Insulin and insulin-like growth factor-I signal transduction requires p21ras

Cancer screening in patients with acromegaly: a plea for a personalized approach and international registries

Acromegaly is a rare condition, and often diagnosis is delayed by several years, for most patients. Acromegaly is characterized by short and long-term respiratory, cardiovascular and metabolic comorbidities, with possible impact on mortality. In the last two decades, life expectancy has progressively increased in part due to a reduction in biochemically active disease, multidisciplinary treatment approaches and a reduction in complications, and the availability of new drugs. Of note, a leading cause of mortality, cardiovascular comorbidity, has been replaced by cancer(s). As such, neoplasms more frequently observed (colon, thyroid, breast, prostate, and stomach) in patients with acromegaly are receiving increased attention. Chronic exposure to increased growth hormone serum levels may contribute to an increase in the occurrence and progression of cancers. Various efforts have been made to determine the pathogenetic mechanisms involved. However, there are no clear medical-related societal agreement(s) in relation to screening methods or timing regarding neoplasm(s) diagnosis in patients with acromegaly. Additionally, independent and dependent risk factor data in patients with acromegaly is lacking. International/national registries could help lay the groundwork to better study the impact of cancer(s) in patients with acromegaly and subsequently lead to and validate the most appropriate diagnostic and therapeutic path forward.

Diabetes mellitus is an independent risk factor for colorectal cancer

BACKGROUND

Epidemiological studies have suggested positive associations between diabetes and colorectal cancer. However, the findings were inconsistent, especially regarding specific sex or cancer subsite.

AIMS

To conduct a systematic review with meta-analysis to examine the association of diabetes mellitus with risk of colorectal cancer and to investigate whether the association is dependent on sex, cancer subsite, race/ethnicity, or duration of diabetes.

METHODS

Relevant studies were identified by searching Pubmed to May 12, 2011. The reference lists of identified articles were also reviewed. Two authors independently selected the studies and extracted relevant data. The study quality was assessed.

RESULTS

Thirty-nine studies (28 cohort studies and 11 case-control studies) were included in the final analysis. Compared with non-diabetic subjects, the pooled risk estimate of colorectal cancer for diabetic subjects was 1.29 (95% CI = 1.23-1.35). The subgroup analyses revealed that the risk was significantly increased for both genders, for every cancer subsite, and for both short duration and long duration of diabetes. Exposure to diabetes increased the risk of colorectal cancer for White and Asian, whereas there was no evidence of such an association either among Black or native Hawaiians.

CONCLUSIONS

Our results confirmed that subjects with diabetes are at modestly increased risk of developing colorectal cancer.

A novel domain of caveolin-2 that controls nuclear targeting: regulation of insulin-specific ERK activation and nuclear translocation by caveolin-2

Herein, we report that insulin-activated extracellular signal-regulated kinase (ERK) is translocated to the nuclear envelope by caveolin-2 (cav-2) and associates with lamin A/C in the inner nuclear membrane in response to insulin. We identified that the Ser¹⁵⁴ -Val¹⁵⁵ -Ser¹⁵⁶ domain on the C-terminal of cav-2 is essential for insulin-induced phosphorylation and nuclear targeting of ERK and cav-2. In human embryonic kidney 293T cells, ERK was not activated and translocated to the nucleus by insulin in comparison to insulin-like growth factor-1 (IGF-1). However, insulin-stimulated activation of ERK was induced by exogenous addition of cav-2. The activated ERK associated and translocated with the cav-2 to the nucleus. In turn, cav-2 promoted phospho-ERK interaction with lamin A/C in the inner nuclear membrane. In contrast, ERK, but not cav-2, was phosphorylated and translocated to the nucleus by IGF-1. The nuclear targeted phospho-ERK failed to localize in the nuclear envelope in response to IGF-1. Together, our data demonstrate that translocation of phospho-ERK to the nuclear envelope is mediated by Ser¹⁵⁴ -Val¹⁵⁵ -Ser¹⁵⁶ domain of cav-2 and this event is an insulin-specific action.

Effects of diet-induced obesity on colitis-associated colon tumor formation in A/J mice

OBJECTIVE

Studies have indicated that obesity is associated with a higher risk of colorectal cancer. This study was performed to determine the effect of diet-induced obesity on the formation of azoxymethane (AOM)/dextran sodium sulfate (DSS)-induced colon tumors and to identify adiposity-related mechanisms.

METHODS

Male A/J mice were placed on either a high-fat diet (HFD; 45% of total calories from fat) or a normal diet (ND; 15% of calories from fat) for 12 weeks. To induce colon tumors, AOM was administered at a dose of 10 mg/kg body weight, followed by two cycles of DSS supply.

RESULTS

Study results indicated that the HFD group had twofold higher numbers of colonic tumors, as compared with the ND group. The HFD group also had significantly increased body weight and epididymal fat weight, which were associated with increases of serum insulin, insulin-like growth factor-1, leptin, epididymal fat pad leptin mRNA and colonic leptin receptor (Ob-R) mRNA. Animals on HFD showed higher expressions of Ob-R, insulin receptor, phosphorylated Akt, phosphorylated extracellular signal-regulated kinases, Bcl-xL and Cyclin D1 proteins in the colon.

CONCLUSION

The results suggest that HFD-induced obesity facilitates colon tumor formation, possibly by regulating downstream targets of circulating adiposity-related factors via receptor-mediated signaling of the phosphatidylinositol 3-kinase/Akt pathway.

Roles of hormones and signaling molecules in describing the relationship between obesity and colon cancer

Colon cancer represents a highly prevalent disease in the Western world. While dietary and lifestyle recommendations remain important factors in disease prevention and treatment, epidemiological data have made it clear that obesity and excess body weight remain significant risk factors for the disease. A number of potential direct and indirect relationships exist between obesity and increased risk of colon cancer. Several mechanisms which appear promising and warrant further investigation are discussed here, specifically the modifying role of insulin and insulin-like growth factors, leptin, adipose-tissue induced changes in estrogens and androgens, and inflammatory molecules. A brief review of these hormones and signaling molecules and their action in colon cancer development is described. A thorough integration and understanding of the mechanisms of action these systems exert on colonic epithelia will be important in designing studies and experiments aimed at elucidating disease etiology for prevention and treatment.

Insulin resistance and hyperinsulinaemia in the development and progression of cancer

Experimental, epidemiological and clinical evidence implicates insulin resistance and its accompanying hyperinsulinaemia in the development of cancer, but the relative importance of these disturbances in cancer remains unclear. There are, however, theoretical mechanisms by which hyperinsulinaemia could amplify such growth-promoting effects as insulin may have, as well as the growth-promoting effects of other, more potent, growth factors. Hyperinsulinaemia may also induce other changes, particularly in the IGF (insulin-like growth factor) system, that could promote cell proliferation and survival. Several factors can independently modify both cancer risk and insulin resistance, including subclinical inflammation and obesity. The possibility that some of the effects of hyperinsulinaemia might then augment pro-carcinogenic changes associated with disturbances in these factors emphasizes how, rather than being a single causative factor, insulin resistance may be most usefully viewed as one strand in a network of interacting disturbances that promote the development and progression of cancer.

Proliferative effect of Apidra (insulin glulisine), a rapid-acting insulin analogue on mammary epithelial cells

The structural modification of insulin results in the generation of insulin analogues that show altered binding affinities to the insulin receptor and/or the IGF-I receptor, and as a consequence insulin analogues may have altered mitogenic potency. We analysed the proliferative effect of the rapid-acting insulin analogue Apidra (insulin glulisine) on mammary epithelial cells. We show that Apidra and Actrapid (recombinant human insulin) have similar proliferative effects on benign MCF10A and tumorigenic MCF7 cells and on epithelial cells of mouse mammary gland. Whereas Apidra and Actrapid induced similar activation of Erk1/2, activation of Akt/PKB by Apidra was significantly weaker compared to regular insulin. As AKT/PKB, an effector of the phosphoinositide 3-kinase pathway, mediates metabolic effects of insulin, we studied induction of hexokinase-2 in MCF7 cells and hexokinase-2 and hexokinase-4 in HepG2 cells by Actrapid and Apidra. Both genes were not significantly induced by Actrapid and Apidra in these cell lines.

