NUCLEAR TARGETING OF BAX DURING APOPTOSIS IN HUMAN COLORECTAL-CANCER CELLS

Homeostasis in colonic epithelial cells is regulated by the balance be tween proliferative activity and cell loss by apoptosis. Because epith elial cells at the apex of colonic crypts undergo apoptosis and prolif erative activity is usually restricted to the base of the crypts, it h as been proposed that the limited availability of growth factor-signal s at the upper portions of the crypts may trigger apoptosis, In the pr esent studies, we investigate the mechanism of apoptosis mediated by g rowth factor deprivation in colorectal carcinoma cells by delineating the possible involvement of Bar and its subcellular localization. We r eport that inhibition of epidermal growth factor receptor (EGFR) tyros ine kinase activity and downregulation of EGFR by anti-EGFR mAb 225 in duces apoptosis in human colorectal carcinoma DiFi and FET cells. Indu ction of apoptosis was preceded by enhanced expression of newly synthe sized Bax protein, and required protein synthesis. In the mAb 225-trea ted cells, Bar was redistributed from the cytosol to the nucleus and s ubsequently, to the nuclear membranes. The observed induction of Bar e xpression by mAb 225 was not associated with p53 induction. However, m Ab 225 treatment also triggered relocalization of p53 from the cytosol to a nuclear membrane-bound form. Induction of Bar and its redistribu tion to the nucleus of DiFi cells during apoptosis was also demonstrat ed in response to butyrate, a physiological relevant molecule in colon ic epithelial cells as it is the principal short-chain fatty acid prod uced by bacterial fermentation of dietary fiber in colonic epithelium, Using immunofluorescence and confocal microscopy, we observed that Ba r is predominantly localized in the cytosol, but during apoptosis it i s localized both inside and along the nuclear membrane. Taken together , these findings suggest that apoptosis induced by growth factor-depri vation or butyrate may involve the subcellular redistribution of Bar i n human colorectal carcinoma cells.