A Previously Unreported Function of β1B Integrin Isoform in Caspase-8-Dependent Integrin-Mediated Keratinocyte Death

Abstract

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 β(1) integrin expression protects keratinocyte stem cells from anoikis, whereas the role of the β(1)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-β(1) 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 β(1) integrin, which in turn internalized and progressively degraded, shedding the cytoplasmic domain. β(1)B was expressed only in the cytoplasm in a perinuclear manner and remained unaltered during suspension. At 24 hours, as β(1)A was located close to the nucleus, β(1)B colocalized with β(1)A and coimmunoprecipitated with caspase-8. Caspase-8 was activated earlier in β(1)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) β(1)B-transfected cells. These results indicate that when β(1)A is unligated, β(1)B is responsible for "integrin-mediated death" in human keratinocytes.