The insulin-like growth factors (IGFs) mediate their effects on cell function through the type 1 IGF receptor (IGFIR) and numerous intracellular signalling molecules including the PI-3K/Akt pathway. The IGFIR also binds to the caveolae protein caveolin-1 but the impact of caveolae on IGF/PI3K/Akt signalling remains controversial. We have examined the effect of complete (knockout) and partial (knockdown) caveolin-1 deficiency on cellular IGF effects mediated via the PI3K/Akt pathway.

Under basal conditions, caveolin-1 deficient (MF^(-/-)) mouse embryonic fibroblast cells incorporated significantly more ³H-thymidine than wildtype (MF^(+/+)) cells, however shRNA-mediated knockdown of caveolin-1 (80% reduction) in 3T3L1 fibroblasts had no affect on basal proliferation. Interestingly, IGF-I induced proliferation was similar in MF^(-/-) and MF^(+/+) cells whilst caveolin-1 knockdown promoted a hyperproliferative response to IGF-I (pkDCav3T3L1(80) 12.4±0.4-fold; pkDShuffle3T3L1 4.3±0.2-fold induction, p<0.01).

Immunoblot analysis showed that caveolin-1 knockdown had no affect on Akt expression or activation. However in MF^(-/-) cells, IGF-I-stimulated phosphorylation of Akt was reduced despite upregulated Akt levels. Further investigation demonstrated that caveolin knockout upregulated Akt-2 and Akt-3 isoform expression but Akt-1 expression was downregulated; interestingly, co-immunoprecipitation studies revealed Akt-1 as the predominant isoform to be phosphorylated in response to IGF-I.

In summary, caveolin-1 deficiency promotes a hyperproliferative response to IGF-I that is unrelated to Akt expression/activation. However, cells which lack caveolin are able to respond appropriately to IGF-1 through compensatory changes in Akt isoform expression. These data posit caveolin-1 as a component of the IGF/PI3K/Akt signalling modulus regulating cellular proliferation with implications for diseases, including cancers, which have altered caveolin expression.