Akt Modulates TGF-beta Activities: R&D Systems

TGF-ß, the prototypical member of the TGF-ß superfamily, acts as a potent growth inhibitor or stimulator of apoptosis in many cell types. The downstream signaling initiated by TGF-ß binding to its serine/threonine kinase receptor is carried out by members of the Smad family. In general, activation of Smad2 or Smad3 results in the formation of a heteromeric complex with Smad4 that translocates to the nucleus to regulate gene expression.^1,2 It has previously been shown that the pro-apoptotic activity of TGF-ß can be down-regulated by growth factors or activation of growth factor signaling. For instance, insulin signaling, including activation of its downstream effector kinases PI 3-Kinase (PI3K) and Akt (also known as PKB), can inhibit TGF-ß-mediated apoptosis.^3,4 Evidence has suggested that these effects result from PI3K/Akt-dependent suppression of Smad3.⁵ How does Akt Suppress Smad3? Recent reports may have shed light on this question by demonstrating that, in several human cell lines, Akt can inhibit TGF-ß activities through direct interaction with Smad3 (Figure 1).^6,7

Figure 1. Insulin initiates signaling cascades that include activation of the kinases PI3K and Akt. Akt binds Smad3 suppressing TGF-ß-mediated phosphorylation, nuclear translocation, and apoptosis. [Note: figure adapted from Remy, I. et al. (2004) Nat. Cell Biol. 6:358.]

Both co-immunoprecipitation and protein-complement fragmentation assays show that Smad3 physically binds Akt.^6,7 This association is constitutive, but can be interrupted by treatment with TGF-ß or enhanced by insulin.^6,7 It has also been shown that insulin/IGF treatment, or transfection with Akt, suppresses TGF-ß-mediated Smad3 phophyorylation^5-7. This suggests that Akt/Smad3 interactions underlie Akt inhibitory effects on TGF-ß.^5-7 In futher support, treatment with insulin or IGF can suppress Smad3 nuclear translocation and/or the transcription of several TGF-ß target genes.^5-7 Interestingly, the mechanism by which Akt inhibits Smad3 does not appear to require kinase activity. In vitro, Smad3 does not act as a substrate for Akt, and little phosphorylated Smad3 is localized to Akt/Smad3 complexes.^6,7 In fact, a mutant form of Smad3 (missing key phosphorylation sites) binds Akt as well as wild-type, and transfection with a catalytically inactive Akt mutant is effective in suppressing TGF-ß-mediated apoptosis.^6,7 Therefore, Akt effects appear to be due to the physical binding and sequestration of Smad3 in the cytoplasm, resulting in its inability to carry out TGF-ß-stimulated nuclear signaling.^6,7 Despite what appear to be specific inhibitory actions on Smad3, Akt/Smad3 binding has little effect on Akt activities. Both insulin-stimulated Akt phosphorylation and its kinase activity remain intact in Smad3-transfected cells.⁷

These studies suggest that a direct interaction between Akt and Smad3 lies at the intersection of TGF-ß and growth factor signaling.^5-7 It will be of interest to determine whether this novel form of modulation affects other TGF-ß activities such as anti-inflammatory responses or tumor suppression.^6,7