α5-integrin is crucial for L1CAM-mediated chemoresistance in pancreatic adenocarcinoma

We recently showed that the adhesion molecule L1CAM (CD171) is overexpressed in pancreatic adenocarcinoma (PDAC) essentially contributing to chemoresistance of PDAC cells. In search of the mechanisms of this effect we now identified α5-integrin as the L1CAM ligand being essential for L1CAM-mediated chemoresistance of these highly malignant tumor cells. Thus, blockade or knock-down of α5-integrin in the L1CAM expressing PDAC cell lines PT45-P1res, Colo357 and Panc1 increased anti-cancer drug sensitivity. In line with the previously reported NO-dependent caspase inhibition resulting from L1CAM induced iNOS expression, the loss of chemoresistance upon α5-integrin inhibition was preceded by decreased iNOS expression and enhanced caspase-3/-7 activation. Accordingly, the loss of anti-cancer drug protection by α5-integrin inhibition could be overcome by administration of the NO-donor SNAP. Moreover, the gain of chemoresistance of parental PT45-P1 cells when transfected with L1CAM was abrogated by α5-integrin inhibition, whereas transfection of PT45-P1 cells with an integrin binding-deficient L1CAM mutant (L1mutRGE) did neither induce chemoresistance or iNOS expression nor conferred sensitivity to α5-integrin inhibition as seen upon transfection with wild-type L1CAM. Thus, mutational loss of the integrin binding site in the L1CAM molecule or the blockade of α5-integrin abolished the induction of iNOS expression and chemoresistance by L1CAM, indicating that both a functional L1CAM and α5-integrin are indispensable of L1CAM-induced drug resistance in PDAC cells.