Vitamin D3 reduces the apoptotic effect of IFN-γ but does not facilitate HLA class II inducibility in RB-defective cells

The retinoblastoma protein (RB) regulates the cell cycle by binding and inactivating the E2F transcription factors, which prevents transcription of genes required for DNA synthesis. RB has been shown to inhibit IFN-γ-mediated apoptosis, possibly by regulating premature entry into S phase. RB is also required for high level IFN-γ induction of HLA class II genes, which encode antigen presenting molecules, but not for other IFN-γ inducible genes as demonstrated in previous reports describing the analysis of RB-transformants of the RB-defective cell lines, MDA-468-S4 (S4) and H2009. The IFN-γ response of the HLA class II genes takes much longer than does the response of the other IFN-γ inducible genes, raising the question of whether RB facilitates 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-γ induced apoptosis in an RB-defective cell line could be prevented independently of RB and whether doing so would facilitate HLA class II inducibility in the RB-defective line. Our results indicated that cotreating the RB-defective S4 cells with IFN-γ and Vitamin D3 decreased the number of cells containing subdiploid DNA compared to cells treated with IFN-γ alone, suggesting that Vitamin D3 reduced IFN-γ-mediated apoptosis. S4 cells cotreated with Vitamin D3 and IFN-γ also had decreased cell detachment, further indicating that Vitamin D3 decreased IFN-γ induced apoptosis. However, Vitamin D3 cotreatment resulted in no detectable increase in HLA-DR, the most prominent HLA class II molecule, indicating that the effect of RB on HLA class II induction is not exclusively due to its ability to inhibit IFN-γ induced apoptosis.