ANGIOTENSIN II MEDIATED REGULATION OF SIGNAL TRANSDUCTION IN METABOLIC SYNDROME AND CANCER
Patients suffering from hypertension often develop non-Insulin dependent diabetes mellitus (NIDDM), a condition caused by Insulin resistance. Though these patients have normal Insulin receptor (IR) and high levels of Insulin in blood, they fail to have IR-mediated signaling essential for glucose uptake and availability. NIDDM usually begins as insulin resistance, a condition in which Insulin Receptor (IR)-mediated signaling that leads to glucose uptake and glucose availability to cells is inhibited even in the presence of high levels of Insulin in blood. Mechanisms for the development of this Insulin resistance in patients suffering from hypertension are unclear. Angiotensin II (Ang II) hormone has been implicated in the pathogenesis of insulin resistance and inhibitors of Ang II receptor AT1 are shown to improve insulin sensitivity. Here we show that in the skeletal muscle tissue of SHR rats, Insulin Receptor (IR) beta- subunit forms a complex with the AT1 receptor and co-immunoprecipitates with IR-beta. Such strong AT1-IR association was not observed in normo-tensive rat tissue. To our knowledge this is the first report that shows AT1 can associate with IR-beta in mammalian tissue and that such association might play a role in the regulation of signaling by IR-beta. We further demonstrate that a 2-hour pre-incubation with Ang II (at concentrations 50pM to 1?ÝM) significantly inhibits 125I-insulin binding to IR in human cell line MCF-7. This effect was not seen when Ang II exposure was performed for shorter periods. The two-hour exposure to Ang II also led to the formation of a protein complex containing AT1 and IR-beta, similar to that seen in skeletal muscle tissue of SHR rats. Both AT1-IR association and differential tyrosine phosphorylation of IR-beta and associated proteins were inhibited when the cells were pre-treated with the AT1 antagonist losartan. These observations suggest that continuous presence of Ang II would result in sequestering IR in the AT1-IR complex and prevent IR from binding insulin. It also coincided with differential tyrosine-phosphorylation of IR beta-subunit and associated proteins, than that generated when IR was activated by insulin. Therefore, we infer that conformational alterations in IR caused by AT1-IR-beta association underlie the development of Ang II-induced insulin resistance. Based on these data we propose a model for AT1-mediated insulin resistance that involves receptor level interaction between the AT1 and the IR. Therefore, Insulin-independent, Ang II-induced tyrosine phosphorylation of IR prevents IR from binding Insulin and contributes to Insulin resistance. The observation that drugs that inhibit Angiotensin II converting enzyme (ACE), or activation of AT1 receptor, not only reduce hypertension, but also induce insulin sensitivity further supports the role for Ang II and AT1 in the development of NIDDM.
Advisor:Dr. Randal Buddington; Dr. Narasaiah Gavini; Dr J. P. Singh; Dr.Donald Downer; Dr. Lakshmi Pulakat
School:Mississippi State University
School Location:USA - Mississippi
Source Type:Master's Thesis
Date of Publication:12/01/2006