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Smad and p38-MAPK signaling mediates apoptotic effects of transforming growth factor-1 in human airway epithelial cells

¹Section of Pulmonary and Critical Care Medicine, University of Chicago, Chicago, Illinois 60637; and ²McDonald Research Laboratories and the iCAPTURE Centre, University of British Columbia, Vancouver, British Columbia, Canada V6Z 1Y6

Submitted 11 February 2004 ; accepted in final form 4 May 2004

Transforming growth factor-1 (TGF-1) belongs to a family of multifunctional cytokines that regulate a variety of biological processes, including cell differentiation, proliferation, and apoptosis. The effects of TGF-1 are cell context and cell cycle specific and may be signaled through several pathways. We examined the effect of TGF-1 on apoptosis of primary human central airway epithelial cells and cell lines. TGF-1 protected human airway epithelial cells from apoptosis induced by either activation of the Fas death receptor (CD95) or by corticosteroids. This protective effect was blocked by inhibition of the Smad pathway via overexpression of inhibitory Smad7. The protective effect is associated with an increase in the cyclin-dependent kinase inhibitor p21 and was blocked by the overexpression of key gatekeeper cyclins for the G₁/S interface, cyclins D1 and E. Blockade of the Smad pathway by overexpression of the inhibitory Smad7 permitted demonstration of a TGF--mediated proapoptotic pathway. This proapoptotic effect was blocked by inhibition of the p38 MAPK kinase signaling with the inhibitor SB-203580 and was associated with an increase in p38 activity as measured by a kinase assay. Here we demonstrate dual signaling pathways involving TGF-1, an antiapoptotic pathway mediated by the Smad pathway involving p21, and an apoptosis-permissive pathway mediated in part by p38 MAPK.

cyclin-dependent kinase; cyclin-dependent kinase inhibitor; extracellular regulated kinase; transforming growth factor- receptor; airway epithelium; apoptosis

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