Smad7 is induced by CD40 and protects WEHI 231 B-lymphocytes from transforming growth factor-beta-induced growth inhibition and apoptosis

Transforming growth factor-beta (TGF-beta) is a potent inducer of apoptosis in B-lymphocytes and is essential for immune regulation and maintenance of self-tolerance, Here we show that concomitant signaling through CD40 susta ins proliferation and rescues the premature B cell line WEHI 231 from both TGF-beta -induced and anti-IgM-induced apoptosis. The anti-apoptotic effect of CD40 is associated with the transcriptional activation of the inhibitor y Smad7 protein. The transactivation of Smad7 by CD40 is NF kappaB-dependen t in that pharmacological inhibitors of this pathway, N-tosyl-L-phenylalani ne chloromethyl ketone and pyrrolidine dithiocarbamate, abrogate CD40-induc ed Smad7 expression. Ectopic overexpression of Smad7 inhibited Smad2 activa tion, TGF-beta -mediated growth inhibition, and apoptosis in WEHI 231 cells . Consistent with this result, dominant negative interference with Smad2 an d Smad3 function also inhibited TGF-beta -induced apoptosis. The inhibitory effects of Smad7 overexpression were specific to TGF-beta -induced apoptos is and were without effect on anti-IgM-induced cell death. These results su ggest a mechanism of suppression of TGF-beta -induced apoptosis by CD40, me diated through activation of NF-kappaB and, consequently, induction of Smad 7 expression.