No abstract

The mechanisms controlling degradation of cytosolic β-catenin are important for regulating β-catenin co-transcriptional activity. Loss of von Hippel-Lindau protein (pVHL) has been shown to stabilize β-catenin, increasing β-catenin transactivation and β-catenin-mediated cell proliferation. However, the role of phosphoinositide 3-kinase (PI3K)/Akt in the regulation of β-catenin signaling downstream from pVHL has never been addressed. Here, we report that hyperactivation of PI3K/Akt in cells lacking pVHL contributes to the stabilization and nuclear accumulation of active β-catenin. PI3K/Akt hyperactivation is facilitated by the up-regulation of 14-3-3ζ and the down-regulation of 14-3-3ε, 14-3-3η and 14-3-3θ. Up-regulation of 14-3-3ζ in response to pVHL is important for the recruitment of PI3K to the cell membrane and for stabilization of soluble β-catenin. In contrast, 14-3-3ε and 14-3-3η enhanced PI3K/Akt signaling by inhibiting PI3K and PDK1, respectively. Thus, our results demonstrated that 14-3-3 family members enhance PI3K/Akt/β-catenin signaling in order to increase proliferation. Inhibition of Akt activation and/or 14-3-3 function strongly reduces β-catenin signaling and decreases cell proliferation. Thus, inhibition of Akt and 14-3-3 function efficiently reduces cell proliferation in 786-0 cells characterized by hyperactivation of β-catenin signaling due to pVHL loss.

Akt activation has been associated with proliferation, differentiation and survival in epithelial cells. Phosphorylation of Thr308 of Akt by PDK1 is critical for optimal activation of its kinase activity. Despite the relevance of the phosphorylation of Akt at Threonine 308, the mechanism(s) regulating this process remain unclear. Here, we report that 14.3.3 proteins control Akt Thr308 phosphorylation during intestinal inflammation. Mechanistically we found that proinflammatory cytokines fully activate Akt by inducing degradation of the PDK1 inhibitor, 14.3.3η via autophagy. Interestingly we also observed that exposure of intestinal epithelial cells to proinflammatory cytokines reduces autophagy, suggesting that during inflammation full Akt activation is prevented. Notably, inhibition of 14.3.3 function by the chemical inhibitor BV02 results in uncontrolled Akt activation and increased cell death. Taken togetherour results show that 14.3.3 proteins tamper Akt activation to regulate its physiological functions, thereby providing a new mechanistic link between cell survival and apoptosis phenotype on intestinal epithelial cells during inflammation.