Type 2 diabetes mellitus as risk factor for colorectal cancer

Colorectal cancer occurs more frequently in patients with type 2 diabetes mellitus. The hyperinsulinemia-hypothesis suggests that elevated levels of insulin and free IGF-1 promote proliferation of colon cells and lead to a survival benefit of transformed cells, ultimately resulting in colorectal cancer. In patients with type 2 diabetes mellitus, epidemiological studies show an increased risk for colorectal cancer and an even higher risk if patients are treated with sulphonylureas or insulin. Moreover, tumour progression at hyperinsulinemia is more rapid and tumour-associated mortality is increased. Colorectal cancer can be avoided by screening. Recommendations for colorectal cancer screening should employ the recent epidemiologic evidence. All patients with type 2 diabetes mellitus should be recommended to undergo colonoscopy before starting insulin therapy, and screening intervals should not exceed 5 years. For this concept, a review of the evidence is presented, and a screening algorithm for colorectal cancer in patients with type 2 diabetes mellitus is proposed.

Ethanol Impairs Insulin's Actions Through Phosphatidylinositol 3-Kinase

Insulin plays an important role in cell metabolism and proliferation. In the present study, we examined the effect of ethanol on insulin actions such as glucose uptake, DNA synthesis, and c-Jun gene expression. Acute treatment with ethanol (200 mM) for 60 minutes inhibited insulin-stimulated 2-deoxyglucose uptake by 50% in 3T3-L1 adipocytes. Insulin-induced DNA synthesis and c-Jun protein expression were also reduced by ethanol treatment in Rat-1 fibroblasts overexpressing normal human insulin receptor. Ethanol has no effect on tyrosine phosphorylation of the insulin receptor and insulin receptor substrate (IRS)-1. However, association of the insulin receptor and IRS-1 with the Src homology 2 domain of the p85 subunit of phosphatidylinositol 3-kinase (PI3-kinase) was reduced by ethanol. Pretreatment with the antidiabetic drug troglitazone, an insulin-sensitizer, reversed ethanol's inhibition. These results suggest that ethanol specifically inhibits the association of the insulin receptor and IRS-1 with the p85 subunit of PI3-kinase, which is required for increased glucose uptake, DNA synthesis, and c-Jun expression by insulin.

Impaired Shc, Ras, and MAPK activation but normal Akt activation in FL5.12 cells expressing an insulin-like growth factor I receptor mutated at tyrosines 1250 and 1251

The Y1250F/Y1251F mutant of the insulin-like growth factor I receptor (IGF-IR) has tyrosines 1250 and 1251 mutated to phenylalanines and is deficient in IGF-I-mediated suppression of apoptosis in FL5.12 lymphocytic cells. To address the mechanism of loss of function in this mutant we investigated signaling responses in FL5.12 cells overexpressing either a wild-type (WT) or Y1250F/Y1251F (mutant) IGF-IR. Cells expressing the mutant receptor were deficient in IGF-I-induced phosphorylation of the JNK pathway and had decreased ERK and p38 phosphorylation. IGF-I induced phosphorylation of Akt was comparable in WT and mutant expressing cells. The decreased activation of the mitogen-activated protein kinase (MAPK) pathways was accompanied by greatly decreased Ras activation in response to IGF-I. Although phosphorylation of Gab2 was similar in WT and mutant cell lines, phosphorylation of Shc on Tyr(313) in response to IGF-I was decreased in cells expressing the mutant receptor, as was recruitment of Grb2 and Ship to Shc. However, phosphorylation of Shc on Tyr(239), the Src phosphorylation site, was normal. A role for JNK in the survival of FL5.12 cells was supported by the observation that the JNK inhibitor SP600125 suppressed IGF-I-mediated protection from apoptosis. Altogether these data demonstrate that phosphorylation of Shc, and assembly of the Shc complex necessary for activation of Ras and the MAPK pathways are deficient in cells expressing the Y1250F/Y1251F mutant IGF-IR. This would explain the loss of IGF-I-mediated survival in FL5.12 cells expressing this mutant and may also explain why this mutant IGF-IR is deficient in functions associated with cellular transformation and cell migration in fibroblasts and epithelial tumor cells.

Kimchi and an Active Component, β-Sitosterol, Reduce Oncogenic H-Rasv12-Induced DNA Synthesis

The Korean fermented vegetable food, kimchi, has been demonstrated to have anticancer functional properties. This study examined the effect of kimchi samples, methanol extracts of commercially grown baechu cabbage kimchi (CK) and organically grown baechu cabbage kimchi (OK), as well as the dichloromethane fraction (DCM fr.) from CK, and the active compound (AC), which has been identified as largely beta-sitosterol, from DCM fr., on the Ras-dependent signaling pathway. CK, OK, and DCM fr. exhibited a greater inhibition against the proliferation of Rat2 fibroblasts transformed with Ras(v12) (HO6) than parental Rat2 fibroblasts. In addition, OK and DCM fr. showed a higher inhibitory effect than CK. Furthermore, we employed the single-cell microinjection technique, combined with 3-bromo-5'-deoxyuridine incorporation, to examine the effects of kimchi samples on DNA synthesis induced by microinjected oncogenic Ras(v12). When the DCM fr. and AC were used to treat Rat1 fibroblasts overexpressing human insulin receptors (HIRc-B) and microinjected with oncogenic H-Ras(v12), the DNA synthesis of injected cells was decreased, suggesting that kimchi might block the signaling pathway of oncogenic Ras(v12), thus preventing the proliferation of transformed cells. This study provides additional evidence that kimchi and its active components, including beta-sitosterol, have potential in both the prevention and treatment of cancer, and presents convincing evidence that the anticancer effects may be a result of an inhibition of Ras oncogene signaling.

Hyperinsulinaemia and hyperglycaemia: possible risk factors of colorectal cancer among diabetic patients

Hyperinsulinaemia and hyperglycaemia are two possible risk factors for colorectal cancer, which constitutes the third leading cause of cancer death in Western countries. Molecular evidence as well as animal models provide support for these associations: Insulin has been shown to be an important growth factor for colonic carcinoma cells, and both insulin and insulin-like growth factor-1 receptors have been detected in colon cancer tissue. The insulin-signal transduction pathway is involved in the regulation of gene expression and apoptosis. The role of hyperglycaemia in carcinogenesis could include pathways via luminal factors (related to fecal bile acid concentrations, stool bulk, and prolonged transit time) or circulatory factors (via glucose as the only energy source for neoplastic cells). This review summarizes the epidemiologic literature with respect to hyperinsulinaemia and hyperglycaemia as risk factors for colorectal cancer, and aims to integrate the biological and epidemiological evidence. Epidemiologic findings to date indicate a slightly increased risk of colorectal cancer for diabetic patients; however, there are some inconsistencies. Possible explanations for these inconsistencies include inadequate information about patients' diabetic disease and treatment states. We suggest that future studies should take medical history, staging and treatment for hyperinsulinaemia and hyperglycaemia into account to further our understanding of the role of hyperglycaemia and hyperinsulinaemia in colorectal carcinogenesis.

