Down-regulation of Insulin Receptor Substrate 1 during Hyperglycemia Induces Vascular Smooth Muscle Cell Dedifferentiation

Abstract

Edited by Jeffrey E. PessinDiabetes is a major risk factor for the development of atherosclerosis, but the mechanism by which hyperglycemia accelerates lesion development is not well defined. Insulin and insulinlike growth factor I (IGF-I) signal through the scaffold protein insulin receptor substrate 1 (IRS-1). In diabetes, IRS-1 is downregulated, and cells become resistant to insulin. Under these conditions, the IGF-I receptor signals through an alternate scaffold protein, SHPS-1, resulting in pathophysiologic stimulation of vascular smooth muscle cell (VSMC) migration and proliferation. These studies were undertaken to determine whether IRS-1 is functioning constitutively to maintain VSMCs in their differentiated state and, thereby, inhibit aberrant signaling. Here we show that deletion of IRS-1 expression in VSMCs in nondiabetic mice results in dedifferentiation, SHPS-1 activation, and aberrant signaling and that these changes parallel those that occur in response to hyperglycemia. The mice showed enhanced sensitivity to IGF-I stimulation of VSMC proliferation and a hyperproliferative response to vascular injury. KLF4, a transcription factor that induces VSMC dedifferentiation, was up-regulated in IRS-1 ؊/؊ mice, and the differentiation inducer myocardin was undetectable. Importantly, these changes were replicated in wild-type mice during hyperglycemia. These findings illuminate a new function of IRS-1: that of maintaining cells in their normal, differentiatedstate.BecauseIRS-1isdown-regulatedinstatesofinsulinresistance that occur in response to metabolic stresses such as obesity and cytokine stimulation, the findings provide a mechanism for understanding how patients with metabolic stress and/or diabetes are predisposed to developing vascular complications.Diabetes is a major predisposing factor for the development of atherosclerosis. The diabetes control complications trial showed that patients with type 1 diabetes who maintained a hemoglobin A-1 C value 1.2% lower than control subjects for 7 years had a significantly reduced rate of vascular disease events during the 20-year follow-up period (1). Similarly the United Kingdom prospective trial demonstrated a significant benefit of lowering glucose in type 2 diabetics on cardiovascular risk (2). Despite intense analysis, the molecular mechanism by which glucose lowering results in a clinical benefit has remained poorly defined. Insulin and insulin-like growth factor I (IGF-I) 2 signal through a scaffold protein termed insulin receptor substrate 1 (IRS-1) (3). The insulin and IGF-I receptor tyrosine kinases directly phosphorylate IRS-1, and these phosphotyrosines recruit the p85 subunit of PI3K and Grb-2, thereby activating the PI3K and MAPK pathways (4). In differentiated cells, these IRS-1-linked signaling cascades promote glucose influx; glycogen, lipid, and protein synthesis; as well as changes in gene expression (5). Under normal physiologic conditions, both insulin and IGF-I stimulate differentiated cell functions through IRS-1 activation (6 -7). In re...

Hyperglycemia induces vascular smooth muscle cell dedifferentiation by suppressing insulin receptor substrate-1–mediated p53/KLF4 complex stabilization

“…Previous work shows that VSMC dedifferentiation is associated with enhanced expression of the transcription factor KLF4 (11) and that enhanced KLF4 expression, which occurs in response to cytokine-induced stress, suppresses transcription of the primary determinant of VSMC differentiation, myocardin (11,12). We demonstrated that KLF4 expression increased in arteries of diabetic or normoglycemic IRS-1 Ϫ/Ϫ mice, and myocardin was suppressed (10). Although the results showed that IRS-1 was required for myocardin expression and differentiation, they did not delineate the mechanism by which maintenance of IRS-1 expression enhances differentiation and inhibits dedifferentiation.…”

“…This reduction is due to enhanced IRS-1 serine phosphorylation and increased ubiquitination that targets it to a proteasome (22). Recently, we reported that down-regulation of IRS-1 in the arteries of diabetic mice was accompanied by the enhanced expression of KLF4, a transcription factor that enhances VSMC dedifferentiation under certain conditions (10). KLF4 suppressed expression of myocardin, a transcription factor that is required to maintain VSMC in the differentiated state.…”

“…essary for normal contractile function (19). Hyperglycemia and insulin resistance predispose to increased VSMC dedifferentiation and atherosclerotic lesion development (10,15,20). Therefore, identification of the factors that regulate the ability of VSMC to maintain the differentiated state as well as those that lead to dedifferentiation will further our understanding of how changes in metabolism lead to arterial dysfunction and atherosclerosis.…”

“…Therefore, identification of the factors that regulate the ability of VSMC to maintain the differentiated state as well as those that lead to dedifferentiation will further our understanding of how changes in metabolism lead to arterial dysfunction and atherosclerosis. Both hyperglycemia and increased insulin resistance down-regulate IRS-1 in VSMCs and in blood vessels of diabetic animals (10,21). This reduction is due to enhanced IRS-1 serine phosphorylation and increased ubiquitination that targets it to a proteasome (22).…”

“…This signaling switch occurs in vivo in VSMCs of diabetic mice in response to hyperglycemia. This change is accompanied by VSMC dedifferentiation (10) and an enhanced cellular proliferative response to injury (10). To determine whether loss of IRS-1 expression in response to hyperglycemia was mediating these changes, we deleted IRS-1 in VSMC in mice.…”