Overexpression of caveolin-1 in adult T-cell leukemia.

Caveolin-1 is implicated in the regulation of signal pathways. Adult T-cell leukemia (ATL) is a T-cell malignancy causatively associated with human T-cell leukemia virus type 1 (HTLV-1). To determine the role of caveolin-1 in leukemogenesis, we examined caveolin-1 expression levels in HTLV-1-infected T-cell lines and ATL cells. These cells expressed high levels of caveolin-1 compared with uninfected T-cell lines and normal peripheral blood mononuclear cells (PBMCs). Caveolin-1-positive ATL cells were detected in ATL lymph nodes and skin lesions, and caveolin-1 was also detected in the plasma of patients with ATL. Infection of a human T-cell line, an epithelial cell line, and normal PBMCs with HTLV-1 induced caveolin-1 expression. The viral protein Tax transcriptionally activated caveolin-1 gene through nuclear factor-kappaB and cAMP response element binding protein signal pathways. HTLV-1-infected T-cell lines, and ATL cells are known to be resistant to transforming growth factor beta (TGF-beta)-induced growth inhibition. Caveolin-1 was colocalized with TGF-beta type I receptor in HTLV-1-infected T-cell lines and suppressed TGF-beta signaling. Caveolin-1 knockdown in an HTLV-1-infected T-cell line exhibited susceptibility to TGF-beta. Thus, we describe a new function for Tax, repression of TGF-beta signaling through caveolin-1 expression, which may play a critical role in ATL leukemogenesis.