Shc and CEACAM1 interact to regulate the mitogenic action of insulin

CEACAM1, a tumor suppressor (previously known as pp120), is a plasma membrane protein that undergoes phosphorylation on Tyr(488) in its cytoplasmic tail by the insulin receptor tyrosine kinase. Co-expression of CEACAM1 with insulin receptors decreased cell growth in response to insulin. Co-immunoprecipitation experiments in intact NIH 3T3 cells and glutathione S-transferase pull-down assays revealed that phosphorylated Tyr(488) in CEACAM1 binds to the SH2 domain of Shc, another substrate of the insulin receptor. Overexpressing Shc SH2 domain relieved endogenous Shc from binding to CEACAM1 and restored MAP kinase activity, growth of cells in response to insulin, and their colonization in soft agar. Thus, by binding to Shc, CEACAM1 sequesters this major coupler of Grb2 to the insulin receptor and down-regulates the Ras/MAP kinase mitogenesis pathway. Additionally, CEACAM1 binding to Shc enhances its ability to compete with IRS-1 for phosphorylation by the insulin receptor. This leads to a decrease in IRS-1 binding to phosphoinositide 3'-kinase and to the down-regulation of the phosphoinositide 3'-kinase/Akt pathway that mediates cell proliferation and survival. Thus, binding to Shc appears to constitute a major mechanism for the down-regulatory effect of CEACAM1 on cell proliferation.

Insulin, insulin-like growth factors and colon cancer: a review of the evidence

Insulin and insulin-like growth factor (IGF) axes are major determinants of proliferation and apoptosis and thus may influence carcinogenesis. In various animal models, modulation of insulin and IGF-1 levels through various means, including direct infusion, energy excess or restriction, genetically induced obesity, dietary quality including fatty acid and sucrose content, inhibition of normal insulin secretion and pharmacologic inhibition of IGF-1, influences colonic carcinogenesis. Human evidence also associates high levels of insulin and IGF-1 with increased risk of colon cancer. Clinical conditions associated with high levels of insulin (noninsulin-dependent diabetes mellitus and hypertriglyceridemia) and IGF-1 (acromegaly) are related to increased risk of colon cancer, and increased circulating concentrations of insulin and IGF-1 are related to a higher risk of colonic neoplasia. Determinants and markers of hyperinsulinemia (physical inactivity, high body mass index, central adiposity) and high IGF-1 levels (tall stature) are also related to higher risk. Many studies indicate that dietary patterns that stimulate insulin resistance or secretion, including high consumption of sucrose, various sources of starch, a high glycemic index and high saturated fatty acid intake, are associated with a higher risk of colon cancer. Although additional environmental and genetic factors affect colon cancer, the incidence of this malignancy was invariably low before the technological advances that rendered sedentary lifestyles and obesity common, and increased availability of highly processed carbohydrates and saturated fatty acids. Efforts to counter these patterns are likely to have the most potential to reduce colon cancer incidence, as well as cardiovascular disease and diabetes mellitus.

Dominant negative farnesyltransferase alpha-subunit inhibits insulin mitogenic effects

Farnesylation of p21Ras is required for translocation to the plasma membrane and subsequent activation by growth factors. Previously we demonstrated that insulin stimulates the phosphorylation of farnesyltransferase (FTase) and its activity, whereby the amount of farnesylated p21Ras anchored at the plasma membrane is increased. Herein we report that substitution of alanine for two serine residues (S60A)(S62A) of the alpha-subunit of FTase creates a dominant negative (DN) mutant. VSMC expressing the FTase alpha-subunit (S60A)(S62A) clone showed a 30% decreased basal FTase activity concurrent with a 15% decrease in the amount of farnesylated p21Ras compared to controls. Expression of alpha-subunit (S60A,S62A) blunted FTase phosphorylation and activity in the presence of hyperinsulinemia, and inhibited insulin-stimulated increases in farnesylated p21Ras. Insulin-stimulated VSMC expressing the FTase alpha-subunit (S60A,S62A) showed decreased (i) phosphorylation of FTase, (ii) FTase activity, (iii) amounts of farnesylated p21Ras, (iv) DNA synthesis, and (v) migration. Thus, down-regulation of FTase activity appears to mitigate the potentially detrimental mitogenic effects of hyperinsulinemia on VSMC.

Insulin signals to prenyltransferases via the Shc branch of intracellular signaling

We assessed the roles of insulin receptor substrate-1 (IRS-1) and Shc in insulin action on farnesyltransferase (FTase) and geranylgeranyltransferase I (GGTase I) using Chinese hamster ovary (CHO) cells that overexpress wild-type human insulin receptors (CHO-hIR-WT) or mutant insulin receptors lacking the NPEY domain (CHO-DeltaNPEY) or 3T3-L1 fibroblasts transfected with adenoviruses that express the PTB or SAIN domain of IRS-1 and Shc, the pleckstrin homology (PH) domain of IRS-1, or the Src homology 2 (SH2) domain of Shc. Insulin promoted phosphorylation of the alpha-subunit of FTase and GGTase I in CHO-hIR-WT cells, but was without effect in CHO-DeltaNPEY cells. Insulin increased FTase and GGTase I activities and the amounts of prenylated Ras and RhoA proteins in CHO-hIR-WT (but not CHO-DeltaNPEY) cells. Overexpression of the PTB or SAIN domain of IRS-1 (which blocked both IRS-1 and Shc signaling) prevented insulin-stimulated phosphorylation of the FTase and GGTase I alpha-subunit activation of FTase and GGTase I and subsequent increases in prenylated Ras and RhoA proteins. In contrast, overexpression of the IRS-1 PH domain, which impairs IRS-1 (but not Shc) signaling, did not alter insulin action on the prenyltransferases, but completely inhibited the insulin effect on the phosphorylation of IRS-1 and on the activation of phosphatidylinositol 3-kinase and Akt. Finally, overexpression of the Shc SH2 domain completely blocked the insulin effect on FTase and GGTase I activities without interfering with insulin signaling to MAPK. These data suggest that insulin signaling from its receptor to the prenyltransferases FTase and GGTase I is mediated by the Shc pathway, but not the IRS-1/phosphatidylinositol 3-kinase pathway. Shc-mediated insulin signaling to MAPK may be necessary (but not sufficient) for activation of prenyltransferase activity. An additional pathway involving the Shc SH2 domain may be necessary to mediate the insulin effect on FTase and GGTase I.

Epidermal growth factor and insulin-induced deoxyribonucleic acid synthesis in primary rat hepatocytes is phosphatidylinositol 3-kinase dependent and dissociated from protooncogene induction

The mitogenic response to insulin and epidermal growth factor (EGF) was studied in subconfluent and confluent cultures of primary rat hepatocytes. In subconfluent cultures, wortmannin, LY294002, and rapamycin reversed insulin- and EGF-induced [3H]thymidine incorporation into DNA. The mitogen-activated protein kinase (MAPK) kinase 1 (MEK1) inhibitor PD98059 was without significant effect on either insulin- or EGF-induced [3H]thymidine incorporation. Insulin treatment did not alter levels of messenger RNAs (mRNAs) for c-fos, c-jun, and c-myc. EGF induced an increase in c-myc, but not c-fos or c-jun, mRNA levels in subconfluent hepatocyte cultures. This increase in c-myc mRNA was abolished by PD98059. In confluent cells that could not be induced to synthesize DNA, EGF treatment also promoted an increase in c-myc mRNA to levels seen in subconfluent cultures. This increase was also abrogated by PD98059. These data indicate that in primary rat hepatocyte cultures, 1) the phosphoinositol 3-kinase pathway, perhaps through p70s6k activation, regulates DNA synthesis in response to insulin and EGF; 2) the MAPKpathway is not involved in insulin- and EGF-induced DNA synthesis; and 3) p44/42 MAPKs are involved the induction of c-myc mRNA levels, although this induction is not required for DNA synthesis. These studies define two distinct signal transduction pathways that independently mediate growth-related responses in a physiologically relevant, normal cell system.

