Imidazoline receptors in insulin signaling and metabolic regulation
The I1-imidazoline receptor is a novel target of drug development for hypertension and insulin resistance. This thesis focused on the molecular basis for I1-imidazoline binding and cell signaling and the mechanisms linking this signaling protein to regulation glucose metabolism. IRAS is a gene candidate for the I1-imidazoline receptor. To investigate the possibility that IRAS is the I1-imidazoline receptor, antisense oligo-nucleotides directly against the initiation site of IRAS sequence were designed and transfected into PC12 cells. Antisense transfection for 48h reduced specific imidazoline radioligand binding to plasma membrane fractions by about 50%, with parallel drops in IRAS protein expression as detected by Western blot. Furthermore, transfection with antisense caused functional impairment of I1-imidazoline receptor signaling. Imidazoline agonist induced ERK1/2 activation was significantly inhibited with antisense transfection without affecting basal ERK level or ERK activation by growth factors. These findings strongly suggested that IRAS encodes an I1-imidazoline receptor or at least an important subunit of it. The mechanism of insulin sensitizing effect from imidazolines was studied in the SHROB rat, an animal model for human metabolic syndrome. Insulin induced Akt activation was found to be severely impaired in isolated adipocytes from SHROB compared to their lean SHR littermates. In addition, insulin induced glucose uptake in these cells from SHROB were also similarly resistant to stimulation by insulin. Chronic treatment of SHROB with the imidazoline agonist moxonidine partially restored both Akt activation and glucose uptake stimulated by insulin in isolated abdominal adipocytes without affecting basal Akt activation level. However, acute in vitro moxonidine administration did not yield similar effects, nor did moxonidine affect basal Akt level in adipocytes from either SHROB or SHR. These results implicate adipose tissue as a locus of insulin resistance in this model of metabolic syndrome, and impairment of insulin signaling through Akt may contribute to this defect. Chronic oral treatment with I1-imidazoline receptor agonists such as moxonidine significantly normalizes insulin resistance in adipose tissue of SHROB in both insulin cell signaling and glucose metabolism. These changes in adipose tissue very likely contribute to the overall insulin sensitizing effect of imidazoline ligands on SHROB.