Inhibition of integrin-linked kinase/protein kinase B/Akt signaling - Mechanism for ganglioside-induced apoptosis

Ganglioside GT1b inhibits keratinocyte attachment to and migration on a fib ronectin matrix by binding to alpha (5)beta (1) and preventing alpha (5)bet a (1) interaction with fibronectin. The role of gangliosides in triggering keratinocyte apoptosis, however, is unknown. Addition of GT1b to keratinocy te-derived SCC12 cells, grown in serum-free medium but exposed to fibronect in, suppressed Bad phosphorylation, activated caspase-9, and inhibited cycl in D and E expression, resulting in cell cycle arrest at G(1) phase and ini tiation of apoptosis. The mechanism of GT1b activation of caspase-9 involve d inhibition of beta (1) integrin serine/threonine phosphorylation and decr eased phosphorylation of both integrin-linked kinase and protein kinase B/A kt at its Ser-473 site, leading to cytochrome c release from mitochondria. Consistently, blockade of GT1b function with anti-GT1b antibody specificall y activated the Ser-473 site of Akt, markedly suppressing apoptosis. The ga nglioside-induced inhibition of Akt phosphorylation was GT1b-specific and w as not observed when cells were treated with other keratinocyte ganglioside s, including GD3. These studies suggest that the modulation of keratinocyte cell cycle and survival by GT1b is mediated by its direct interaction with alpha (5)beta (1) and resultant inhibition of the integrin/integrin-linked kinase/protein kinase B/Akt signaling pathway.