Insulin-like growth factor I as a cardiac hormone: physiological and pathophysiological implications in heart disease

Accumulating evidence has indicated that insulin-like growth factor-1 (IGF-1) plays a specific role in the intricate cascade of events of cardiovascular function, in addition to its well established growth-promoting and metabolic effects. IGF-1 is believed to mediate many effects of growth hormone (GH), IGF-1 promotes cardiac growth, improves cardiac contractility, cardiac output, stroke volume, and ejection fraction. In humans, IGF-1 improves cardiac function after myocardial infarction by stimulating contractility and promoting tissue remodeling. Furthermore, IGF-1 facilitates glucose metabolism, lowers insulin levels, increases insulin sensitivity, and improves the lipid profile. These data suggest an attractive therapeutic potential of IGF-1. Both clinically observed and experimentally induced impairments of cardiac function are also found to be associated with abnormal IGF-1 levels. IGF-1 and its binding proteins have been considered as markers for the presence of certain cardiac abnormalities, indicating that IGF-1 may be a risk factor for certain cardiac disorders. The present review will emphasize the role of IGF-1 in the regulation of cardiac growth and function, and the potential pathophysiological role of IGF-1 in cardiac function.

Increased activity and expression of MAP kinase in HCC model rats induced by 3'-methyl-4-dimethylamino-azobenzene

BACKGROUND/AIMS

The ras-mitogen-activated protein kinase (MAPK) cascade plays an important role not only in the mitogenic signal transduction pathway but also in the development of cancer, and it is believed to be one of the important regulators in normal hepatocytes and hepatocellular carcinoma. The aim of this study was to determine the role of insulin receptor substrate-1 and the MAPK cascade in rats with hepatocellular carcinoma induced by 3'-methyl-4-dimethylamino-azobenzene (3'-MeDAB).

METHODS

Liver cancer was induced in rats by feeding 3'-MeDAB, and the changes in expression of IRS-1 and MAPK were analyzed in tumorous, non-tumorous and control liver.

RESULTS

Expression of insulin receptor substrate-1 (IRS-1) showed a 1.4-fold increase at protein level in the tumors (p<0.01), but the tyrosine phosphorylation of IRS-1 did not differ between the tumor and control liver. Expression of MAPK and its activity were elevated 4.5-7.5-fold (p<0.01) and 4.6-fold (p<0.01) in the tumor compared with control liver. In non-tumorous lesions from rats fed with 3'-MeDAB, expression of MAPK, but not IRS-1, increased significantly (p<0.01). Between tumorous and adjacent non-tumorous lesions, there was a significant difference in MAPK expression (p<0.05) and activities (p<0.05).

CONCLUSIONS

The increased expression of MAPK may play an important role in the progression or initiation of HCC in this rat model.

Quantification of Fos immunoreactivity in cortical cultures treated with growth factors

Among the effects mediated by growth factors in vivo are the switch to growth arrest and differentiation during normal development of the nervous system, survival during the period of naturally occurring cell death, and plasticity and repair of neurons in the adult brain. Much interest has focused on the signalling pathways utilised by growth factors with a large proportion of experiments carried out using the phaeochromocytoma cell line. Here we have quantified Fos immunoreactivity following stimulation of primary cultures of rat cortical neurons with a variety of growth factors including neurotrophins and cytokines. Expression of Fos has been quantified in these cultures using an ELISA technique, and immunocytochemistry followed by digital stereology. Treatment of cultures with brain derived neurotrophic factor (BDNF) or neurotrophin-4 (NT-4) causes a dose-dependent increase in Fos expression, while neurotrophin-3 (NT-3) causes an increase but at high concentrations only. A sub-population of cortical neurons within the cultures express Fos in response to fibroblast growth factor-1 or fibroblast growth factor-2 but no cells respond with Fos expression on treatment with insulin-like growth factor-1. We conclude that BDNF and NT-4 cause dose-dependent increases in the number of Fos immunoreactive cells.

Inhibition of phosphatidylinositol 3-kinase activity by adenovirus-mediated gene transfer and its effect on insulin action

Phosphatidylinositol 3-kinase (PI 3-K) is implicated in cellular events including glucose transport, glycogen synthesis, and protein synthesis. It is activated in insulin-stimulated cells by binding of the Src homology 2 (SH2) domains in its 85-kDa regulatory subunit to insulin receptor substrate-1 (IRS-1), and, others. We have previously shown that IRS-1-associated PI 3-kinase activity is not essential for insulin-stimulated glucose transport in 3T3-L1 adipocytes, and that alternate pathways exist in these cells. We now show that adenovirus-mediated overexpression of the p85N-SH2 domain in these cells behaves in a dominant-negative manner, interfering with complex formation between endogenous PI 3-K and its SH2 binding targets. This not only inhibited insulin-stimulated IRS-1-associated PI 3-kinase activity, but also completely blocked anti-phosphotyrosine-associated PI 3-kinase activity, which would include the non-IRS-1-associated activity. This resulted in inhibition of insulin-stimulated glucose transport, glycogen synthase activity and DNA synthesis. Further, Ser/Thr phosphorylation of downstream molecules Akt and p70 S6 kinase was inhibited. However, co-expression of a membrane-targeted p110(C) with the p85N-SH2 protein rescued glucose transport, supporting our argument that the p85N-SH2 protein specifically blocks insulin-mediated PI 3-kinase activity, and, that the signaling pathways downstream of PI 3-kinase are intact. Unexpectedly, GTP-bound Ras was elevated in the basal state. Since p85 is known to interact with GTPase-activating protein in 3T3-L1 adipocytes, the overexpressed p85N-SH2 peptide could titrate out cellular GTPase-activating protein by direct association, such that it is unavailable to hydrolyze GTP-bound Ras. However, insulin-induced mitogen-activated protein kinase phosphorylation was inhibited. Thus, PI 3-kinase may be required for this action at a step independent of and downstream of Ras. We conclude that, in 3T3-L1 adipocytes, non-IRS-1-associated PI 3-kinase activity is crucial for insulin's metabolic signaling, and that overexpressed p85N-SH2 protein inhibits a variety of insulin's ultimate biological effects.

Replacement of tyrosine 1251 in the carboxyl terminus of the insulin-like growth factor-I receptor disrupts the actin cytoskeleton and inhibits proliferation and anchorage-independent growth

Insulin-like growth factor (IGF)-I signaling through the IGF-I receptor modulates cellular adhesion and proliferation and the transforming ability of cells overexpressing the IGF-I receptor. Tyrosine phosphorylation of intracellular proteins is essential for this transduction of the IGF-I-induced mitogenic and tumorigenic signals. IGF-I induces specific cytoskeletal structure and the phosphorylation of proteins in the associated focal adhesion complexes. The determination of the exact pathways emanating from the IGF-I receptor that are involved in mediating these signals will contribute greatly to the understanding of IGF-I action. We have previously shown that replacement of tyrosine residues 1250 and 1251 in the carboxyl terminus of the IGF-I receptor abrogates IGF-I-induced cellular proliferation and tumor formation in nude mice. In this study, replacement of either tyrosine 1250 or 1251 similarly reduces the cells ability to grow in an anchorage-independent manner. The actin cytoskeleton and cellular localization of vinculin are disrupted by replacement of tyrosine 1251. Tyrosine residues 1250 and 1251 are not essential for tyrosine phosphorylation of two known substrates; insulin receptor substrate-1 and SHC, nor association of known downstream adaptor proteins to these substrates. In addition, these mutant IGF-I receptors do not affect IGF-I-stimulated p42/p44 mitogen-activated protein kinase activation or phosphatidylinositol (PI) 3'-kinase activity. Thus, it appears that in fibroblasts expressing tyrosine 1250 and 1251 mutant IGF-I receptors, the signal transduction pathways impacting on mitogenesis and tumorigenesis do not occur exclusively through the PI 3'-kinase or mitogen-activated protein kinase pathways.

