Regulation of vimentin expression and protease-mediated vimentin degradation during differentiation of human monocytic leukemia cells.

Terminal differentiation of human monocytic leukemia THP-1 cells is induced in vitro by 12-O-tetradecanoylphorbol-13-acetate (TPA). We investigated the effects of TPA on the expression of vimentin during the differentiation of THP-1 cells at both the mRNA and the protein level. On northern blotting analysis, a 2.1 kb vimentin mRNA was up-regulated by TPA. On western blotting, small vimentin molecules with a molecular mass of approximately 40 kDa were observed in the soluble fraction and increased with TPA-induction of cellular differentiation. Since larger, including intact, vimentin molecules were detectable at a high TPA dose, we assessed the possible existence of protease activity directed against vimentin in THP-1 cells. With incubation of the cellular lysates of THP-1 cells, the endogenous vimentin became increasingly smaller over time, suggesting the presence of a vimentin-degrading protease. Phenylmethylsulfonyl fluoride inhibited this apparent protease activity against vimentin, suggesting the enzyme involved to be a serine protease. Interestingly, the protease activity was down-regulated by TPA treatment. TPA-treated THP-1 cells were found to express a vimentin-filament network based on immunocytochemical analysis using an anti-vimentin monoclonal antibody, V9. Taken together, these observations suggest that post-translational mechanisms work in cooperation with transcriptional regulation to maintain the vimentin-intermediate filament structure in differentiated THP-1 cells.