THE ROLE OF BCL-2 AND BAX PROTEIN IN MONOCYTE-MEDIATED APOPTOSIS IN HUMAN LEUKEMIC-CELL LINES

Monocytes or monocyte-derived supernatants are able to kill leukemic c ells via apoptosis, thereby preferentially effecting more mature leuke mic cells. In the present study, the relationship between apoptosis an d the apoptosis related proteins, bcl-2 and bar, was investigated in a number of human leukemic cell lines. Monocyte-derived supernatant ind uces extensive apoptosis in U937 myeloid leukemia cells and minor apop tosis in HL60 cells. No apoptosis was seen in four other cell lines (T HP1, HL60-D3, KG1, and K562). The expression of bcl-2 and bax protein was determined in both groups of leukemic cell lines by flow cytometry (bcl-2 and bas) and Western blotting (bcl-2) at baseline level and af ter incubation with monocyte supernatant after different time periods. No clear relation was found between baseline bcl-2 or bas protein exp ression and the occurrence of apoptosis after incubation with monocyte supernatant. After different incubation time periods, no change was f ound in bcl-2 protein expression in U937 and K562 cells, whereas in KG 1, HL60, and especially in THP1 cells, a significant decrease could be noticed. On the otter hand, there was an increase in bcl-2 expression in HL60-D3 cells. Bas protein expression, measured at the same time p oints, remained essentially unchanged in HL60-D3 cells, decreased sign ificantly in U937, HL60, and THP1 cells and slightly in K562 cells, an d increased significantly in KG1 cells. Also, the ratio bax/bcl-2 decr eased in ML60D3, but especially in U937 and HL60 cells, increased slig htly in THP1 and KG1 cells, and remained essentially unchanged in K562 cells. Rh-tumor necrosis factor-alpha (TNF-alpha), the main mediator of monocyte mediated cytotoxicity, induced apoptosis in U937, HL60, an d THP1 cells, thereby showing changes in bcl-2 expression similar to t hose found for monocyte derived supernatants. We concluded that in hum an leukemic cell lines, there is no relation between either bcl-2 or b ax protein expression or the ratio of both, and apoptosis mediated by monocyte derived supernatant or TNF-alpha.