Involvement of growth factor receptor-bound protein-2 in rat hepatocyte growth

Growth factor receptor-bound protein-2 (GRB-2) is a protein linking receptor tyrosine kinase and Sos (Son of Sevenless gene; Ras GDP/GTP exchange protein), leading to activation of the Ras-mitogen-activated protein kinase (MAPK) cascade. So far, it remains unclear how GRB-2 plays a role in signal transduction pathways evoked by hepatotrophic factors. This study was attempted to evaluate the involvement of GRB-2 in signalling in rat hepatocyte growth. Using rat cultured hepatocytes stimulated by hepatotrophic factors and regenerating livers after partial hepatectomy (PH) we examined GRB-2-mediated linkage of hepatotrophic factor receptors to signal transducing molecules such as Sos or dynamin-II by immunoprecipitation and western blot analysis. In primary cultured hepatocytes stimulated with hepatocyte growth factor (HGF) or epidermal growth factor (EGF), GRB-2 linked HGF receptor or EGF receptor, respectively, to Sos which activated the mitogen-activated protein kinase (MAPK) cascade. In contrast, in primary cultured hepatocytes stimulated with insulin, GRB-2 linked insulin receptor substrate-1 (IRS-1) to dynamin-II as well as Sos. In the early phase after PH, GRB-2 activated the Ras-MAPK cascade by linking HGF receptor, IRS-1, or EGF receptor to Sos. In the late phase after PH, a complex of IRS-1-GRB-2 associated with dynamin-II, indicating that GRB-2 may transduce signals from IRS-1 to dynamin-II. We conclude that GRB-2 may play a role in transmitting signals from hepatotrophic factors to not only MAPK but also to other signalling pathways in hepatocyte growth.

What does insulin do to Ras?

The Ras pathway lies in the center of signalling cascades of numerous growth-promoting factors. The Ras pathway appears to connect signalling events that begin at the plasma membrane with nuclear events. Insulin is one of the major stimulants of the Ras signalling pathway. The influence of insulin on this pathway consists of five important events: (1) p21Ras activation is promoted by insulin stimulation of the guanine nucleotide exchange factor, Sos, resulting in increased GTP-loading of p21Ras; (2) p21Ras deactivation involves the hyperphosphorylation of Sos; (3) insulin increases farnesyltransferase (FTase) activity that farnesylates p21Ras; (4) increased amounts of farnesylated p21Ras translocate to the plasma membrane where they can be activated by other growth-promoting agents; and (5) cellular responses to other growth factors are potentiated by insulin-stimulated pre-loading of the plasma membrane with farnesylated p21Ras.

Receptor-like protein-tyrosine phosphatase alpha specifically inhibits insulin-increased prolactin gene expression

A physiologically relevant response to insulin, stimulation of prolactin promoter activity in GH4 pituitary cells, was used as an assay to study the specificity of protein-tyrosine phosphatase function. Receptor-like protein-tyrosine phosphatase alpha (RPTPalpha) blocks the effect of insulin to increase prolactin gene expression but potentiates the effects of epidermal growth factor and cAMP on prolactin promoter activity. RPTPalpha was the only protein-tyrosine phosphatase tested that did this. Thus, the effect of RPTPalpha on prolactin-chloramphenicol acetyltransferase (CAT) promoter activity is specific by two criteria. A number of potential RPTPalpha targets were ruled out by finding (a) that they are not affected or (b) that they are not on the pathway to insulin-increased prolactin-CAT activity. The negative effect of RPTPalpha on insulin activation of the prolactin promoter is not due to reduced phosphorylation or kinase activity of the insulin receptor or to reduced phosphorylation of insulin receptor substrate-1 or Shc. Inhibitor studies suggest that insulin-increased prolactin gene expression is mediated by a Ras-like GTPase but is not mitogen-activated protein kinase dependent. Experiments with inhibitors of phosphatidylinositol 3-kinase suggest that insulin-increased prolactin-CAT expression is phosphatidylinositol 3-kinase-independent. These results suggest that RPTPalpha may be a physiological regulator of insulin action.

Mitogenic signaling of insulin-like growth factor I in MCF-7 human breast cancer cells requires phosphatidylinositol 3-kinase and is independent of mitogen-activated protein kinase

Addition of insulin-like growth factor I (IGF-I) to quiescent breast tumor-derived MCF-7 cells causes stimulation of cyclin D1 synthesis, hyperphosphorylation of the retinoblastoma protein pRb, DNA synthesis, and cell division. All of these effects are independent of the mitogen-activated protein kinase (MAPK) pathway since none of them is blocked by PD098059, the specific inhibitor of the MAPK activating kinase MEK1. This observation is consistent with the finding that the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), a strong inducer of MAPK activity in MCF-7 cells, effectively inhibits proliferation. The anti-proliferative effect of TPA in these cells may be accounted for, at least in part, by the MAPK-dependent stimulation of the synthesis of p21(WAF1/CIP1), an inhibitor of cyclin/cyclin-dependent kinase complexes. In contrast, all of the observed stimulatory effects of IGF-I on cell cycle progression, cyclin D1 synthesis, and pRb hyperphosphorylation were blocked by the specific phosphatidylinositol 3-kinase inhibitor LY294002, suggesting that phosphatidylinositol 3-kinase activity but not MAPK activity is required for transduction of the mitogenic IGF-I signal in MCF-7 cells.

Insulin stimulates the phosphorylation and activity of farnesyltransferase via the Ras-mitogen-activated protein kinase pathway

Farnesylation of p21Ras by farnesyltransferase (FTase) is obligatory for anchoring p21Ras to the plasma membrane, where it can be activated by growth factors. Insulin significantly stimulates the phosphorylation of the alpha-subunit of FTase (4-fold) and the enzymatic activity of FTase in 3T3-L1 fibroblasts and adipocytes. FTase activity was assessed by the amount of [3H] mevalonate (a precursor of farnesyl) incorporated into p21Ras in vivo and by quantitating the amount of farnesylated p21Ras before and after insulin administration. Insulin-stimulated phosphorylation of the alpha-subunit of FTase in 3T3-L1 fibroblasts and adipocytes was blocked by the mitogen-activated protein/extracellular-signal regulated kinase-kinase inhibitor, PD98059, but not by wortmannin or bisindolylmaleimide. Additionally, PD98059 blocked insulin-stimulated [3H]mevalonic incorporation and farnesylation of unprocessed p21Ras in both cell lines. Furthermore, expression of the dominant negative mutant of p21Ras precluded insulin-stimulated phosphorylation of the FTase alpha-subunit and activation of its enzymatic activity. In contrast, 3T3-L1 fibroblasts, expressing the constitutively active Raf-1, exhibited enhanced phosphorylation of the FTase alpha-subunit. It seems that insulin's effect on the phosphorylation and activation of FTase in both fibroblasts and adipocytes is mediated via the Ras pathway, resulting in a positive feedback augmentation of the cellular pool of farnesylated p21Ras.

Bovine fetuin is an inhibitor of insulin receptor tyrosine kinase

Fetuin has been identified earlier as the bovine homolog of the human plasma protein, alpha2-Heremans Schmid glycoprotein (alpha2-HSG). Although bovine fetuin shares over 70% amino acid sequence similarity with alpha2-HSG and rat fetuin, no common function(s) have been identified. We report that immunoaffinity purified bovine fetuin acts as an inhibitor of insulin receptor tyrosine kinase activity (IR-TKA) with half-maximal inhibition at 1.5 microM. In vitro, bovine fetuin (1.5 microM) blocked insulin-induced autophosphorylation of the human IR completely and the half-maximal inhibitory effect was observed at 0.5 microM. Incubation of HIRcB cells (rat1 fibroblasts transfected with wild-type human insulin receptor cDNA) with bovine fetuin (1.5 microM) inhibited insulin-induced tyrosine phosphorylation of the IR beta-subunit by 40%. In addition, bovine fetuin (2 microM) completely blocked insulin-stimulated DNA synthesis in H-35 rat hepatoma cells. Our results, together with earlier reports on rat fetuin and human alpha2-HSG, indicate a common IR-TK inhibitory function for fetuin homologs.

