p42/44 MAP kinase-dependent and -independent signaling pathways regulate caveolin-1 gene expression - Activation of Ras-MAP kinase and protein kinaseA signaling cascades transcriptionally down-regulates caveolin-1 promoter activity

Caveolin-1 is a principal component of caveolae membranes in vivo. Caveolin -1 mRNA and protein expression are down-regulated in NIH 3T3 cells in respo nse to transformation by activated oncogenes, such as H-Ras(G12V) and v-Abl , The mechanisms governing this down-regulation event remain unknown. Here, we show that caveolin-1 gene expression is directly regulated by activatio n of the Ras-p42/44 MAP kinase cascade, Down regulation of caveolin-1 prote in expression by Ras is independent of (i) the type of activating mutation (G12V versus Q61L) and (ii) the form of activated Ras transfected (H-Ras ve rsus H-Ras versus N-Ras), Treatment of Ras or Raf-transformed MH 3T3 cells with a well characterized MEK inhibitor (PD 98059) restores caveolin-1 prot ein expression. In contrast, treatment of v-Src and v-Abl transformed NIH 3 T3 cells with PD 98059 does not restore caveolin-1 expression. Thus, there must be at least two pathways for down-regulating caveolin-1 expression: on e that is p42/44 MAP kinase-dependent and another that is p42/44 MAP kinase -independent. We focused our efforts on the p42/44 MAP kinase-dependent pat hway. The activity of a panel of caveolin-1 promoter constructs was evaluat ed using transient expression in H-Ras(G12V) transformed MR 3T3 cells. We s how that caveolin-1 promoter activity is up-regulated similar to 5-fold by inhibition of the p42/44 MAP kinase cascade. Using electrophoretic mobility shift assays we provide evidence that the caveolin-1 promoter (from -156 t o -561) is differentially bound by transcription factors in normal and H-Ra s(G12V)-transformed cells. We also show that activation of protein kinase A (PKA) signaling is sufficient to down-regulate caveolin-1 protein expressi on and promoter activity. Thus, we have identified two signaling pathways ( Ras-p42/44MAP kinase and PKA) that transcriptionally down-regulate caveolin -1 gene expression.