Downregulation of the proteasome subunits, transporter, and antigen presentation in hepatocellular carcinoma, and their restoration by interferon-gamma.

Abstract

In our previous study, expressions of human histocompatibility leukocyte antigens class I molecules (HLA-I) and the transporter associated with antigen processing (TAP) 1/2 genes were investigated in seven hepatocellular carcinoma (HCC) cell lines. Two cell lines, Hep-3B and HuH-7, showed a reduced level of TAP, which might cause the low surface expression of HLA-I. In order to understand the downregulation mechanism of antigen presentation in tumors, the two cell lines were further investigated. Expressions of HLA-I and antigen presentation-related genes were analyzed by flow cytometry and polymerase chain reaction, respectively. Antigen presentation was tested in 51Cr-release assays. Flow cytometric analyses revealed low surface expression of HLA-I on Hep-3B and HuH-7 cells. Introduction of HLA-A2 gene did not result in a high surface expression of HLA-A2. This suggested the downregulation of HLA-I expression might be related to defects in the antigen presentation machinery. We then examined expression levels of various antigen presentation-related genes. Hep-3B and HuH-7 demonstrated low expression of the low-molecular-weight protein (LMP) 2, LMP7, TAP1, and HLA-I heavy-chain transcripts. The downregulation of these genes was dissolved by treatment with gamma-interferon. Furthermore, allo-specific cytotoxic T lymphocyte (CTL) lines failed to recognize Hep-3B and HuH-7 cells, while they killed IFN-gamma-treated Hep-3B and HuH-7 cells. Our results suggest that defects in the antigen presentation-related molecules might cause downregulation of HLA-I expression, antigen presentation, and subsequently, escape from specific CTL killing. The downregulation could be restored by IFN-gamma treatment, suggesting the potential use of IFN-gamma for therapeutic purposes.