Evidence for functional roles of Crk-II in insulin and epidermal growth factor signaling in Rat-1 fibroblasts overexpressing insulin receptors

We examined the potential role of Crk-II in insulin and epidermal growth factor (EGF) signaling in Rat-1 fibroblasts overexpressing insulin receptors. Crk is an SH2 and SH3 domain-containing adaptor protein that has been reported to associate with p130cas, paxillin, c-cbl, c-abl, Sos, and C3G in vitro. Insulin- and EGF-induced association of Crk-II with these molecules was assessed by immunoblotting of anti-Crk-II precipitates in Rat-1 fibroblasts overexpressing insulin receptors. Neither insulin nor EGF treatment induced Crk-II association with either Sos or C3G. Basal tyrosine phosphorylation of c-abl and its constitutive association with Crk-II were not further increased by insulin or EGF. p130cas and paxillin were heavily tyrosine phosphorylated in the basal state. Both insulin and EGF stimulated their dephosphorylation, followed by p130cas-Crk-II dissociation and paxillin-Crk-II association, although the magnitude of these effects was greater with insulin than with EGF. Interestingly, EGF, but not insulin, stimulated tyrosine phosphorylation of c-cbl and its association with Crk-II. To investigate the functional roles of Crk-II in mitogenesis and cytoskeletal rearrangement, we performed microinjection analysis. Cellular microinjection of anti-Crk-II antibody inhibited EGF-induced, but not insulin-induced, DNA synthesis. Insulin, but not EGF, stimulated cytoskeletal rearrangement in the cells, and microinjection of anti-Crk-II antibody effectively inhibited insulin-induced membrane ruffling, suggesting that Crk-II is involved in insulin-induced cytoskeletal rearrangement. These results indicate that Crk-II functions as a multifunctional adaptor molecule linking insulin and EGF receptors to their downstream signals. The presence of c-cbl-Crk-II association may partly determine the signal specificities initiated by insulin and EGF.

v-Ras and v-Raf block differentiation of transformable C3H10T1/2-derived preadipocytes at lower levels than required for neoplastic transformation

To investigate the functional relationship between the transforming ability of Ras and its role as an integral component of the differentiative insulin signaling pathway, we introduced a leu61-activated ras gene into a Ras-transformable, C3H10T1/2-derived preadipocytic cell line. The results demonstrate that rasleu61 expression in this line blocks differentiation and that this block appears at lower levels than required for full neoplastic transformation. In addition, to examine whether the inability of Rasleu61 to induce differentiation by replacing the insulin signal could be attributed to its transforming effect in this system, we examined the effect of Rasleu61 at levels below the baseline, by expressing rasleu61 in a series of preadipocytes which were rendered deficient in endogenous c-Ras activity. The results show that even very low Rasleu61 levels, insufficient to restore the growth rate of these cells to normal, blocked rather than enhanced differentiation, indicating that rasleu61 expression alone is not sufficient to promote adipocytic differentiation in this system, even in the absence of neoplastic transformation. Consistent with its established role as a downstream effector of Ras, v-Raf expression mirrored the v-Ras effects upon adipocytic differentiation and transformation.

Phosphatidylinositol 3'-kinase and p70s6k are required for insulin but not bisperoxovanadium 1,10-phenanthroline (bpV(phen)) inhibition of insulin-like growth factor binding protein gene expression. Evidence for MEK-independent activation of mitogen-activated protein kinase by bpV(phen)

The hormonal regulation of insulin-like growth factor binding protein (IGFBP)-1 and -4 mRNA was compared in serum-free primary rat hepatocyte cultures. The combination of dexamethasone and glucagon (Dex/Gluc) strongly increased IGFBP-1 and IGFBP-4 mRNA levels. Insulin suppressed Dex/Gluc-stimulated IGFBP-1 but not IGFBP-4 mRNA levels. In contrast, the peroxovanadium compound, bisperoxovanadium 1,10-phenanthroline (bpV(phen)), completely abrogated Dex/Gluc induction of both IGFBP mRNA species. Wortmannin and rapamycin blocked the inhibitory effect of insulin but not that of bpV(phen) on Dex/Gluc-stimulated IGFBP mRNA. Thus, although phosphatidylinositol 3'-kinase and p70s6k are necessary for insulin-mediated transcriptional inhibition of the IGFBP-1 gene, a signaling pathway, independent of phosphatidyloinositol 3'-kinase and p70s6k, is activated by bpV(phen) and mediates IGFBP-1 as well as IGFBP-4 mRNA inhibition. Mitogen-activated protein (MAP) kinase activity induced by insulin was suppressed to below basal levels in the presence of Dex/Gluc, whereas in response to bpV(phen), MAP kinase activity was high and unaffected by Dex/Gluc, consistent with a role of MAP kinases in bpV(phen)-mediated inhibition of IGFBP mRNA. The specific MAP kinase kinase (MEK) inhibitor, PD98059, inhibited insulin but not bpV(phen)-stimulated MAP kinase activity, suggesting that MAP kinases can be activated in a MEK-independent fashion. Peroxovanadium compounds are strong inhibitors of tyrosine phosphatases, which may inhibit specific tyrosine/threonine phosphatases involved in the negative regulation of MAP kinases.

Insulin-induced egr-1 and c-fos expression in 32D cells requires insulin receptor, Shc, and mitogen-activated protein kinase, but not insulin receptor substrate-1 and phosphatidylinositol 3-kinase activation

Many studies suggest that insulin utilizes multiple signal transduction pathways. Insulin's effects are initiated by insulin binding to the insulin receptor, resulting in tyrosine phosphorylation of insulin receptor and intracellular substrates, such as insulin receptor substrate-1 (IRS-1), IRS-2, or Shc. We recently demonstrated that immediate-early gene egr-1 transcription was fully induced without phosphorylation of IRS-1 in Chinese hamster ovary cells (Harada, S., Smith, R. M., Smith, J. A., Shah, N. , Hu, D.-Q. & Jarett, L. (1995) J. Biol. Chem. 270, 26632-26638). In the present study, we examined the effects of insulin on immediate-early gene egr-1 and c-fos expression in 32D cells overexpressing the insulin receptor (32D/IR), IRS-1 (32D/IRS), or both (32D/IR+IRS) and compared these effects with insulin-induced tyrosine phosphorylation. Insulin (17 nM) increased egr-1 and c-fos expression in 32D/IR and 32D/IR+IRS cells, but not in parental cells or 32D/IRS cells, as determined by Northern blot analysis. Insulin treatment (5 min at 37 degrees C) markedly increased tyrosine phosphorylation of several proteins, including the insulin receptor, IRS-1, and Shc, in 32D/IR+IRS cells as determined by immunoprecipitation and Western blot analysis with anti-phosphotyrosine antibody. In contrast, only two tyrosine-phosphorylated proteins, i.e. insulin receptor and Shc, were detected in 32D/IR cells. These data suggest that insulin receptor and Shc phosphorylation is necessary for insulin-induced egr-1 and c-fos expression, but IRS-1 phosphorylation is not necessary or sufficient for the expression of these genes. Furthermore, the effect of specific inhibitors on insulin-induced egr-1 expression was examined. Wortmannin (25 nM), a phosphatidylinositol 3-kinase inhibitor, had no effect on insulin-induced egr-1 expression. In contrast, PD 98059 (30 microM), a mitogen-activated protein kinase kinase inhibitor, totally blocked egr-1 expression induced by insulin. These data indicate that mitogen-activated protein kinase activation, but not phosphatidylinositol 3-kinase activation, is involved in insulin-induced egr-1 expression. Taken together, insulin receptor tyrosine phosphorylation, Shc tyrosine phosphorylation, and mitogen-activated protein kinase activation appear to be the signal transduction pathway responsible for insulin-induced egr-1 expression in 32D cells. These data demonstrate that insulin has multiple signal transduction pathways that vary from cell to cell.

