We have recently reported that receptor-interacting protein kinase 1 (RIPK1) is commonly up-regulated through cellular inhibitor apoptosis protein (cIAP)-mediated stabilization and functions as an oncogenic driver via activation of NF-κB in melanoma cells. Here we show that α-actinin-4 (ACTN4), an isoform of α-actinins of spectrin superfamily proteins that is emerging as an oncogenic regulator, is required for cIAP-mediated stabilization of RIPK1 and plays a pro-oncogenic role in melanoma. Similar to RIPK1, ACTN4 was found to be commonly increased in melanoma cells. While knockdown of ACTN4 inhibited melanoma cell proliferation that was associated with down-regulation of RIPK1 and reduction in the basal levels of NF-κB activation, overexpression of ACTN4 resulted in enhanced proliferation of melanocytes that was associated with elevation in RIPK1 expression and increased NF-κB activation. The inhibitory effect of ACTN4 knockdown on melanoma cell proliferation and activation of NF-κB was due to decreased expression of RIPK1, as it was abolished by overexpression of RIPK1. On the other hand, knockdown of RIPK1 diminished ACTN4 overexpression-triggered enhancement in proliferation in melanocytes. Strikingly, knockdown of ACTN4 attenuated the association between cIAPs and RIPK1 and reduced K63-linked ubiquitination of the protein, leading to RIPK1 protein degradation. Mechanistic studies showed that ACTN4 bound to RIPK1 through its N-terminus, whereas it was associated with cIAPs via its C-terminus, and that dimerization of ACTN4 in an anti-parallel manner is required for the interaction between cIAPs and RIPK1. Collectively, ACTN4 is necessary for stabilization of RIPK1 by cIAPs and functions as an oncogenic driver in melanoma.

Citation Format: Lei Jin, Hessam Tabatabaeehatambakhsh, Chen Chen Jiang, Xu Guang Yan, Jia Yu Wang, Yuan Yuan Zhang, Hamed Yari, Chun Yan Wang, Ting La, Fu Xi Lei, Yu Chen Feng, Su Tang Guo, Xu Dong Zhang. ACTN4 stabilises RIPK1 to function as an oncogenic driver in melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4462. doi:10.1158/1538-7445.AM2017-4462