In vascular endothelial cells, specialized microdomains of plasma membrane termed caveolae modulate various receptor signal transduction pathways regulated by caveolin-1, a resident protein of caveolae. We examined whether transforming growth factor-beta1 (TGF-beta1), a multifunctional cytokine, alters expression levels of caveolin-1 and influences heterologous receptor signaling. Treatment of cultured bovine aortic endothelial cells (BAEC) with TGF-beta1 induces marked decreases in caveolin-1 expression in a time- and dose-dependent fashion at both levels of protein and mRNA. A pharmacological inhibitor of activin receptor-like kinase 5 (ALK-5) counteracts caveolin-1 downregulation by TGF-beta1, indicating the involvement of ALK-5 receptor subtype for TGF-beta1. Sphingosine 1-phosphate (S1P) is a serum-borne angiogenic lipid growth factor that exerts a wide variety of biological actions. S1P modulates G protein-coupled S1P receptors, activating downstream molecules kinases AMP-activated protein kinase (AMPK), and Akt as well as a small G protein Rac1, ultimately to promote migration. Because S1P receptor signaling is associated with caveolae/caveolin-1, we examined whether pretreatment with TGF-beta1 enhances effects of S1P on BAEC. Whereas S1P alone evokes robust BAEC responses to S1P, pretreatment with TGF-beta1 leads to even higher magnitudes of S1P-elicited signaling responses and cell migration. Conversely, genetic knockdown of caveolin-1 using small interfering RNA mimics TGF-beta1-induced promotion of BAEC responses to S1P. Collectively, these data demonstrate that TGF-beta1 downregulates caveolin-1 of cultured endothelial cells, involving ALK-5 receptor subtype. Because downregulation of caveolin-1 by TGF-beta1 promotes subsequent heterologous receptor signaling by S1P, these results may also identify novel point of cross-talk between cytokines and sphingolipids within endothelial signal transduction machineries.
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