Effect of insulin on farnesyltransferase activity in 3T3-L1 adipocytes

Activation of p21(ras) by GTP loading is a critical step in a cascade of intracellular insulin signaling. Farnesylation of p21(ras) protein is an obligatory event that facilitates Ras migration to the plasma membrane and subsequent activation. Farnesyltransferase (FTase) is a ubiquitous enzyme that catalyzes the lipid modification of p21(ras) by the addition of farnesyl to the C-terminal "CAAX" motif. In vitro and in vivo FTase activities were studied in 3T3-L1 adipocytes in response to insulin challenge. Insulin exerted a biphasic stimulatory effect on FTase activity measured in vitro with a 31% increase at 5 min and a 130% increase at 60 min. Insulin-stimulated farnesylation of p21(ras) pools in vivo correlated with FTase activity seen in vitro by displaying an increase in farnesylated p21(ras) from 40% of total cellular Ras in control cells to 63% by 5 min and 80% by 60 min (p < 0.05) in insulin-treated cells. Insulin challenge of 3T3-L1 adipocytes increased incorporation of tritiated mevalonic acid in p21(ras) in a dose-dependent manner and stimulated a 2-fold increase in phosphorylation of the alpha-subunit of FTase at 5 min and a 4-fold increase at 60 min.

The signal-dependent coactivator CBP is a nuclear target for pp90RSK

We have examined the mechanism by which growth factor-mediated induction of the Ras pathway interferes with signaling via the second messenger cAMP. Activation of cellular Ras with insulin or NGF stimulated recruitment of the S6 kinase pp90RSK to the signal-dependent coactivator CBP. Formation of the pp90RSK-CBP complex occurred with high stoichiometry and persisted for 6-8 hr following growth factor addition. pp90RSK specifically recognized the E1A-binding domain of the coactivator CBP. In addition, like E1A, binding of pp90RSK to CBP was sufficient to repress transcription of cAMP-responsive genes via the cAMP-inducible factor CREB. By contrast with its effects on the cAMP pathway, formation of the pp90RSK-CBP complex was required for induction of Ras-responsive genes. These results provide a demonstration of cross-coupling between two signaling pathways that occurs at the level of a signal-dependent coactivator.

Interactions of protein kinase C with insulin signaling. Influence on GAP and Sos activities

In this study, we investigated the influence of the protein kinase C (PKC)-dependent system upon the ability of insulin to stimulate p21(ras).GTP loading in 3T3-L1 adipocytes. Activation of PKC by 12-0-tetradecanoylphorbol-13-acetate (TPA) did not affect the basal amount of p21(ras).GTP but significantly reduced insulin-induced increases in p21(ras).GTP. This reduction was due to inhibition of the insulin's ability to stimulate guanine nucleotide exchange activity of Sos in cells incubated with 100 nM TPA for either 30 min or 3 h. TPA had no effect on basal activity of Sos. Depletion of PKC by an 18-h incubation with TPA or inhibition by bisindolylmaleimide resulted in profound inhibition of the insulin-induced p21(ras).GTP loading. In contrast to PKC activation, removal of PKC did not influence Sos activity but resulted in a 2-fold stimulation of GTPase activating protein (GAP). This effect of PKC depletion is unique to 3T3-L1 adipocytes and was not observed in either 3T3-L1 fibroblasts or Rat-1 fibroblasts. Thus, it appears that in 3T3-L1 adipocytes, PKC has a constitutive inhibitory effect on GAP that permits insulin to activate Sos and p21(ras). Removal of this inhibitory influence activates GAP and reduces insulin-stimulated p21(ras).GTP loading.

Long-term treatment of Swiss 3T3 fibroblasts with dexamethasone attenuates MAP kinase activation induced by insulin-like growth factor-I (IGF-I)

Bone formation is reduced in hyperglucocorticoid states, e.g. Cushing's syndrome or long-term treatment with synthetic glucocorticoids during rheumatic diseases. possibly related to decreased sensitivity of the target to insulin-like growth factor-I (IGF-I). In this study, we have sought to identify postreceptor-mechanisms for glucocorticoid-induced resistance to insulin-like peptides in a model system. Treatment of Swiss 3T3 fibroblasts with 100 nM dexamethasone for 48h reduced IGF-I-induced activation of mitogen-activated protein kinase (MAP kinase). The level of insulin receptor substrate-1 (IRS-1) was reduced in dexamethasone-treated cells, as measured by Western blot; however, the pattern of tyrosine-phosphorylated protein subsequent to stimulation with IGF-I (1 min) was not altered. No inhibitory effect of dexamethasone was observed on the level of phosphotyrosine in IRS-1 in extracts from IGF-I-treated cells. The amount of IGF-I-induced association of insulin receptor substrate-1 and phosphatidylinositol 3-kinase was increased in steroid treated cells. Addition of IGF-I increased the synthesis of lipid, glycogen and protein, and the reduction of a tetrazolium dye, MTS, in untreated cells. The response to IGF-I in terms of glycogen synthesis was blunted, whereas the effect of IGF-I was unaffected for the other three parameters in cells pretreated with dexamethasone. These findings indicate that the activation of MAP kinase may be dissociated from IGF-I-induced anabolic pathways and tyrosine phosphorylation of IRS-1. The results agree with the previously proposed role for the activation of MAP kinase in the regulation of glycogen synthesis. Furthermore, they suggest that dexamethasone-induced reduction of IRS-1 expression may be important for the impaired activation of MAP kinase by insulin-like peptides in steroid-treated cells.

Involvement of ErbB2 in the signaling pathway leading to cell cycle progression from a truncated epidermal growth factor receptor lacking the C-terminal autophosphorylation sites

To investigate the mechanisms underlying the enhanced mitogenic activity of the truncated epidermal growth factor receptor (EGFR) lacking the C-terminal autophosphorylation sites (Delta973-EGFR), we studied the intracellular signaling pathways in NR6 cells expressing human wild type EGFR and Delta973-EGFR. Microinjection of dominant/negative p21ras(N17) completely inhibited EGF-induced DNA synthesis in both cell types. EGF stimulated Shc phosphorylation as well as the formation of wild type EGFR.Shc complexes. In contrast, EGF stimulated Shc phosphorylation without formation of Delta973-EGFR.Shc complexes. Tyrosine-phosphorylated Shc formed complexes with Grb2.Sos, and microinjection of anti-Shc antibody and Shc-SH2 GST fusion protein inhibited EGF stimulation of DNA synthesis in both cell lines. EGF markedly increased ErbB2 tyrosine phosphorylation in wild type EGFR cells. In Delta973-EGFR cells, ErbB2 was tyrosine phosphorylated in the basal state and EGFR stimulated further phosphorylation of ErbB2. In addition to ErbB2, additional proteins were tyrosine phosphorylated in Delta973-EGFR cells, mostly in the molecular mass range of 120 170 kDa. Taken together with our findings indicating coupling of ErbB2 to Shc, these data suggest the importance of an alternative signaling pathway in Delta973-EGFR cells mediated by the formation of heterodimeric structures between the truncated EGFR and ErbB2, followed by coupling through Shc to Grb2.Sos and the p21ras pathway, ultimately leading to mitogenesis.

The effects of insulin-like growth factor analogues on survival of cultured cerebral cortex and cerebellar granule neurones

Insulin and insulin-like growth factors (IGF-I, IGF-II) are closely related polypeptides which are found in the CNS and which promote neuronal survival and neurite outgrowth. They are each associated with specific cell surface receptors and several soluble binding proteins (IGFBPs) which are involved in regulating function and availability. Two analogues of IGF-I were produced by site directed mutagenesis: (Gln3, Ala4, Tyr15, (Leu16)IGF-1 (QAYL-IGF) and a B-chain mutant in which the first 16 amino acids of IGF-1 were replaced by the first 17 amino acids of insulin. These analogues have significantly reduced binding affinity for IGFBPs. Using glucose deprivation as a damaging stimulus and assaying lactate dehydrogenase released from cultures as a marker for cell death, we have investigated the effect of IGF analogues on cell death of cerebrocortical and cerebellar granule cell cultures. In the presence of IGF-I, QAYL-IGF or B-chain mutant, the amount of LDH released from cortical and cerebellar granule cell cultures was significantly reduced compared to control (no glucose), indicating that these molecules promote survival. Both QAYL and B-chain mutants, which have reduced affinity for IGFBPs, are as effective as IGF-I in promoting cell survival in conditions of glucose deprivation and their reduced affinity for IGFBPs has no apparent deleterious effect on their neuroprotective function. We also show that the neuroprotective effect of the IGF analogues is due to a direct effect on the neurones in these cultures and is independent of the presence of glia.

