Proteolysis of the tumour suppressor hDlg in response to osmotic stress is mediated by caspases and independent of phosphorylation

Human disc-large (hDlg) is a scaffold protein critical for the maintenance of cell polarity and adhesion. hDlg is a component of the p38γ MAP kinase pathway, which is important for the adaptation of mammalian cells to changes in environmental osmolarity. Here we report a strong decrease in the levels of hDlg protein in the human epithelial cell line HeLa when exposed to osmotic shock. This is independent of the phosphorylation state of hDlg, is prevented by preincubating the cell with the caspase inhibitor z-VAD and is part of the apoptotic process triggered by cellular stress. Although, both caspase 3 and caspase 6 are strongly activated by osmotic shock, the time course of caspase 6 activation parallels hDlg degradation, suggesting that this caspase may be responsible for the proteolysis. Mutating hDlg Asp747 to Ala abolishes caspase-induced cleavage, but does not affect the early stage of apoptosis or cell attachment. Our findings show that osmotic stress triggers hDlg degradation through a mechanism different from the one mediated by proteasomes, and we identify hDlg as a caspase substrate during the apoptotic process, although its proteolysis may not be implicated in the progression of early apoptosis.