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Modulation of Caveolin-1 Expression Can Affect Signalling through the Phosphatidylinositol 3-Kinase/Akt Pathway and Cellular Proliferation in Response to Insulin-Like Growth Factor I

Endocrine Sciences (L.C.M.), Maternal and Fetal Health Research Group (M.J.T., M.W.), University of Manchester, Manchester M13 9PT, United Kingdom

Address all correspondence and requests for reprints to: Dr. Melissa Westwood, Maternal and Fetal Health Research Group, University of Manchester, St. Mary’s Hospital, Hathersage Road, Manchester M13 0JH, United Kingdom. E-mail: melissa.westwood{at}manchester.ac.uk.

The IGFs mediate their effects on cell function through the type I IGF receptor and numerous intracellular signalling molecules, including the phosphatidylinositol 3-kinase (PI-3K)/Akt pathway. The type I IGF receptor also binds to the caveolae protein caveolin-1, but the impact of caveolae on IGF/PI-3K/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 PI-3K/Akt pathway. Under basal conditions, caveolin-1-deficient mouse embryonic fibroblast cells [MF^(–/–)] incorporated significantly more [³H]thymidine than wild-type mouse embryonic fibroblast cells [MF^(+/+)]; however, small hairpin RNA-mediated knockdown of caveolin-1 (80% reduction) in 3T3L1 fibroblasts had no effect on basal proliferation. Interestingly, IGF-I induced proliferation was similar in MF^(–/–) and MF^(+/+) cells, whereas 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 up-regulated Akt levels. Further investigation demonstrated that caveolin knockout up-regulated Akt-2 and Akt-3 isoform expression, but Akt-1 expression was down-regulated; interestingly, coimmunoprecipitation 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 that lack caveolin are able to respond appropriately to IGF-I through compensatory changes in Akt isoform expression. These data posit caveolin-1 as a component of the IGF/PI-3K/Akt signalling modulus regulating cellular proliferation with implications for diseases, including cancers, which have altered caveolin expression.