Insulin regulation of phosphoenolpyruvate carboxykinase gene expression does not require activation of the Ras/mitogen-activated protein kinase signaling pathway

Expression of phosphoenolpyruvate carboxykinase (PEPCK), the rate-limiting step in hepatic gluconeogenesis, is primarily regulated at the level of gene transcription. Insulin and phorbol esters inhibit basal PEPCK transcription and antagonize the induction of PEPCK gene expression by glucocorticoids and glucagon (or its second messenger cAMP). Insulin activates a signaling cascade involving Ras --> Raf --> p42/p44 mitogen-activated protein (MAP) kinase kinase (MEK) --> p42/p44 MAP kinase (ERK 1 and 2). Recent reports suggest that activation of this Ras/MAP kinase pathway is critical for the effects of insulin on mitogenesis and c-fos transcription but is not required for insulin action on metabolic processes such as glycogen synthesis, lipogenesis, and Glut-4-mediated glucose transport. We have used three distinct approaches to examine the role of the Ras/MAP kinase pathway in the regulation of PEPCK transcription by insulin in H4IIE-derived liver cells: (i) chemical inhibition of Ras farnesylation, (ii) infection of cells with an adenovirus vector encoding a dominant-negative mutant of Ras, and (iii) use of a chemical inhibitor of MEK. Although each of these methods blocks insulin activation of MAP kinase, none alters insulin antagonism of cAMP- and glucocorticoid-stimulated PEPCK transcription. Although phorbol esters activate MAP kinase and mimic the effects of insulin on PEPCK gene transcription, inhibition of MEK has no effect on phorbol ester inhibition of PEPCK gene transcription. Using the structurally and mechanistically distinct phosphatidylinositol 3-kinase (PI 3-kinase) inhibitors, wortmannin and LY 294002, we provide further evidence supporting a role for PI 3-kinase activation in the regulation of PEPCK gene transcription by insulin. We conclude that neither insulin nor phorbol ester regulation of PEPCK gene transcription requires activation of the Ras/MAP kinase pathway and that insulin signaling to the PEPCK promoter is dependent on PI 3-kinase activation.

IRS-I expression on the luteinized rat ovary: IGF-I and cyclic AMP effects on IRS-I tyrosine phosphorylation

The expression of insulin receptor substrate-I (IRS-I) mRNA was demonstrated in rat luteal cells by Northern blot analysis, in situ hybridization as well as by reverse transcriptase polymerase chain reaction. Western blot with a polyclonal anti IRS-I antibody showed the presence of a 183 kDa protein which corresponds to the size of IRS-I reported in other tissues. Further studies were performed to determine whether human chorionic gonadotropin (hCG) can interact with the insulin-like growth factor-I (IGF-I) signaling pathway to increase tyrosine phosphorylation of IRS-I. While hCG alone was ineffective in stimulating the phosphorylation of IRS-I, IGF-I mediated phosphorylation of IRS-I was increased by prior exposure to hCG. These results were further confirmed by the immunoprecipitation of IRS-I from the lysate of hCG- and IGF-I-treated luteal cell cultures followed by Western blotting with anti-phosphotyrosine antibody. Similarly, pretreatment with forskolin also increased IGF-I stimulated IRS-I phosphorylation. The increased tyrosine phosphorylation of IRS-I seen in response to IGF-I stimulation following treatment with either hCG or forskolin was not due to an increase in IRS-I content. Furthermore, IGF-I receptor tyrosine kinase activity was not affected by forskolin, suggesting that the increase in IRS-I tyrosine phosphorylation was not the result of an increase in its activity. Thus, we conclude that hCG/LH and IGF-I signaling pathways 'cross-talk' to increase the levels of IRS-I tyrosine phosphorylation. The observed increase in IRS-I tyrosine phosphorylation may be the result of an increase in the stability of the phosphorylated form of IRS-I.

Insulin-stimulated GLUT4 translocation is mediated by a divergent intracellular signaling pathway

Insulin stimulates glucose transport largely by mediating translocation of the insulin-sensitive glucose transporter (GLUT4) from an intracellular compartment to the plasma membrane. Using single cell microinjection of 3T3-L1 adipocytes, coupled with immunofluorescence detection of GLUT4 proteins, we have determined that inhibition of endogenous p21ras or injection of oncogenic p21ras has no effect on insulin-stimulated GLUT4 translocation. On the other hand, microinjection of anti-phosphotyrosine antibodies or inhibition of endogenous phosphatidylinositol 3-kinase by microinjection of a GST-p85 SH2 fusion protein markedly inhibits this biologic effect of insulin. These data suggest that the p21ras/mitogen-activated protein kinase pathway is not involved in this metabolic effect of insulin, whereas tyrosine phosphorylation and stimulation of phosphatidylinositol 3-kinase activity are critical components of this signaling pathway.

Negative feedback regulation and desensitization of insulin- and epidermal growth factor-stimulated p21ras activation

Insulin and epidermal growth factor receptors transmit signals for cell proliferation and gene regulation through formation of active GTP-bound p21ras mediated by the guanine nucleotide exchange factor Sos. Sos is constitutively bound to the adaptor protein Grb2 and growth factor stimulation induces association of the Grb2/Sos complex with Shc and movement of Sos to the plasma membrane location of p21ras. Insulin or epidermal growth factor stimulation induces a rapid increase in p21ras levels, but after several minutes levels decline toward basal despite ongoing hormone stimulation. Here we show that deactivation of p21ras correlates closely with phosphorylation of Sos and dissociation of Sos from Grb2, and that inhibition of mitogen-activated protein (MAP) kinase kinase (also known as extracellular signal-related kinase (ERK) kinase, or MEK) blocks both events, resulting in prolonged p21ras activation. These data suggest that a negative feedback loop exists whereby activation of the Raf/MEK/MAP kinase cascade by p21ras causes Sos phosphorylation and, therefore, Sos/Grb2 dissociation, limiting the duration of p21ras activation by growth factors. A serine/threonine kinase downstream of MEK (probably MAP kinase) mediates this desensitization feedback pathway.

Insulin and cyclic adenosine monophosphate increase prolactin gene expression through different response pathways

Insulin and cAMP stimulate prolactin gene transcription and prolactin-CAT expression in rat pituitary tumor GH cells. Expression of prolactin-CAT construct, pPrl(-173/+75)CAT, is stimulated 10- to 30-fold by either insulin or cAMP. Addition of both insulin and cAMP resulted in an additive 20- to 60-fold stimulation. Although the regulatory sequences have not been defined precisely, both insulin and cAMP appear to stimulate transcription of prolactin-CAT expression through possibly identical sequences in the -106/-87 region of the promoter. Insulin mediated increases in prolactin-CAT expression are not ras-dependent in GH4 cells. Thus, a number of experiments were performed to determine that the effects of insulin and cAMP are independent. First, insulin does not stimulate cAMP levels in GH4 cells. Second, cAMP action was inhibited by expression of a mutant regulatory subunit of cAMP-dependent protein kinase A that does not bind cAMP and by expression of an inhibitor of cAMP-dependent protein kinase A, while insulin action was not affected by expression of these proteins. Thus, although the regulatory sequences for insulin and cAMP may be identical, the effects of insulin and cAMP on the prolactin gene are clearly mediated through distinct response pathway.