Phorbol myristate acetate and Bryostatin 1 rescue IFN-gamma inducibility of MHC class II molecules in LS1034 colorectal carcinoma cell line

BACKGROUND: The expression of major histocompatibility complex class II (MHCII) antigens in both mouse and human tumors is rare, and these antigens are not easily inducible by IFN-gamma (IFNg). Since MHCII may play an important role in the development of host antitumor immune response, we explored the possibility of restoring MHCII inducibility in several IFNg-resistant tumor cell lines using protein kinase C (PKC) agonists phorbol myristate acetate (PMA) or Bryostatin. RESULTS: Tumor cells were co-cultured with various concentrations of PMA and IFNg for 48 hr. The expression of MHCII antigens and receptors IFNgR1 and IFNgR2 was determined by flow cytometry. We showed that the presence of as little as 0.1 ng/ml of PMA in tissue culture restored the ability of weakly inducible LS1034 colon carcinoma cells to express MHCII in response to IFNg (100 – 10,000 IU/ml) in a dose-dependent manner. Likewise, Bryostatin 1, as low as 10 ng/ml produced a 5–6 fold upregulation of MHCII. The effect of PMA was not observed in two other poorly responding cell lines, MSTO-211H mesothelioma and HepG2 hepatocellular carcinoma, and was abrogated by relatively high concentrations of PKC inhibitors staurosporine (100 nM) and GF 109203X (1,000 nM). Both surface and intracellular staining of all cell lines with antibodies against IFNgR1 and IFNgR2 failed to detect any increase in IFNg receptor expression following incubation with PMA. CONCLUSION: In this study we showed that IFNg-inducibility of MHCII antigens in weakly inducible LS1034 colorectal carcinoma cell line can be rescued by concomitant incubation with PKC agonists. Bryostatin 1 may be considered for further investigation of IFNg-dependent MHCII induction in resistant tumors in vivo.