Hodgkin lymphoma in temporal association with growth hormone replacement

The association between growth hormone (GH) replacement and malignancy has long been debated. We report a case of Hodgkin lymphoma that developed in temporal association with the initiation of GH replacement in a 57-year-old woman with panhypopituitarism secondary to a non-secretory pituitary macroadenoma. Treatment of her pituitary tumor included transphenoidal surgery, external beam radiation, Bromocriptine and Cabergaline therapy. In addition to replacement steroid, thyroid and sex hormones, she insisted on GH replacement. Approximately 2 years after GH initiation, the diagnosis of Hodgkin lymphoma was made. Although the exact contribution of GH to the development of Hodgkin disease in our patient is unclear and a causal effect cannot be concluded, the temporal association is suggestive, and warrants reporting as part of ongoing surveillance for potential complications of GH replacement.

Leptin improves insulin resistance and hyperglycemia in a mouse model of type 2 diabetes

Leptin has metabolic effects on peripheral tissues including muscle, liver, and pancreas, and it has been successfully used to treat lipodystrophic diabetes, a leptin-deficient state. To study whether leptin therapy can be used for treatment of more common cases of type 2 diabetes, we used a mouse model of type 2 diabetes (MKR mice) that show normal leptin levels and are diabetic due to a primary defect in both IGF-I and insulin receptors signaling in skeletal muscle. Here we show that leptin administration to the MKR mice resulted in improvement of diabetes, an effect that was independent of the reduced food intake. The main effect of leptin therapy was enhanced hepatic insulin responsiveness possibly through decreasing gluconeogenesis. In addition, the reduction of lipid stores in liver and muscle induced by enhancing fatty acid oxidation and inhibiting lipogenesis led to an improvement of the lipotoxic condition. Our data suggest that leptin could be a potent antidiabetic drug in cases of type 2 diabetes that are not leptin resistant.

Metabolic memory in diabetes--focus on insulin

Large-scale clinical trials have demonstrated that metabolic control achieved early in the course of diabetes substantially reduces development and progression of diabetes and the associated microvascular complications. Additionally, prospective observational studies have demonstrated that atherogenic and inflammatory mediators are elevated even prior to the onset of diabetes and significantly contribute to subsequent development of macrovascular complications. Collectively, these data suggest that metabolic memories are stored early in the course of diabetes. We believe that insulin suppresses inflammation and also suppresses glucotoxicity and lipotoxicity (and the consequences thereof, such as the formation of advanced glycation end products and epigenetic phenomena), and thus has a pivotal and beneficial role. Comprehensive metabolic control, especially when instituted early, may alter the natural history of diabetic complications by affecting this metabolic memory. Thus, our overall goal is to understand in more detail the molecular mechanisms involved in these changes, thereby affording us opportunities to reduce the long-term effects of diabetes.

Clinical relevance of systemic and local IGF-I

The insulin-like growth factor family of ligands, receptors and binding proteins are critical for many normal physiological functions. These include normal development during fetal and post-natal development and maintenance of organ function in adult life. Circulating IGF-I is produced primarily by the liver under GH control, whereas the production of tissue IGF-I has other controls. Recent studies have demonstrated that both circulating and tissue IGF-I are important for maintaining the normal structure-function of complex organs such as bone. Circulating IGF-I is important for maintaining ambient GH levels; in its absence GH elevation is seen leading to insulin resistance. In addition, low levels of circulating IGF-I retard the progression and metastatic potential of a number of cancers.

Studies involving the GH-IGF axis: Lessons from IGF-I and IGF-I receptor gene targeting mouse models

The IGFs are ubiquitous and have pleoitropic effects. They are critical for normal growth and development, and for normal functioning of adult tissues. A liver-specific gene-deletion knockout of the IGF-I gene resulted in a mouse model with reduced circulating IGF-I levels, that led to insulin resistance due to the secondary elevation of circulating GH levels. The reduction in circulating IGF-I levels was also associated with a reduction in cancer growth and metastases in three cancer models, one for colon cancer and two for breast cancer. A second mouse model, using the transgenic approach, inhibited the IGF-I and insulin receptor function in skeletal muscle, and resulted in severe insulin resistance in muscle followed by insulin resistance in fat and liver and, eventually, beta-cell dysfunction and development of Type 2 diabetes. This progression from insulin resistance to Type 2 diabetes was most likely due to lipotoxicity with elevated serum and tissue triglyceride levels. Evidence supporting the hypothesis came from the use of fibrates and leptin injections, each of which enhanced fatty acid (FA) oxidation in liver and muscle and was associated with a reversal of the insulin resistance and diabetes.

