The Effects of Smad3 on Adrenocorticotropic Hormone-Secreting Pituitary Adenoma Development, Cell Proliferation, Apoptosis, and Hormone Secretion.

Objective: Down-regulation of mothers against decapentaplegic homolog 3 (Smad3) results in the formation of tumors both in vivo and in vitro. However, little is known about the effect of Smad3 on adrenocorticotropic hormone-secreting pituitary adenomas (ACTH-PAs). Our objective was to study the expression and effect of Smad3 in ACTH-PAs and its possible mechanisms.

Methods: Smad3, COOH-terminally phosphorylated mothers against decapentaplegic homolog 3 (pSmad3), and mothers against decapentaplegic homolog 2 proteins (Smad2) were detected in samples from 5 normal anterior pituitaries and 18 ACTH-PAs by Western blot and immunohistochemical analysis. Then, Smad3 expression was up-regulated by Smad3-CMV plasmid or down-regulated by small interfering RNA in ACTH tumor cells (AtT-20) in vitro. Cell proliferation, apoptosis, ACTH level, and pSmad3, B-cell lymphoma/lewkmia-2 (BCL-2), and pro-opiomelanocortin (POMC) protein expression in the AtT-20 cells were measured to investigate the antitumor effects of Smad3.

Results: Reduced expression of Smad3 and pSmad3 but unchanged Smad2 levels were found in ACTH-PAs compared with normal pituitaries. In vitro, the overexpression of Smad3 inhibited cell proliferation, promoted cell apoptosis, and decreased ACTH secretion; in contrast, Smad3 knockdown increased cell proliferation and decreased cell apoptosis but had no significant effect on ACTH secretion. At the same time, overexpression of Smad3 increased pSmad3 but inhibited BCL-2 and POMC protein expression. On the contrary, underexpression of Smad3 inhibited pSmad3 but promoted BCL-2 and POMC protein expression.

Conclusions: Smad3 is underexpressed in ACTH-PAs. Reversing the expression of Smad3 in AtT-20 cells could suppress cell growth, promote tumor apoptosis, and decrease ACTH secretion. Tumor suppression was possibly mediated by the promotion of pSmad3 and the reduction of BCL-2 and POMC expression.