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.
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.
Matthews
Laura C
LC
Maternal and Fetal Health Research Group, University of Manchester, St. Mary's Hospital, Hathersage Road, Manchester M13 0JH, United Kingdom.
Taggart
Michael J
MJ
Westwood
Melissa
M
D041721:3T3-L1 Cells; D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics; D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000502:physiology; D051242:Caveolin 1 / Q000235:genetics / Q000378:metabolism*; D002455:Cell Division / Q000187:drug effects / Q000502:physiology; D002470:Cell Survival / Q000187:drug effects / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D005347:Fibroblasts / Q000166:cytology / Q000378:metabolism; D015870:Gene Expression / Q000502:physiology; D055504:Insulin Receptor Substrate Proteins; D007334:Insulin-Like Growth Factor I / Q000378:metabolism* / Q000494:pharmacology; D051379:Mice; D018345:Mice, Knockout; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt / Q000235:genetics / Q000378:metabolism*; D034741:RNA, Small Interfering; D017526:Receptor, IGF Type 1 / Q000235:genetics; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D015854:Up-Regulation / Q000187:drug effects / Q000502:physiology
D048868:Adaptor Proteins, Signal Transducing; C495541:Cav1 protein, mouse; D051242:Caveolin 1; D055504:Insulin Receptor Substrate Proteins; C526295:Irs1 protein, mouse; D034741:RNA, Small Interfering; D007334:Insulin-Like Growth Factor I; D019869:Phosphatidylinositol 3-Kinases; D017526:Receptor, IGF Type 1; D051057:Proto-Oncogene Proteins c-akt