Thiazolidinediones improve insulin sensitivity in adipose tissue and reduce the hyperlipidaemia without affecting the hyperglycaemia in a transgenic model of type 2 diabetes

AIM/HYPOTHESIS

The aim of this study was to examine the effects of thiazolidinediones on the MKR mouse model of type 2 diabetes.

METHODS

Six-week-old wild-type (WT) and MKR mice were fed with or without rosiglitazone or pioglitazone for 3 weeks. Blood was collected from the tail vein for serum biochemistry analysis. Hyperinsulinaemic-euglycaemic clamp analysis was performed to study effects of thiazolidinediones on insulin sensitivity of tissues in MKR mice. Northern blot analysis was performed to measure levels of target genes of PPAR gamma agonists in white adipose tissue and hepatic gluconeogenic genes.

RESULTS

Thiazolidinedione treatment of MKR mice significantly lowered serum lipid levels and increased serum adiponectin levels but did not affect levels of blood glucose and serum insulin. Hyperinsulinaemic-euglycaemic clamp showed that whole-body insulin sensitivity and glucose homeostasis failed to improve in MKR mice after rosiglitazone treatment. Insulin suppression of hepatic endogenous glucose production failed to improve in MKR mice following rosiglitazone treatment. This lack of change in hepatic insulin insensitivity was associated with no change in the ratio of HMW : total adiponectin, hepatic triglyceride content, and sustained hepatic expression of PPAR gamma and stearoyl-CoA desaturase 1 mRNA. Interestingly, rosiglitazone markedly enhanced glucose uptake by white adipose tissue with a parallel increase in CD36, aP2 and GLUT4 gene expression.

CONCLUSIONS/INTERPRETATION

These data suggest that potentiation of insulin action on tissues other than adipose tissue is required to mediate the antidiabetic effects of thiazolidinediones in our MKR diabetic mice.

Pancreatic-specific inactivation of IGF-I gene causes enlarged pancreatic islets and significant resistance to diabetes

The dogma that IGF-I stimulates pancreatic islet growth has been challenged by combinational targeting of IGF or IGF-IR (IGF receptor) genes as well as beta-cell-specific IGF-IR gene deficiency, which caused no defect in islet cell growth. To assess the physiological role of locally produced IGF-I, we have developed pancreatic-specific IGF-I gene deficiency (PID) by crossing Pdx1-Cre and IGF-I/loxP mice. PID mice are normal except for decreased blood glucose level and a 2.3-fold enlarged islet cell mass. When challenged with low doses of streptozotocin, control mice developed hyperglycemia after 6 days that was maintained at high levels for at least 2 months. In contrast, PID mice only exhibited marginal hyperglycemia after 12 days, maintained throughout the experiment. Fifteen days after streptozotocin, PID mice demonstrated significantly higher levels of insulin production. Furthermore, streptozotocin-induced beta-cell apoptosis (transferase-mediated dUTP nick-end labeling [TUNEL] assay) was significantly prevented in PID mice. Finally, PID mice exhibited a delayed onset of type 2 diabetes induced by a high-fat diet, accompanied by super enlarged pancreatic islets, increased insulin mRNA levels, and preserved sensitivity to insulin. Our results suggest that locally produced IGF-I within the pancreas inhibits islet cell growth; its deficiency provides a protective environment to the beta-cells and potential in combating diabetes.

