Occludin limits epithelial survival by inducing caspase‐3 expression

The tight junction protein occludin regulates the paracellular barrier, but is also implicated in other epithelial functions. To determine if occludin regulates survival, responses of occludin knockdown (KD) Caco‐2 and enterocytes in occludin knockout (KO) mice to pro‐apoptotic stimuli was assessed. Occludin‐deficiency reduced extrinsic pathway apoptosis (TNF) and intrinsic pathway apoptosis (5FU, staurosporine, camptothecin), both in vitro and in vivo. Caspase‐8 and ‐9 were activated normally in both occludin‐deficient models, but caspase‐3 activation was reduced. Consistent with this, cytochrome C and dATP failed to efficiently activate caspase‐3 in cytosolic extracts of occludin‐deficient cells. Further, occludin‐deficient intestinal epithelia demonstrated decreased caspase‐3 promoter activity as well as mRNA and protein expression. Exogenous caspase‐3 expression reversed apoptotic resistance in vitro. Thus, occludin‐dependent caspase‐3 expression is required for intestinal epithelial apoptosis. The biologic relevance of this observation is emphasized by the resistance of occludin KO mice to DSS‐induced epithelial apoptosis, colonic damage, and systemic disease. Thus, disease‐associated occludin downregulation may limit epithelial apoptosis. While acutely beneficial, we speculate that this may allow damaged cells to survive and increase neoplastic risk in chronic disease.