BCL-2 AND BAK MAY PLAY A PIVOTAL ROLE IN SODIUM BUTYRATE-INDUCED APOPTOSIS IN COLONIC EPITHELIAL-CELLS - HOWEVER OVEREXPRESSION OF BCL-2 DOES NOT PROTECT AGAINST BAK-MEDIATED APOPTOSIS

Butyrate, a short chain fatty acid produced in the colon as a result o f fermentation of dietary fibre by symbiotic bacteria, induces apoptos is in colonic tumour cell lines. Three human colonic adenoma cell line s (AA/C1, RG/C2, and BH/C1) and one carcinoma cell line (S/KS/FI) were used to determine the effects of butyrate on the expression of bcl-2, bax and bak to examine the possible role of these proteins in the ind uction of apoptosis, RG/C2, and BH/C1 cells express p-26-bcl-2 and but yrate treatment decreased p26-bcl-2 levels in association with apoptos is, whereas bar and bak levels remained constant. AA/C1 and S/KS/F1 ce lls have no detectable p26-bcl-2. In S/KS/F1 cells, bax or bak levels did not change in response to butyrate. However, in AA/C1 cells, butyr ate-induced apoptosis was associated with increased bak levels. Theref ore, in AA/C1 cells butyrate-induced apoptosis appears to be mediated through bak. Furthermore, butyrate also induced apoptosis and increase d bak levels in AA/C1 cells transfected with a bcl-2 expression vector which expressed high levels of p26-bcl-2. For S/KS/F1 cells, two bcl- 2 transfectants gave different results, bcl-2 protected against apopto sis in one transfectant in which bak levels were not elevated in respo nse to butyrate, whereas it did not protect in the other transfectant in which bak levels were increased after butyrate treatment. The resul ts suggest that expression of constitutively high levels of p26-bcl-2 only conferred protection against apoptosis when bak levels were not e levated in response to butyrate and that expression of constitutively high levels of p26-bcl-2 does not counter the effects of bak, Differen t mechanisms appear to be involved in cell death signalling in differe nt tumours since butyrate may induce apoptosis via elevated levels of bak or reduced levels of p26-bcl-2.