Phloridzin improves hyperglycemia but not hepatic insulin resistance in a transgenic mouse model of type 2 diabetes

The chronic hyperglycemia that occurs in type 2 diabetes may cause deterioration of beta-cell function and insulin resistance in peripheral tissues. Mice that express a dominant-negative IGF-1 receptor, specifically in skeletal muscle (MKR mice), exhibit severe insulin resistance, hyperinsulinemia, dyslipidemia, and hyper-glycemia. To determine the role of hyperglycemia in the worsening of the diabetes state in these animals, MKR mice were treated with phloridzin (PHZ), which inhibits intestinal glucose uptake and renal glucose reabsorption. Blood glucose levels were decreased and urine glucose levels were increased in response to PHZ treatment in MKR mice. PHZ treatment also increased food intake in MKR mice; however, the fat mass was decreased and lean body mass did not change. Serum insulin, fatty acid, and triglyceride levels were not affected by PHZ treatment in MKR mice. Hyperinsulinemic-euglycemic clamp analysis demonstrated that glucose uptake in white adipose tissue was significantly increased in response to PHZ treatment. Despite the reduction in blood glucose following PHZ treatment, there was no improvement in insulin-stimulated whole-body glucose uptake in MKR mice and neither was there suppression of endogenous glucose production by insulin. These results suggest that glucotoxicity plays little or no role in the worsening of insulin resistance that occurs in the MKR mouse model of type 2 diabetes.

Muscle-specific overexpression of CD36 reverses the insulin resistance and diabetes of MKR mice

Insulin resistance is one of the primary characteristics of type 2 diabetes. Mice overexpressing a dominant-negative IGF-I receptor specifically in muscle (MKR mice) demonstrate severe insulin resistance with high levels of serum and tissue lipids and eventually develop type 2 diabetes at 5-6 wk of age. To determine whether lipotoxicity plays a role in the progression of the disease, we crossed MKR mice with mice overexpressing a fatty acid translocase, CD36, in skeletal muscle. The double-transgenic MKR/CD36 mice showed normalization of the hyperglycemia and the hyperinsulinemia as well as a marked improvement in liver insulin sensitivity. The MKR/CD36 mice also exhibited normal rates of fatty acid oxidation in skeletal muscle when compared with the decreased rate of fatty acid oxidation in MKR. With the reduction in insulin resistance, beta-cell function returned to normal. These and other results suggest that the insulin resistance in the MKR mice is associated with increased muscle triglycerides levels and that whole-body insulin resistance can be, at least partially, reversed in association with a reduction in muscle triglycerides levels, although the mechanisms are yet to be determined.

Role of growth hormone (GH) in liver regeneration

Liver regeneration is a fundamental mechanism by which the liver responds to injury. This process is regulated by endogenous growth factors and cytokines, and it involves proliferation of all mature cells that exist within the intact organ. To understand the role of the GH/IGF-I axis in liver regeneration, we performed partial hepatectomies in three groups of mice: GH antagonist (GHa) transgenic mice, in which the action of GH is blocked; liver IGF-I-deficient mice that lack IGF-I specifically in the liver and also lack the acid-labile subunit (ALS; LID+ALSKO mice), in which IGF-I levels are very low and GH secretion is increased; and control mice. Interestingly, the survival rate of GHa transgenic mice was dramatically reduced after partial hepatectomy (57%) compared with the survival rate of controls (100%) or LID+ALSKO mice (88%). In control mice, the liver was completely regenerated after 4 d, whereas liver regeneration required 7 d in LID+ALSKO mice. In contrast, in GHa mice, liver regeneration reached only 70% of the original liver mass after 4 d and did not improve thereafter. Strikingly, 36 and 48 h after hepatectomy, the livers of control and LID+ALSKO mice, respectively, exhibited intense 5-bromo-2'-deoxyuridine (BrdU) staining, whereas BrdU staining was dramatically decreased in the livers of GHa-treated mice. These results suggest that GH plays a critical role in liver regeneration, although whether it acts directly or indirectly remains to be determined.

