Increased tumor growth in mice with diet-induced obesity: impact of ovarian hormones.

Obesity increases the risk of many cancers in both males and females. This study describes a link between obesity, obesity-associated metabolic alterations, and the risk of developing cancer in male and female mice. The goal of this study was to evaluate the relationship between gender and obesity and to determine the role of estrogen status in obese females and its effect on tumor growth. We examined the susceptibility of C57BL/6 mice to diet-induced obesity, insulin resistance/glucose intolerance, and tumors. Mice were injected sc with one of two tumorigenic cell lines, Lewis lung carcinoma, or mouse colon 38-adenocarcinoma. Results show that tumor growth rate was increased in obese mice vs. control mice irrespective of the tumor cell type. To investigate the effect of estrogen status on tumor development in obese females, we compared metabolic parameters and tumor growth in ovariectomized (ovx) and intact obese female mice. Obese ovx female mice developed insulin resistance and glucose intolerance similar to that observed in obese males. Our results demonstrate that body adiposity increased in ovx females irrespective of the diet administered and that tumor growth correlated positively with body adiposity. Overall, these data point to more rapid tumor growth in obese mice and suggest that endogenous sex steroids, together with diet, affect adiposity, insulin sensitivity, and tumor growth in female mice.

Metabolic effects of IGF-I deficiency: lessons from mouse models.

Insulin and insulin-like growth factors (IGFs) belong to the most biologically characterized family of peptides involved in metabolism, growth and development. The cellular responses to the IGFs are mediated primarily by the IGF-I receptor. The IGF-I receptor is a member of the family of tyrosine kinase growth factor receptors, and is highly homologous (70%) to the insulin receptor, especially in the tyrosine kinase domain (84%) ADDIN. Upon ligand binding to the extracellular region, the intrinsic tyrosine kinase domain of the receptor is activated. In the past it was believed that insulin activates primarily metabolic processes while IGFs promote cell growth and differentiation. However, in the last two decades many animal models of IGFI deficiency and excess revealed the importance of IGF-I in carbohydrate and lipid metabolism and now it is clear that these peptide hormones together with growth hormone (GH) work in a coordinate and interdependent manner. In the circulation, IGFs are bound in a binary complex with a family of high affinity IGF-binding proteins (IGFBPs) ADDIN. However, most of the circulating IGF-I associates with a high molecular weight complex approximately 150 KDa consisting of IGFBP-3 and the acid labile subunit (ALS) ADDIN. Once the ternary complex dissociates, the binary complexes of IGFBP-IGFs are removed from the circulation and by crossing the endothelium to reach the target tissues and to interact with cell surface receptors. In the present review we will summarize the role of GH and IGF in somatic growth and focus on the metabolic effects of IGF-I deficiency as assessed in various mouse models.

Genetic background (C57BL/6J versus FVB/N) strongly influences the severity of diabetes and insulin resistance in ob/ob mice.

We studied the effects of genetic background on the phenotype of ob/ob mice, a model of severe obesity, insulin resistance, and diabetes caused by leptin deficiency. Despite a comparable degree of obesity and hyperinsulinemia, C57BL/6J ob/ob mice had much milder hyperglycemia and, surprisingly, normal circulating adiponectin levels despite still-prominent signs of insulin resistance. Hyperinsulinemic-euglycemic clamp revealed relatively less whole-body and muscle insulin resistance in C57BL/6J ob/ob mice, whereas liver insulin resistance tended to be more severe than in FVB/N ob/ob mice. C57BL/6J ob/ob mice had also more rapid clearance of circulating triglycerides and more severe hepatic steatosis. We suggest that strain-related distinction in lipid handling is the most important player in the differences in diabetic phenotype and insulin sensitivity, whereas the impact of circulating adiponectin levels on the overall phenotype of ob/ob mice is less important.

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.

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.

Peroxisome proliferator-activated receptor-alpha agonist treatment in a transgenic model of type 2 diabetes reverses the lipotoxic state and improves glucose homeostasis.

