysate, cell-free system. The level of glucose-6-phosphate dehydrogenase mRNA was about 5-fold higher in the presence of insulin than in its absence. This increase of glucose-6-phosphate dehydrogenase mRNA reached a maximum 12 h after the addition of insulin. The maximum level of induction of glucose-6-phosphate dehydrogenase mRNA required 10(-8) M insulin. Glucagon and triiodothyronine had no effect on the glucose-6-phosphate dehydrogenase mRNA level. The increase of glucose-6-phosphate dehydrogenase activity correlated with the increase in level of mRNA of this enzyme. This suggests that the changes in glucose-6-phosphate dehydrogenase activity in response to the above hormonal changes are primarily due to changes in the amount of mRNA coding for this enzyme.

Inhibition of transcription of the phosphoenolpyruvate carboxykinase gene by insulin

Insulin regulates the synthesis of several proteins in a variety of tissues. Before techniques were available to quantify the amount of specific mRNAs, insulin was thought to regulate the synthesis of proteins by influencing the rate of translation of a fixed amount of mRNA. A very different interpretation is called for by experiments which show that insulin alters the amount of several specific mRNAs, but little is known about the mechanism. Insulin decreases the rate of synthesis of the critical gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK) in both liver and H4IIE heptoma cells. We recently showed that insulin acts directly on H4IIE cells to decrease mRNAPEPCK activity without any other hormone intermediaries. This effect is mediated by the insulin receptor and occurs at insulin concentrations which are well within the physiological range range (10(-12)--10(-9) M). Here we extend these studies to show that insulin specifically inhibits transcription of the PEPCK gene. This inhibition results in a rapid decrease in the concentration of nuclear PEPCK transcripts which is followed, in turn, by a proportionate decline in cytoplasmic mRNAPEPCK and synthesis of the protein.

Insulin resistance in H-35 rat hepatoma cells is mediated by post-receptor mechanisms

Incubation of H-35 cells with 300 ng/ml (50 nM) of insulin causes a 3-4-fold induction of tyrosine aminotransferase at 4-6 h of incubation. At 24 h the activity of transaminase returns to basal levels despite the presence of sufficient insulin to stimulate a maximal response. Furthermore, addition of 300 ng/ml of fresh insulin fails to stimulate the induction of transaminase. In contrast, the addition of 0.1 microM dexamethasone to insulin-treated cells stimulates the induction of tyrosine aminotransferase, indicating that the loss of responsiveness is specific to insulin action. Incubation of H-35 cells with insulin also causes a 25-30% decrease in insulin binding. This modest decrease in receptor binding is not sufficient to explain the virtually complete loss of insulin responsiveness. Hence, in H-35 hepatoma cells insulin-induced desensitization to insulin action is mediated primarily by post-receptor events.

Factors that control insulin action at the receptor

The binding of insulin to its plasma membrane receptors is an important component of insulin action at the cellular level. Insulin binding is altered in a number of clinical disease states in humans, and several specific regulators of the receptor have been identified. Recent in vitro studies of receptor regulators have furthered our understanding of the interaction of insulin with its receptors and also increased our awareness of the complexity of this interaction. The importance of the plasma membrane environment around the receptor, the potential importance of receptor microaggregation to information transfer, and the coupling of receptor binding to insulin action are reviewed.