A Previously Unreported Function of β1B 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 β1B 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. β1B was expressed only in the cytoplasm in a perinuclear manner and remained unaltered during suspension. At 24 hours, as β1A was located close to the nucleus, β1B colocalized with β1A and coimmunoprecipitated with caspase-8. Caspase-8 was activated earlier in β1B 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) β1B-transfected cells. These results indicate that when β1A is unligated, β1B is responsible for “integrin-mediated death” in human keratinocytes.