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.

Overcoming anoikis – pathways to anchorage-independent growth in cancer

“…In contrast to 'classical' anoikis, IMD does not necessarily imply loss of ECM attachment because it can be triggered by unligated integrins in an otherwise normal adherent situation. IMD has been described in epithelial cells, for both integrins 1 and 3; it involves the recruitment of caspase-8 to the integrin--subunit cytoplasmic domain and the onset of an apoptotic response (Lotti et al, 2010;Stupack et al, 2001;Zhang et al, 2011). A similar direct interaction of integrins and caspase-3 has been described in fibroblasts undergoing anoikis (Rajeswari and Pande, 2006).…”

Keratinocyte stem cells: Friends and foes

“…While in epidermis these two events are clearly different at the molecular level (Lippens et al, 2005), it is interesting to note that blocking b 1 integrin induces keratinocyte anoikis without affecting the markers of differentiation and vice versa (Mitra et al, 1997;Tiberio et al, 2002). The role of b 1 integrin within the KSC niche is even more complex, since the b 1 B isoform cooperates with unligated b 1 A to induce anoikis (Lotti et al, 2010). The best example of the close correlation between KSC and apoptosis derives from survivin, an antiapoptotic molecule that also regulates cell cycle (Mita et al, 2008).…”

Cyclin D1 localizes in the cytoplasm of keratinocytes during skin differentiation and regulates cell–matrix adhesion

“…13 Moreover, we and others have shown that CycD1 exerts cytoplasmic functions collaborating with Ral and Rho GTPases to control cell-matrix adhesion and motility. 4,7 RalGTPases, in turn, control exocyst formation and activity, which increases β1 integrin recycling.…”

“…12,13 We analyzed the co-localization of CycD1 with β1 integrin in cultured keratinocytes a short time (16 h) after calcium addition (akin to skin epithelia in the initial steps of differentiation). Under these conditions, a significant number of keratinocytes showed β1 integrin, forming a punctuate pattern in their cytoplasm, whereas cells incubated without calcium did not exhibit these foci 13 ( Fig. 5A and B).…”

“…During differentiation, β1 integrin is internalized to the cytoplasm of keratinocytes and degraded. 13 Alternatively, in order to initiate their differentiation, basal epidermal cells can undergo asymmetrical divisions, but also in these conditions, interaction of keratinocytes with the extracellular matrix is needed for the regulation of the commitment to differentiation.…”