Regulation and Mechanistic Functions of Caspase-9 RNA Splicing

• Main Author: Vu, Ngoc T
• Format: Others
• Published: VCU Scholars Compass 2014
• Subjects: caspase-9b; caspase-9; RNA splicing; hnRNP U; NF-kappaB pathway; Molecular Biology
• Online Access: http://scholarscompass.vcu.edu/etd/3578; http://scholarscompass.vcu.edu/cgi/viewcontent.cgi?article=4582&context=etd

Caspase-9 has two splice variants, pro-apoptotic caspase-9a and anti-apoptotic caspase-9b, and dysregulation of caspase-9 splice variant ratio or expression of caspase-9b isoform has been linked to augmentation of the anchorage-independent growth and tumorigenic capacity of non-small cell lung cancer (NSCLC) cells. This study focuses on cell signaling pathway(s) regulating the alternative splicing of caspase-9 pre-mRNA and mechanistic roles of caspase-9b in a certain oncogenic/survival pathway. In regards to the former, we have identified hnRNP U as a novel splice-enhancer associated with exon 3 of caspase-9 (C9/E3). Moreover, hnRNP U binds specifically to C9/E3 at an RNA cis-element previously reported as the binding site for the splicing repressor, hnRNP L. Phosphorylated hnRNP L interferes with hnRNP U for binding to C9/E3, and our results demonstrate the importance of the phosphoinositide 3-kinase/AKT pathway in modulating the association of hnRNP U to C9/E3. Overall, a mechanistic model has been revealed where hnRNP U competes with hnRNP L for C9/E3 binding to enhance the inclusion of the four-exon cassette, and this splice-enhancing effect is blocked by the AKT pathway via phosphorylation of hnRNP L. As to the latter aim, it is unknown about the mechanistic roles of caspase-9b besides the inhibitory effect on caspase-9a processing. In this study, caspase-9b has been demonstrated to have a dual function in regulating the survival/oncogenic nuclear factor κB (NF-κB) pathway, which is independent from modulating caspase-9a activation. In particular, caspase-9b has been shown to activate the canonical arm and inhibit the non-canonical arm of the NF-κB pathway by destabilizing NF-κB inhibitor alpha (IκB-α) and NF-κB-inducing kinase (NIK). Importantly, this new role for caspase-9b contributes to the enhanced survival and anchorage-independent growth of NSCLC cells conferred by caspase-9b expression. Further mechanistic studies have demonstrated a direct association of caspase-9b with the cellular inhibitor of apoptosis 1 (cIAP1), a regulatory factor in both arms of the NF-κB network, via its IAP-binding motif. Through this interaction, caspase-9b induces the E3 ligase activity of cIAP1, which regulates NF-κB activation, and promotes the survival, anchorage-independent growth and tumorigenicity of NSCLC cells. Overall, a novel tumorigenic mechanism has been identified, by which alternative mRNA processing regulates the NF-κB signaling independent of external agonist.