Polarity proteins PAR6 and aPKC regulate cell death through GSK-3β in 3D epithelial morphogenesis

Epithelial cells are polarized, with an apical surface facing a lumen or outer surface and a basolateral surface facing other cells and extracellular matrix (ECM). Hallmarks of epithelial carcinogenesis include loss of polarity, as well as uncontrolled proliferation and resistance to apoptosis. Are these features controlled by a common molecular mechanism? The partitioning- defective 3 (PAR3)-PAR6-atypical PKC (aPKC) complex is a master regulator that controls polarization in many animal cells. Here we show that PAR6 is involved in apoptosis by regulating aPKC and glycogen synthase kinase 3 beta (GSK-3 beta) activity. During epithelial morphogenesis in 3D culture of Madin- Darby canine kidney ( MDCK) cells, expression of an N- terminally deleted PAR6 (PAR6 Delta N) leads to a significant increase in caspase- dependent cell death by downregulating aPKC activity. Accordingly, inhibition of aPKC in wild- type WT) MDCK cells with either a cell- permeable PKC xi pseudosubstrate or RNAi promotes apoptosis, which suggests that PAR6 regulates apoptosis via an aPKC-mediated pathway. GSK-3 beta, a substrate of aPKC, is hyperactivated by expressing PAR6 Delta N. GSK-3 beta inhibitors block PAR6 Delta N-induced apoptosis while expression of constitutively active GSK- 3 beta (S9A) promotes apoptosis, which is rescued by ectopic expression of aPKC. We conclude that a PAR6-aPKC-GSK-3 beta mechanism links cell polarity and apoptosis.