VITAMIN-D3 REDUCES THE APOPTOTIC EFFECT OF IFN-GAMMA BUT DOES NOT FACILITATE HLA CLASS-II INDUCIBILITY IN RB-DEFECTIVE CELLS

The retinoblastoma protein (RB) regulates the cell cycle by binding an d inactivating the E2F transcription factors, which prevents transcrip tion of genes required for DNA synthesis. RB has been shown to inhibit IFN-gamma-mediated apoptosis, possibly by regulating premature entry into S phase. RB is also required for high level IFN-gamma induction o f HLA class II genes, which encode antigen presenting molecules, but n ot for other IFN-gamma inducible genes as demonstrated in previous rep orts describing the analysis of RB-transformants of the RB-defective c ell lines, MDA-468-S4 (S4) and H2009. The IFN-gamma response of the HL A class II genes takes much longer than does the response of the other IFN-gamma inducible genes, raising the question of whether RB facilit ates HLA class II inducibility by maintaining cell viability over the long time course required for HLA class II induction. Thus, we sought to learn whether IFN-gamma induced apoptosis in an RB-defective cell l ine could be prevented independently of RB and whether doing so would facilitate HLA class II inducibility in the RB-defective line. Our res ults indicated that cotreating the RB-defective S4 cells with IFN-gamm a and Vitamin D3 decreased the number of cells containing subdiploid D NA compared to cells treated with IFN-gamma alone, suggesting that Vit amin D3 reduced IFN-gamma-mediated apoptosis. S4 cells cotreated with Vitamin D3 and IFN-gamma also had decreased cell detachment, further i ndicating that Vitamin D3 decreased IFN-gamma induced apoptosis. Howev er, Vitamin D3 cotreatment resulted in no detectable increase in HLA-D R, the most prominent HLA class II molecule, indicating that the effec t of RB on HLA class II induction is not exclusively due to its abilit y to inhibit IFN-gamma induced apoptosis.