A previously unreported function of Β₁ B integrin isoform in caspase-8-dependent integrin-mediated keratinocyte death

Integrins regulate adhesive cell-matrix interactions and mediate survival signals. On the other hand, unligated or free cytoplasmic fragments of integrins induce apoptosis in many cell types (integrin-mediated death). We have previously shown that Β₁ integrin expression protects keratinocyte stem cells from anoikis, whereas the role of the Β₁ B integrin isoform has not been clarified. In this study we report that suspended keratinocytes undergo apoptosis through the activation of caspase-8, independently of the Fas/Fas ligand system. Indeed, anti-Β₁ integrin-neutralizing antibodies induced apoptosis in short hairpin RNA Fas-associated death domain-treated cells. Moreover, before and during suspension, caspase-8 directly associated with Β₁ integrin, which in turn internalized and progressively degraded, shedding the cytoplasmic domain. Β₁ B was expressed only in the cytoplasm in a perinuclear manner and remained unaltered during suspension. At 24 hours, as Β₁ A was located close to the nucleus, Β₁ B colocalized with Β₁ A and coimmunoprecipitated with caspase-8. Caspase-8 was activated earlier in Β₁ B integrin-transfected keratinocytes, and these cells underwent a higher rate of apoptosis than mock cells. In contrast, caspase-8 was not activated in small interfering RNA (siRNA) Β₁ B-transfected cells. These results indicate that when Β₁ A is unligated, Β₁ B is responsible for integrin-mediated death in human keratinocytes.