Modulation of caveolin-1 expression can effect signalling through the PI-3K/Akt pathway and cellular
June 29th, 2008 | by admin |Modulation of caveolin-1 expression can effect signalling through the PI-3K/Akt pathway and cellular proliferation in response to IGF-I.
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 (3)H-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.
Matthews LC, Taggart MJ, Westwood M.
Endocrine Sciences, Maternal and Fetal Health Research Group, University of Manchester, Manchester, M13 9PT, UK.