Effects of organ-specific loss of insulin-like growth factor-I production on murine hematopoiesis

To determine whether circulating insulin-like growth factor (IGF)-I has a role in hematopoiesis, we examined hematologic parameters in mice with markedly reduced serum levels resulting from a liver-specific inactivation of the IGF-I gene. These mice have normal postnatal growth and development, suggesting that local production of IGF-I can maintain anabolic effects. Liver-specific IGF-I-deficient (LID) mice were compared with control littermates with regard to hematopoietic parameters. Spleen cellularity was decreased in the LID mice compared with control mice. Spleen myeloid progenitors, as determined by colony-forming units-granulocyte/monocyte (CFU-GM) and colony-forming units-high proliferative potential (CFU-HPP), were significantly decreased in the LID mice. Immune parameters, as indicated by the absolute number of B and T cells, did not significantly differ between the knockout and control mice. In contrast to the decreased cellularity and myelopoiesis in the spleen, bone marrow cellularity was not different between the 2 groups, but the total femoral content of CFU-GM and CFU-HPP was significantly increased in the LID mice. The decrease in splenic myelopoiesis was not due to the inability of progenitors to exit the bone marrow, because CFU-GM and burst-forming units-erythroid were significantly increased in the blood of LID mice compared with normal littermates. Administration of exogenous IGF-I to the LID mice for 4 days partially restored myelopoietic parameters in the spleen. Liver production of IGF-I and, therefore, normal serum levels of this hormone, although not necessary for general organ growth and development, seems necessary for survival or transition of myeloid progenitors into the spleen.

The luteinizing hormone-releasing hormone inhibits the anti-apoptotic activity of insulin-like growth factor-1 in pituitary alphaT3 cells by protein kinase Calpha-mediated negative regulation of Akt

The luteinizing hormone-releasing hormone (LHRH) receptor is a G protein-coupled receptor involved in the synthesis and release of pituitary gonadotropins and in the proliferation and apoptosis of pituitary cells. Insulin-like growth factor-1 receptor (IGF-1R) is a tyrosine kinase receptor that has a mitogenic effect on pituitary cells. In this study, we used the alphaT3 gonadotrope cell line as a model to characterize the IGF-1R signaling pathways and to investigate whether this receptor interacts with the LHRH cascade. We found that IGF-1 activated the IGF-1R, insulin receptor substrate (IRS)-1, phosphatidylinositol 3-kinase, and Akt in a time-dependent manner in alphaT3 cells. The MAPK (ERK1/2, p38, and JNK) pathways were only weakly activated by IGF-1. In contrast, LHRH strongly stimulated the MAPK pathways but had no effect on Akt activation. Cotreatment with IGF-1 and LHRH had various effects on these signaling pathways. 1) It strongly increased IGF-1-induced tyrosine phosphorylation of IRS-1 and IRS-1-associated phosphatidylinositol 3-kinase through activation of the epidermal growth factor receptor. 2) It had an additive effect on ERK1/2 activation without modifying the phosphorylation of p38 and JNK1/2. 3) It strongly reduced IGF-1 activation of Akt. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays and cell cycle analysis revealed that, in addition to having an additive effect on ERK1/2 activation, cotreatment with IGF-1 and LHRH also had an additive effect on cell proliferation. The LHRH-induced inhibition of Akt stimulated by IGF-1 was completely blocked by Safingol, a protein kinase C (PKC) alpha-specific inhibitor, and by a dominant negative form of PKCalpha. Finally, we showed that the inhibitory effect of LHRH on IGF-1-induced PKCalpha-mediated Akt activation was associated with a marked reduction in Bad phosphorylation and a substantial decrease in the ability of IGF-1 to rescue alphaT3 cells from apoptosis induced by serum starvation. Our results demonstrate for the first time that several interactions take place between IGF-1 and LHRH receptors in gonadotrope cells.

Hepatic CCAAT/enhancer binding protein alpha mediates induction of lipogenesis and regulation of glucose homeostasis in leptin-deficient mice

