RNA splicing of caspase-8 : a pathophysiological role in autoimmune disease and its implification for new therapy

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RNA splicing has two different processes: namely, constitutive splicing and alternative splicing. The latter has now emerged as an important mechanism which produces a high degree of protein diversity at a low genetic cost Importantly, alternative splicing generates functionally distinct products from the same apoptosis-related genes including Bcl-x, caspase-9, caspase-2, Fas and capase-8. Furthermore, many genes in the immune system are alternatively spliced including CD45, CD44, CTLA-4 and ICAM-1, thus indicating that alternative splicing plays a critical role in both T cell homeostasis and activation-induced cell death (AICD). A critical issue in alternative splicing regulation is the specific recognition of the splice site, which mainly depends on the phosphorylation level of serine/arginine splicing factors (SR proteins). Caspase activation induces apoptosis (at full activation)or proliferation (at limited activation) of lymphocytes. mRNA splicing of caspase-8 generates normal transcript (NT)and splice variant (SV). We sought to elucidate how mRNA splicing of caspase-8 is regulated and affects its enzymatic activity. Peripheral blood mononuclear cell (PBMC)were stimulated with either staurosporine or phorbol 12-myristate 13-acetate (PMA)followed by analysis of mRNA splicing, protein expression, caspase activation and cell proliferation. Activation of protein phosphate-1 (PP-1)and protein kinase C (PKC)had opposite effects on mRNA splicing, thereby increasing and decreasing the ratio of SV to NT, respectively. Downregulation of SV of caspase-8 induced its limited activation, non-apoptotic cleavage of cellular FLICE-inhibitory protein (cFLIP), which was associated with augmentation in phosphorylation of splicing factors. Blockade of caspase activation prevented downregulation of SV, IL-2 production, and cell proliferation. Thus, activation of PP-1 and PKC regulates mRNA splicing of caspase-8 and its enzymatic activity.