Abstract 2023: The role of RASSF1A in FasL expression and function

Abstract

Abstract RASSF1A is a tumor suppressor protein that is epigenetically silenced in a variety of tumor cells, including those of lymphocytic origin. Several previous studies have identified a role for RASSF1A in death receptor-dependent apoptosis in abnormal cells of epithelial origin downstream of tumor necrosis factor receptor 1 (TNF-R1) engagement; however relatively little is known about a possible role for Rassf1A in lymphocytic cells. Interaction of Fas ligand (FasL) on activated T lymphocytes with the Fas receptor on other immune cells plays a major role in down-modulating immune responses through activation-induced cell death (AICD). Apoptosis of T cells that have undergone clonal expansion in response to antigenic stimulation through the TCR is a key regulatory feature of the adaptive immune response that prevents the accumulation of excessive lymphocytes once the antigen has been cleared. We have examined the effect of RASSF1A overexpression on FasL transcriptional activation and function in the Jurkat human T cell line. We have demonstrated that over-expression of RASSF1A induced a significant increase in FasL promoter (FasLp) activity. Similar-fold increases in transcriptional activity were observed when the distal NFAT binding site (3X NFAT plus minimal promoter) within FasLp was used in co-transfection reporter assays; however, other transcription reporters (AP1, NFkB) were unaffected. Experiments with mutant Jurkat cell lines indicate that early signaling mediators, including tyrosine kinases and PLCγ, are critical in the RASSF1A-induced enhancement of FasL production. Poly (ADP-ribose) polymerase (PARP) cleavage is mediated by caspases, and is indicative of apoptosis. Supernatants from anti-CD3 stimulated Jurkat cells that stably overexpress RASSF1A show an increased ability to induce cleavage of PARP in apoptosis-sensitive J16 target cells when compared to anti-CD3 stimulated vector-transfected cells. Taken together, these results indicate that RASSF1A plays a role in expression of soluble FasL leading to increased apoptosis-inducing activity by these lymphocytic cells. The loss of this activity could contribute to outgrowth of malignant clones of lymphocytic origin in which RASSF1A is silenced. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2023. doi:1538-7445.AM2012-2023