CCAAT/enhancer binding protein alpha (C/EBP alpha) is a critical factor in glucose metabolism in the neonate as revealed by conventional C/EBP alpha-null mice that do not survive beyond the first day after birth because of severe hypoglycemia and a deficiency in hepatic glycogen accumulation. To elucidate the function of C/EBP alpha in leptin-deficient mouse (ob/ob) liver, a C/EBP alpha-liver null mouse on an ob/ob background (ob/ob-C/EBP alpha/Cre(+)) was produced using a floxed C/EBP alpha allele and Cre recombinase under control of the albumin promoter (AlbCre). The C/EBP alpha-deficient liver in ob/ob mice had significantly decreased triglyceride content compared with equivalent mice lacking the AlbCre transgene (ob/ob-C/EBP alpha/Cre(-)). Expression of genes involved in lipogenesis including fatty acid synthase, acetyl-coenzyme A carboxylase, stearoyl-coenzyme A desaturase 1 and ATP-citrate lyase dramatically decreased in ob/ob-C/EBP alpha/Cre(+) mouse liver. Induction of these lipogenic genes by a high-carbohydrate diet caused an exacerbation in the development of fatty liver and an increase in liver size, hepatic triglyceride, and cholesterol contents in ob/ob-C/EBP alpha/Cre(-) mice but not in ob/ob-C/EBP alpha/Cre(+) mice. Deficiency in hepatic C/EBP alpha expression caused an exacerbation of hyperglycemia because of decreased insulin secretion. Taken together, these results indicate that hepatic C/EBP alpha plays a critical role in the acceleration of lipogenesis in ob/ob mice and in glucose homeostasis by the indirect regulation of insulin secretion.

14-3-3σ Mediation of Cell Cycle Progression Is p53-independent in Response to Insulin-like Growth Factor-I Receptor Activation

We investigated the role of 14-3-3sigma protein in insulin-like growth factor-I (IGF-I) receptor signaling. It has been previously shown that 14-3-3sigma negatively regulates cell cycle especially in response to p53-sensitive DNA damage. In this study we demonstrated that 14-3-3sigma is a positive mediator of IGF-I receptor-induced cell proliferation. Treatment with IGF-I increased 14-3-3sigma mRNA and protein levels about 4-fold, in a time-dependent manner in MCF-7 breast cancer cells. Preincubation with the phosphoinositide 3'-kinase inhibitor LY294002 significantly reduced the effects of IGF-I on 14-3-3sigma gene expression in these cells, suggesting that this effect of IGF-I occurs via the phosphoinositide 3'-kinase pathway. 14-3-3sigma is induced by IGF-I in MCF-7 cells, which express wild-type p53, as well as in MCF-7 cells transfected with a small interference RNA targeting duplex that reduced p53 expression levels. These results suggest that IGF-I induces 14-3-3sigma expression in a manner that is independent of p53. Using the small interference RNA strategy, we demonstrated that a 70-75% reduction of 14-3-3sigma mRNA levels resulted in a similar decrease in the effects of IGF-I on cell cycle progression and proliferation in MCF-7 cells. This effect was also associated with a reduction in IGF-I-induced cyclin D1 expression. Taken together, these results suggest that 14-3-3sigma positively mediates IGF-I-induced cell cycle progression.

Insulin-like growth factor I is required for vessel remodeling in the adult brain

Although vascular dysfunction is a major suspect in the etiology of several important neurodegenerative diseases, the signals involved in vessel homeostasis in the brain are still poorly understood. We have determined whether insulin-like growth factor I (IGF-I), a wide-spectrum growth factor with angiogenic actions, participates in vascular remodeling in the adult brain. IGF-I induces the growth of cultured brain endothelial cells through hypoxiainducible factor 1 alpha and vascular endothelial growth factor, a canonical angiogenic pathway. Furthermore, the systemic injection of IGF-I in adult mice increases brain vessel density. Physical exercise that stimulates widespread brain vessel growth in normal mice fails to do so in mice with low serum IGF-I. Brain injury that stimulates angiogenesis at the injury site also requires IGF-I to promote perilesion vessel growth, because blockade of IGF-I input by an anti-IGF-I abrogates vascular growth at the injury site. Thus, IGF-I participates in vessel remodeling in the adult brain. Low serum/brain IGF-I levels that are associated with old age and with several neurodegenerative diseases may be related to an increased risk of vascular dysfunction.

