LMP-associated proteolytic activities and TAP-dependent peptide transport for class 1 MHC molecules are suppressed in cell lines transformed by the highly oncogenic adenovirus 12.

Abstract

SummaryExpression of class I major histocompatibility complex antigens on the surface of cells transformed by adenovirus 12 (Ad12) is generally very low, and correlates with the in vivo oncogenicity of this virus. In primary embryonal fibroblasts (H-26) that express a transgenic swine class I antigen (PD 1), Ad12-mediated transformation results in inhibition in transport of newly synthesized class I molecules, as well as significant reduction in transporter associated with antigen presentation (TAP) gene expression. In this report we show that reexpression of TAP molecules either by stable transfection of mouse TAP genes or by infection with recombinant vaccinia viruses expressing human TAP genes, only partially reconstitutes the expression and transport of the class I molecueles. Further analysis of Ad12-transformed cells revealed that the expression of both LMP2 and LMP7, but not of other proteasome complex components, was downregulated, resulting in altered proteolytic activities of the 20S proteasomes. Reconstitution of both TAP and LMP expression resulted in complete restoration of PD1 cell surface expression and enhanced expression of the endogenous H-2D 6 molecules. Despite high expression of TAP, LMP, and class I MHC molecules encoded by recombinant vaccinia viruses, in reconstituted Ad12-transformed cells, efficient transport of H-2 class I molecules could only be achieved by treatment of the cells with ~/-interferon. These data suggest that an additional factor(s) that is interferon-regulated plays a role in the biosynthetic pathway of the class I complex, and that its function is deficient in this cell system. Thus, Ad12 viral transformation appears to suppress the expression of multiple genes that are important for antigen processing and presentation, which allows such transformed cells to escape immune surveillance. This coordinate downregnlation of immune response genes must likely occur through their use of common regulatory elements.

Organization and Functional Analysis of the Mouse Transporter Associated with Antigen Processing 2 Promoter

“…Moreover, in some tumors all of these genes are down-regulated, resulting in a loss of recognition by CTL (26). For example, Ad12-mediated transformation results in transcriptional down-regulation of class I, peptide transporter, and LMP genes (27,28). Some of these genes, such as those encoding class I heavy chains, ␤ 2 m, and TAP1, are partially suppressed, whereas transcripts encoded by TAP2, LMP2, and LMP7 genes are undetectable, as are the corresponding protein products.…”

The ubiquitin-proteasome pathway in cancer

“…Interferon-a and, in particular, interferon-y could partly suppress these defects (Seliger et al, 1996). The potential importance of subunits LMP-2 and LMP-7 for MHC class Irestricted antigen presentation is also underscored by the fact that they are specifically down-regulated after viral transformation in vitro by oncogenic viruses (Rotem Yehudar et al, 1996 ionizing radiation or oxidative stress but not by the protein kinase C inhibitor staurosporine (Wang et al, 1996). The activation of NF-KB requires two steps of proteasome-dependent proteolysis.…”

“…Because of the broad involvement of ubiquitin-proteasome proteolysis in fundamental biochemical processes, this pathway is a potential target for cancer-related deregulation, and alterations of proteasome function have indeed been described in events, such as cellular transformation by oncogenic viruses (Scheffner et Finley, 1995Finley, ) 1990Finley, , 1993Ciechanover et al, 1994) and immune escape (Restifo et al, 1993;Sibille et al, 1995;Rotem Yehudar et al, 1996;Seliger et al, 1996). Furthermore, alterations of proteasome activity in tumour samples have been reported recently to confer in colon and possibly breast cancer a phenotype of clinical aggressiveness associated with poor prognosis (Catzavelos et al, 1997;Loda et al, 1997;Porter et al, 1997).…”

Molecular mechanisms of HLA class I antigen abnormalities following viral infection and transformation

“…LMP2 mRNA levels have been shown to be reduced because of transcriptional suppression of the bidirectional LMP2/TAP1 promoter activity. 18,19 Viruses can also inhibit proteasomal-mediated protein degradation by changing the proteolytic cleavage site within a viral CTL epitope. As a result, the antigen is not processed.…”