Abnormalities in insulin action are the characteristics of type 2 diabetes. Dominant-negative muscle-specific IGF-I receptor (MKR) mice exhibit elevated lipid levels at an early age and eventually develop type 2 diabetes. To evaluate the role of elevated lipids in the progression of the diabetic state, MKR mice were treated with WY14,643, a peroxisome proliferator-activated receptor (PPAR)-alpha agonist. WY14,643 treatment markedly reduced serum fatty acid and triglyceride levels within a few days, as well as muscle triglyceride levels, and subsequently normalized glucose and insulin levels in MKR mice. Hyperinsulinemic-euglycemic clamp analysis showed that WY14,643 treatment enhanced muscle and adipose tissue glucose uptake by improving whole-body insulin sensitivity. Insulin suppression of endogenous glucose production by the liver of MKR mice was also improved. The expression of genes involved in fatty acid oxidation was increased in liver and skeletal muscle, whereas gene expression levels of hepatic gluconeogenic enzymes were decreased in WY14,643-treated MKR mice. WY14,643 treatment also improved the pattern of glucose-stimulated insulin secretion from the perfused pancreata of MKR mice and reduced the beta-cell mass. Taken together, these findings suggest that the reduction in circulating or intracellular lipids by activation of PPAR-alpha improved insulin sensitivity and the diabetic condition of MKR mice.

Reduced expression of insulin-like growth factor I receptors in MCF-7 breast cancer cells leads to a more metastatic phenotype.

Several lines of evidence support an important role for the insulin-like growth factor system in breast cancer. Alterations in insulin-like growth factor I receptor (IGF-IR) have been associated with breast cancer metastasis; however, the specific role played by the IGF-IR in this process remains unclear. To address this issue, we evaluated MCF-7 breast cancer cells stably transfected either with an antisense construct to the IGF-IR, which reduced the expression of the IGF-IRs by approximately 50% (SX13 cells), or with the empty vector as control (NEO cells). Using functional assays for motility, attachment, and aggregation, we found a 3-fold increase in migration using both the wounding assay and the Boyden chamber migration assay. In addition, the SX13 cells attached less, and there was a reduction in cellular aggregation. These functional changes were accompanied by approximately 50% decrease in expression of E-cadherin and approximately 80% increase in p120 protein levels. Moreover, there was a significant reduction in p120 present in the E-cadherin-catenin-p120 complex. There was a 2-fold increase in active Rac1 and Cdc42 and a 35% decrease in active Rho in the SX13 cells. Our findings strongly suggest that the IGF-IR plays a role in the stabilization of the E-cadherin-catenin complex, thereby providing one possible explanation for the association between low levels of IGF-IR and a higher risk of mammary tumor metastasis.

Muscle-specific inactivation of the IGF-I receptor induces compensatory hyperplasia in skeletal muscle.

During the development of skeletal muscle, myoblasts withdraw from the cell cycle and differentiate into myotubes. The insulin-like growth factors IGF-I and IGF-II, through their cognate tyrosine kinase receptor (IGF-I receptor), are known to play a role in this process. After withdrawal of myoblasts from the cell cycle, IGF-I promotes muscle differentiation by inducing the expression or activity of myogenic regulatory factors (MyoD, myogenin) and effectors (p21). However, little is known about the intracellular mechanisms by which the IGF-I system regulates these factors during the process of myogenesis. Here we show that MKR mice, which express a dominant negative IGF-I receptor specifically in skeletal muscle, have marked muscle hypoplasia from birth to 3 weeks of age. This hypoplasia occurs concomitantly with a decrease in ERK immunoreactivity levels and decreases in MyoD and myogenin expression. BrdU immunocytochemistry showed a compensatory hyperplasia as MKR mice grew to adulthood. Interestingly, hyperplasia occurred concomitantly with an increase in p38, MyoD, myogenin, and p21 immunoreactivity levels, as well as a decrease in Twist levels. These findings suggest that regulation of these cellular elements by IGF-I may play a role in the development and differentiation of skeletal muscle in vivo.