Protection against hypoxic–ischemic injury in transgenic mice overexpressing Kir6.2 channel pore in forebrain

The role of the K-ATP channel pore-forming subunit Kir6.2 on protection from cerebral hypoxic-ischemic injury was assessed in transgenic mice overexpressing normal Kir6.2 or a dominant negative form (AFA) of this subunit in the forebrain. The resulting mice overexpress either the Kir6.2 or the AFA transgene mainly in the cerebral cortex and hippocampus. The Kir6.2 transgenic mice are resistant to hypoxic-ischemic injury showing a decreased region of cortical damage as compared to the dominant negative AFA and the wild-type mice. Moreover, the overexpression of Kir6.2 allowed an important silencing of the neurons present in forebrain regions thus protecting them from ischemic injury. Interestingly, the phenotype observed in Kir6.2 transgenic mice was observed without increased sulfonylurea binding. Taken together, these results indicate that the transgenic overexpression of Kir6.2 in forebrain significantly protects mice from hypoxic-ischemic injury and neuronal damage seen in stroke.

Peroxisome proliferator-activated receptor-alpha deficiency does not alter insulin sensitivity in mice maintained on regular or high-fat diet: hyperinsulinemic-euglycemic clamp studies

Chronic peroxisome proliferator-activated receptor (PPAR)-alpha activation improves glucose metabolism in rodent models of insulin resistance and diabetes; however, PPAR-alpha deficiency was also reported to protect against high-fat diet (HFD)-induced insulin resistance. The aim of this study was to clarify the role of PPAR-alpha in the development of insulin resistance using PPAR-alpha knockout (KO) mice and wild-type controls (WT). Both WT and PPAR-alpha KO mice on HFD gained significantly more weight relative to chow-fed groups and displayed an increase in insulin levels and a decrease in adiponectin levels. Hyperinsulinemic-euglycemic clamp performed in the nonfasting state demonstrated that HFD caused a marked reduction in whole body, muscle, and white and brown adipose tissue glucose uptake in both WT and PPAR-alpha KO mice relative to chow-fed groups. Suppression of endogenous glucose production during the clamp was markedly blunted in both WT and PPAR-alpha KO HFD-fed mice, indicating liver insulin resistance. The magnitude of HFD-induced changes in the clamp parameters of insulin sensitivity was comparable in PPAR-alpha KO and WT mice. In conclusion, these data show that PPAR-alpha deficiency does not alter insulin sensitivity in mice fed normal chow diet and does not protect against HFD-induced insulin resistance as measured by hyperinsulinemic-euglycemic clamp in nonfasted state.

TDAG51 mediates the effects of insulin-like growth factor I (IGF-I) on cell survival

Insulin-like growth factor-I (IGF-I) receptors and insulin receptors belong to the same subfamily of receptor tyrosine kinases and share a similar set of intracellular signaling pathways, despite their distinct biological actions. In the present study, we evaluated T cell death-associated gene 51 (TDAG51), which we previously identified by cDNA microarray analysis as a gene specifically induced by IGF-I. We characterized the signaling pathways by which IGF-I induces TDAG51 gene expression and the functional role of TDAG51 in IGF-I signaling in NIH-3T3 (NWTb3) cells, which overexpress the human IGF-I receptor. Treatment with IGF-I increased TDAG51 mRNA and protein levels in NWTb3 cells. This effect of IGF-I was specifically mediated by the IGF-IR, because IGF-I did not induce TDAG51 expression in NIH-3T3 cells overexpressing a dominant-negative IGF-I receptor. Through the use of specific inhibitors of various protein kinases, we found that IGF-I induced TDAG51 expression via the p38 MAPK pathway. The ERK, JNK, and phosphatidylinositol 3-kinase pathways were not involved in IGF-I-induced regulation of TDAG51. To assess the role of TDAG51 in IGF-I signaling, we used small interfering RNA (siRNA) expression vectors directed at two different target sites to reduce the level of TDAG51 protein. In cells expressing these siRNA vectors, TDAG51 protein levels were decreased by 75-80%. Furthermore, TDAG51 siRNA expression abolished the ability of IGF-I to rescue cells from serum starvation-induced apoptosis. These findings suggest that TDAG51 plays an important role in the anti-apoptotic effects of IGF-I.

