PubMed2004
Although melanocytes are devoid of the human major histocompatibility complex class II (HLA II) molecules, melanomas often display constitutive expression of these molecules, particularly HLA-DR. This constitutive expression of HLA-DR molecules is associated with tumor progression and poor prognosis but the molecular basis for this association remains poorly understood. Within the hypothesis of a role in immune escape, we analyzed the regulation of Fas-mediated apoptosis by HLA-DR signaling in the HLA-DR-positive malignant melanoma cell line A375. Our study demonstrates that engagement of HLA-DR molecules with anti-HLA-DR-specific monoclonal antibody L243 significantly reduces Fas-mediated apoptosis; DNA fragmentation and cell death were decreased by 50% and 40%, respectively. We found that while HLA-DR signaling does not affect Fas receptor expression, it significantly reduces Fas-induced activation of caspase-8 and Bid. Furthermore, inhibition studies and expression of dominant negative form of Mek-1 demonstrated that HLA-DR-mediated inhibition of caspase-8/Bid activation and apoptosis are dependent on the activation of the MAPK/Erk pathway. Together, our results provide evidence that HLA-DR signaling activates the MAPK/Erk pathway in A375 melanoma cells, which has a functional role in the resistance of these cells to Fas-mediated apoptosis. These observations underline the potential importance that HLA-DR signaling might have in melanoma immune escape and tumor progression.