The overexpression of insulin receptor makes CHO cells resistant to the action of IGF-1: role of IRS-1

Emerging role for RNA binding motif protein 4 in the development of brown adipocytes

RNA-binding motif protein 4 (RBM4) reportedly reprograms the tissue-specific splicing network which modulates the development of muscles and pancreatic β-islets. Herein, we report that Rbm4a(-/-) mice exhibited hyperlipidemia accompanied with reduced mass of interscapular brown adipose tissue (iBAT). Elevated RBM4a led to the isoform shift of IR, Ppar-γ, and Pref-1 genes which play pivotal roles in the different stages of adipogenesis. Overexpression of RBM4a enhanced the mitochondrial activity of brown adipocyte-like lineage in the presence of uncoupling agent. RBM4a-ablated adipocytes inversely exhibited impaired development and inefficient energy expenditure. Intriguingly, overexpressed RBM4a induced the expression of brown adipocyte-specific factors (Prdm16 and Bmp7) in white adipocyte-like lineage, which suggested the potential action of RBM4a on the white-to-brown trans-differentiation of adipocytes. In differentiating adipocytes, RBM4a constituted a feed-forward circuit through autoregulating the splicing pattern of its own transcript. Based on these results, we propose the emerging role of RBM4 in regulating the adipocyte-specific splicing events and transcription cascade, which subsequently facilitate the development and function of brown adipocyte-like cells.

Insulin receptor isoforms and insulin receptor/insulin-like growth factor receptor hybrids in physiology and disease

In mammals, the insulin receptor (IR) gene has acquired an additional exon, exon 11. This exon may be skipped in a developmental and tissue-specific manner. The IR, therefore, occurs in two isoforms (exon 11 minus IR-A and exon 11 plus IR-B). The most relevant functional difference between these two isoforms is the high affinity of IR-A for IGF-II. IR-A is predominantly expressed during prenatal life. It enhances the effects of IGF-II during embryogenesis and fetal development. It is also significantly expressed in adult tissues, especially in the brain. Conversely, IR-B is predominantly expressed in adult, well-differentiated tissues, including the liver, where it enhances the metabolic effects of insulin. Dysregulation of IR splicing in insulin target tissues may occur in patients with insulin resistance; however, its role in type 2 diabetes is unclear. IR-A is often aberrantly expressed in cancer cells, thus increasing their responsiveness to IGF-II and to insulin and explaining the cancer-promoting effect of hyperinsulinemia observed in obese and type 2 diabetic patients. Aberrant IR-A expression may favor cancer resistance to both conventional and targeted therapies by a variety of mechanisms. Finally, IR isoforms form heterodimers, IR-A/IR-B, and hybrid IR/IGF-IR receptors (HR-A and HR-B). The functional characteristics of such hybrid receptors and their role in physiology, in diabetes, and in malignant cells are not yet fully understood. These receptors seem to enhance cell responsiveness to IGFs.

Bi-directional regulation of brown fat adipogenesis by the insulin receptor

Insulin is a potent inducer of adipogenesis, and differentiation of adipocytes requires many components of the insulin signaling pathway, including the insulin receptor substrate IRS-1 and phosphatidylinositol 3-kinase (PI3K). Brown pre-adipocytes in culture exhibit low levels of insulin receptor (IR), and during differentiation there is both an increase in total IR levels and a shift in the alternatively spliced forms of IR from the A isoform (-exon 11) to the B isoform (+exon 11). Brown pre-adipocyte cell lines from insulin receptor-deficient mice exhibit dramatically impaired differentiation and an inability to regulate alternative splicing of the insulin receptor. Surprisingly, re-expression of either splice isoform of IR in the IR-deficient cells fails to rescue differentiation in these cells. Likewise, overexpression of IR in control IRlox cells also results in inhibition of differentiation and a failure to accumulate expression of the adipogenic markers peroxisome proliferator-activated receptor gamma, Glut4, and fatty acid synthase, although cells overexpressing IR retain the ability to activate PI3K and down-regulate mitogen-activated protein kinase (MAPK) phosphorylation. Thus, differentiation of brown adipocytes requires a timed and regulated expression of IR, and either the absence or overabundance of insulin receptors in these cells dramatically inhibits differentiation.

Insulin-induced egr-1 expression in Chinese hamster ovary cells is insulin receptor and insulin receptor substrate-1 phosphorylation-independent. Evidence of an alternative signal transduction pathway

Insulin's effects primarily are initiated by insulin binding to its plasma membrane receptor and the sequential tyrosine phosphorylation of the insulin receptor and intracellular substrates, such as insulin receptor substrate-1 (IRS-1). However, studies suggest some insulin effects, including those at the nucleus, may not be regulated by this pathway. The present study compared the levels of insulin binding, insulin receptor and IRS-1 tyrosine phosphorylation, and phosphatidylinositol 3'-kinase activity to immediate early gene c-fos and egr-1 mRNA expression in Chinese hamster ovary (CHO) cells expressing only neomycin-resistant plasmid (CHONEO), overexpressing wild type human insulin receptor (CHOHIRc) or ATP binding site-mutated insulin receptors (CHOA1018K). Insulin binding in CHONEO cells was markedly lower than that in other cell types. 10 nM insulin significantly increased tyrosine phosphorylation of insulin receptor and IRS-1 in CHOHIRc cells. Phosphorylation of insulin receptor and IRS-1 in CHONEO and CHOA1018K cells was not detected in the presence or absence of insulin. Similarly, insulin increased phosphatidylinositol 3-kinase activity only in CHOHIRc cells. As determined by Northern blot, nuclear run-on analysis, and in situ hybridization, insulin induced c-fos mRNA expression, through transcription, in CHOHIRc cells but not in CHONEO and CHOA1018K cells, consistent with previous reports. In contrast, all three cell types showed a similar insulin dose-dependent increase of egr-1 mRNA expression through transcription. These data indicated that insulin-induced egr-1 mRNA expression did not correlate with the levels of insulin binding to insulin receptor or phosphorylation of insulin receptor and IRS-1. These results suggest that different mechanisms are involved in induction of c-fos and egr-1 mRNA expression by insulin, the former by the more classic insulin receptor tyrosine kinase pathway and the latter by a yet to be determined alternative signal transduction pathway.