Adiponectin, a peptide hormone mainly produced by adipocytes, is now widely recognized as an insulin sensitizer that possesses anti-diabetic, anti-inflammatory and cardioprotective properties. However, the molecular mechanisms by which adiponectin sensitizes insulin signaling and action remain largely unknown. We have recently identified a pleckstrin homology (PH) and phosphotyrosine binding (PTB) domain- containing protein, APPL1 that interacts directly with the adiponectin receptors AdipoR1 and AdipoR2. Suppression of APPL1 by RNAi not only inhibits adiponectin signaling but also insulin-stimulated Akt phosphorylation, suggesting that APPL1 may play an essential role in the crosstalk between the adiponectin and insulin signaling pathways (Mao et al, 2006, Nat. Cell Biol, 8, 516-523). Consistent with this view, our preliminary data have shown that APPL1 interacts directly with the insulin receptor and more interestingly, this interaction is enhanced by adiponectin stimulation. In addition, we have found that APPL1 undergoes adiponectin-stimulated phosphorylation at Ser430;this suggests a potential mechanism by which adiponectin regulates the interaction between APPL1 and the insulin receptor. Furthermore, we have found that APPL2, an APPL1 isoform which shares 54% identity in protein sequence with APPL1, dimerizes with APPL1 and inhibits adiponectin signaling when overexpressed in cells. Based on these novel findings, we hypothesize that the interaction between APPL ispforms and signaling molecules in the adiponectin and insulin signaling pathways may play a key role in the crosstalk between these two important signaling pathways. To test this hypothesis, we will: 1) Characterize the interaction between APPL1 and signaling molecules in the insulin receptor (IR) signaling pathway and the roles of the interaction in insulin signaling;2) Elucidate the roles of APPL2 in regulating adiponectin and insulin signaling and function;and 3) Determine whether APPL1 plays a role in insulin signaling and the insulin sensitizing effect of adiponectin in vivo.
We believe that results from the proposed study will provide critical information on the molecular mechanism underlying the insulin sensitizing effect of adiponectin, which should have significant implications in the development of new therapeutic drugs for the treatment of obesity, insulin resistance, and type 2 diabetes.
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