Endocrine Abstracts

The insulin-like growth factors (IGFs) signal through the type 1 IGF receptor (IGFIR) to regulate cellular proliferation and survival. A current theory suggests that a subset of plasmalemmal lipid rafts, termed caveolae, may orchestrate such IGF-mediated signalling.

In order to investigate the role of caveolae and the marker protein caveolin in controlling IGF signals, two cell models of caveolin deficiency were generated. Using shRNA, caveolin-1 expression by 3T3L1 cells was downregulated (80% reduction by immunoblot) and clonal fibroblast cell lines were established from wildtype and caveolin-1 knockout mice. In these cell systems, IGF-dependent changes in proliferation, survival and activation of intracellular signalling proteins were assessed by thymidine incorporation, chromatin fragmentation and phospho-immunoblot respectively. Neither the downregulation nor knockout of caveolin-1 significantly altered the ability of 5 nM IGF-I to promote cellular survival. However, in vitro downregulation of caveolin-1 promoted a hyperproliferative response to 5 nM IGF-I (control3T3L1 4.3±0.2-fold; shRNA3T3L1 12.4±0.4-fold induction, n=4, P<0.01), despite no change in the expression or activation of IGFIR, IRS-1, Akt or p42/44 MAPK. In contrast, in vivo knockout of caveolin resulted in clonal fibroblasts with unaltered rates of IGF-dependent proliferation, even though the expression and activation of IGFIR, IRS-1 and Akt were clearly altered. Immunoprecipitation studies further demonstrated that in caveolin-containing cells, IGF interacts with the Akt1 isoform of Akt; expression of this isoform was reduced in the caveolin-1 knockout cells, but not the downregulated, cells. In summary, short-term downregulation of caveolin promotes a hyperproliferative response to IGF; however, cells with long-term knockout of caveolin revert to appropriate proliferative responses through compensatory changes in the expression of Akt isoforms. This data posits an IGFIR/caveolin/Akt pathway as an important component of the IGF signalling modulus regulating cellular proliferation with implications for diseases, such as cancers, in which caveolin expression is abnormal.