Integrin clipping: A novel adhesion switch

Abstract

We previously identified a novel structural variant of the alpha6 integrin called alpha6p. This variant was produced on the cell surface and was missing the β-barrel extracellular domain. Using several different concentrations of amiloride, aminobenzamidine and PAI-1 and the urokinase-type plasminogen Activator (uPA) function blocking antibody (3689) we showed that uPA, acting as a protease, is responsible for production of α6p. We also showed that addition of uPA in the culture media of cells that do not produce α6p, resulted in a dose dependent α6p production. In contrast, the addition of uPA did not result in the cleavage of other integrins. Using α2-antiplasmin and plasmin depleted media, we observed that uPA cleaves the alpha6 integrin directly. Further, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) induced the production of alpha6p, and this induction was abolished by PAI-1 but not α2-antiplasmin. Using site directed mutagenesis we have identified the site of cleavage to be at arginines 594 and 595. We have also shown that while a fraction of α6 integrin is normally associated with CD151, the α6p form is not. In order to determine whether α6 integrin clipping occurs in tissue, we have found that α6p is present in human prostate cancer tissue, in normal mouse epidermis, in mouse papillomas and squamous cell carcinomas induced by DMBA, TPA and MNNG treatments and in mouse melanomas induced by activated ras. Interestingly, subcutaneous injection into athymic nude mice of a malignant mouse keratinocyte derived cell line (6M90) that is α6p negative, results in the development of tumors that contain α6p integrin. Furthermore, we have shown that PC3N cells transfected with an uncleavable mutant of the α6 integrin grew smaller tumors when injected subcutaneously in SCID mice compared to wildtype α6 transfected cells. In addition, the tumors from the uncleavable mutant alpha6 transfected PC3N cells had higher levels of activated caspase 3 indicating higher levels of apoptosis. This finding suggests that the α6 integrin clipping is important for integrin signaling for survival. Collectively, all these data suggest that the cell surface clipping of the α6 integrin is a novel mechanism for altering integrin-laminin interactions during skin tissue remodeling and carcinogenesis.