Prostaglandin E₂Down-Regulates the Expression of HLA-DR Antigen in Human Colon Adenocarcinoma Cell Lines

Prostaglandins (PG) have been implicated in the pathogenesis of cancer and play an important role in immune regulation. Colon cancer is associated with elevated levels of PGE₂, while aspirin, the prototypical inhibitor of PG synthesis, appears to reduce the incidence of colon cancer by 50%. We have observed that in human colon cancer the expression of HLA class I and II antigens is reduced or lost; loss of HLA antigens is suspected to be a mechanism by which the malignant cell escapes the immune surveillance. We investigated the effect of these eicosanoids on the expression of HLA antigens in human colon adenocarcinoma cell lines. PGE₂down-regulated the expression of the class II antigen HLA-DR in SW1116 cells (65% reduction at 2.8 × 10^-8M). This effect was dose- and time-dependent, reversible, and specific (PGF_2αand LTB₄had no effect; the expression of carcinoembryonic antigen and class I genes were not affected). Aspirin induced the expression of HLA-DR in HT29 cells, a cell line not expressing constitutively HLA-DR. The reduction of HLA-DR by PGE2 was accompanied by reduced messenger RNA (mRNA) levels of HLA-DRa and reduced transcription of the corresponding gene. In contrast to HLA-DR, none of these three eicosanoids affected the expression of HLA class I genes, as assessed via determination of protein expression by fluorescence flow cytometric analysis and evaluation of the corresponding class I mRNA levels. We conclude that PGE₂specifically down-regulates the expression of HLA-DR, while it does not affect the expression of class I antigens. These findings may be relevant to the general problem of immune surveillance of cancer and the mode of action of aspirin in protecting from colon cancer.