Regulation of TGF-beta-induced apoptosis by the serine/threonine kinase Akt/protein kinase B

Transforming growth factor-beta (TGF-beta) induces both apoptosis and cell cycle arrest in a number of epithelial, lymphoid and myeloid cell lines, but only cell cycle arrest in others. It is not clear how this differential response to TGF-beta is specified. Smad proteins are critical mediators of TGF-beta signaling. Upon stimulation by TGF-beta, Smad2 and Smad3 become phosphorylated by the activated TGF-beta receptor kinases, oligomerize with Smad4 and translocate to the nucleus. The Smad heteromeric complexes then interact with various transcription factors and regulate the expression of TGF-beta target genes. In this dissertation it is reported that the response of cells to TGF-beta is modulated by crosstalk between the Akt/PKB serine/threonine kinase and Smad3 via a mechanism that is independent of the kinase activity of Akt. Akt physically interacts with unphosphorylated Smad3 to sequester it on the cell membrane and in the cytoplasm, preventing its phosphorylation and downstream signaling. This results in a decrease in Smad3-induced activation of transcription and inhibition of the Smad3-mediated apoptotic response to TGF-beta. The inhibition of Smad3 by Akt requires Akt phosphorylation and plasma membrane localization. Since Akt negatively modulates the activity of Smad3, the ratio of Smad3 to Akt expression in a given cell correlates with the sensitivity of the cell to TGF-beta-induced apoptosis. Alteration of this ratio by changing the expression of Akt or Smad3 results in changes in the apoptotic responses of cells to TGF-beta, but has no effect on the ability of these cells to undergo growth arrest. These findings reveal an important determinant of sensitivity to TGF-beta-induced apoptosis involving crosstalk between the TGF-beta and PI3-kinase pathways. This crosstalk may also have broad implications in the understanding of many pathological conditions including cancer and inflammation where upregulation of Akt has been detected.