Blocking the expression of
integrin α2β1, which was accomplished by transduction of α2-specific
shRNA, resulted in significant inhibition of proliferation and clonal activity in human MCF-7
breast carcinoma and SK-Mel-147
melanoma cells. Along with these changes, deprivation of α2β1 caused a sharp decrease in
melanoma cell invasion in vitro. Analysis of
integrin-mediating signal pathways that control cell behavior revealed a significant increase in activity of Akt
protein kinase in response to depletion of α2β1. The increase in Akt activity that accompanies a suppressive effect on cell invasion contradicts well-known Akt function aimed at stimulation of
tumor progression. This contradiction could be explained by the "reversed" (noncanonical) role played by Akt in some cells that consists in suppression rather than promotion of invasive phenotype. To test this suggestion, the effects of Akt inhibitors on invasive activity of SK-Mel-147 cells were investigated. If the above suggestion is true, then inhibition of Akt in cells depleted of α2β1 should result in the restoration of their invasive activity. It appeared that treatment with
LY294002, which inhibits all Akt
isoforms (Akt1, Akt2, Akt3), not only failed to restore the invasive phenotype of
melanoma cells but further attenuated their invasive activity. However, treatment of the cells with an Akt1-specific inhibitor significantly increased their invasion. Thus, the stimulating effect of α2β1
integrin on invasion of
melanoma cells is realized through a mechanism based on inhibition of one of the Akt
isoforms, which in these cells exhibits a noncanonical function consisting in suppression of invasion.