Distinct roles of transforming growth factor-β signaling and transforming growth factor-β receptor inhibitor SB431542 in the regulation of p21 expression.

Transforming growth factor-β (TGF-β) has both tumor suppressive and oncogenic activities. Autocrine TGF-β signaling supports tumor survival and growth in certain types of cancer, and the TGF-β signaling pathway is a potential therapeutic target for these types of cancer. TGF-β induces p21 expression, and p21 is considered as an oncogene as well as a tumor suppressor, due to its anti-apoptotic activity. Thus, we hypothesized that autocrine TGF-β signaling maintains the expression of p21 at levels that can support cell growth. To verify this hypothesis, we sought to examine p21 expression and cell growth in various cancer cells following the inhibition of autocrine TGF-β signaling using siRNAs targeting TGF-β signaling components and SB431542, a TGF-β receptor inhibitor. Results from the present study show that p21 expression and cell growth were reduced by knockdown of TGF-β signaling components using siRNA in MDA-MB231 and A549 cells. Cell growth was also reduced in p21 siRNA-transfected cells. Downregulation of p21 expression induced cellular senescence in MDA-MB231 cells but did not induce apoptosis in both cells. These data suggest that autocrine TGF-β signaling is required to sustain p21 levels for positive regulation of cell cycle. On the other hand, treatment with SB431542 up-regulated p21 expression while inhibiting cell growth. The TGF-β signaling pathway was not associated with the SB431542-mediated induction of p21 expression. Specificity protein 1 (Sp1) was downregulated by treatment with SB431542, and p21 expression was increased by Sp1 knockdown. These findings suggest that downregulation of Sp1 expression is responsible for SB43154-induced p21 expression.