PTEN inhibits cell proliferation and induces apoptosis by downregulating cell surface IGF-IR expression in prostate cancer cells

PTEN is a tumor suppressor gene that is frequently mutated in human tumors. It functions primarily as a lipid phosphatase and plays a key role in the regulation of phosphatidylinositol-3'-kinase. PTEN appears to play a crucial role in modulating apoptosis by reducing the levels of PtdIns(3,4,5)P3, a phospholipid that activates AKT, a central regulator of apoptosis. To understand the role of PTEN in regulating cell proliferation and apoptosis, we stably overexpressed PTEN in PC3 cells, which are prostate cancer cells that lack PTEN. Overexpression of PTEN in two different clones inhibited cell proliferation and increased serum starvation-induced apoptosis, as compared to control cells. Interestingly, PTEN overexpression resulted in a 44-60% reduction in total insulin-like growth factor-I receptor (IGF-IR) protein levels and a 49-64% reduction in cell surface IGF-IR expression. [35S]methionine pulse experiments in PC3 cells overexpressing PTEN demonstrated that these cells synthesize significantly lower levels of the IGF-IR precursor, whereas PTEN overexpression had no effect on IGF-IR degradation. Taken together, our results show that PTEN can regulate cell proliferation and apoptosis through inhibition of IGF-IR synthesis. These results have important implications for understanding the roles of PTEN and the IGF-IR in prostate cancer cell tumorigenesis.

Inhibition of growth hormone action improves insulin sensitivity in liver IGF-1-deficient mice

Liver IGF-1-deficient (LID) mice have a 75% reduction in circulating IGF-1 levels and, as a result, a fourfold increase in growth hormone (GH) secretion. To block GH action, LID mice were crossed with GH antagonist (GHa) transgenic mice. Inactivation of GH action in the resulting LID + GHa mice led to decreased blood glucose and insulin levels and improved peripheral insulin sensitivity. Hyperinsulinemic-euglycemic clamp studies showed that LID mice exhibit severe insulin resistance. In contrast, expression of the GH antagonist transgene in LID + GHa mice led to enhanced insulin sensitivity and increased insulin-stimulated glucose uptake in muscle and white adipose tissue. Interestingly, LID + GHa mice exhibit a twofold increase in white adipose tissue mass, as well as increased levels of serum-free fatty acids and triglycerides, but no increase in the triglyceride content of liver and muscle. In conclusion, these results show that despite low levels of circulating IGF-1, insulin sensitivity in LID mice could be improved by inactivating GH action, suggesting that chronic elevation of GH levels plays a major role in insulin resistance. These results suggest that IGF-1 plays a role in maintaining a fine balance between GH and insulin to promote normal carbohydrate and lipid metabolism.

The new kid on the block(ade) of the IGF-1 receptor

The insulin-like growth factors (IGF) system, and particularly the IGF-1 receptor, has recently become the subject of major interest in the arena of cancer research. Its involvement in cancer cell growth and survival makes the system an excellent target as potential adjunct therapy to standard chemotherapy.

Inhibition of growth hormone action improves insulin sensitivity in liver IGF-1-deficient mice

Liver IGF-1-deficient (LID) mice have a 75% reduction in circulating IGF-1 levels and, as a result, a fourfold increase in growth hormone (GH) secretion. To block GH action, LID mice were crossed with GH antagonist (GHa) transgenic mice. Inactivation of GH action in the resulting LID + GHa mice led to decreased blood glucose and insulin levels and improved peripheral insulin sensitivity. Hyperinsulinemic-euglycemic clamp studies showed that LID mice exhibit severe insulin resistance. In contrast, expression of the GH antagonist transgene in LID + GHa mice led to enhanced insulin sensitivity and increased insulin-stimulated glucose uptake in muscle and white adipose tissue. Interestingly, LID + GHa mice exhibit a twofold increase in white adipose tissue mass, as well as increased levels of serum-free fatty acids and triglycerides, but no increase in the triglyceride content of liver and muscle. In conclusion, these results show that despite low levels of circulating IGF-1, insulin sensitivity in LID mice could be improved by inactivating GH action, suggesting that chronic elevation of GH levels plays a major role in insulin resistance. These results suggest that IGF-1 plays a role in maintaining a fine balance between GH and insulin to promote normal carbohydrate and lipid metabolism.