Abstract 4366: The small GTPase Rac1 controls the stability of the Yes-associated protein 1 (YAP1) independently of the LATS1/2 kinases and SCF-βTRCP ubiquitin ligase

Activating mutations in the K-Ras gene are the earliest and most common genetic alterations detected in human pancreatic cancer (PC) specimens. In genetically engineered mouse models of PC, oncogenic K-Ras drives the formation of PanIN precursor lesions and their progression to invasive PC. Oncogenic K-Ras has at least four effector pathways that play distinct roles in malignant transformation: MAPK pathway, PI3K pathway, Ral-GDS, and the Rac1 GTPase (Ras-related C3 botulinum toxin substrate 1). Among these effectors, the least well understood is the Rac1 pathway. Yet, this pathway has now been shown to be critical for Ras-driven PC development and malignant transformation. However, the specific mechanisms by which Rac1 promotes anchorage-independent growth and tumorigenicity are still unclear. In recent studies, we have discovered that the Rac1 pathway regulates the stability of the Yes-associated protein 1 (YAP1). YAP1 is a transcriptional co-activator that controls the transcription of genes involved in proliferation and survival. The stability, location, and activity of YAP are controlled by signaling pathways involved in sensing cell-cell and cell-matrix interactions, such as the Hippo and Wnt pathways. In mouse models of Ras-driven PC, YAP1 is required for the formation of PanIN precursor lesions and their progression to invasive PC. In our telomerase-immortalized human pancreatic ductal cells, overexpressed YAP alone is sufficient to allow anchorage-independent growth. Our results show that in PC treated with Rac1 inhibitors (NSC23766, EHT-1864, AZA1, and Rac1T17N), YAP1 is rapidly and almost completely degraded within 24 hours. Importantly, this degradation of YAP1 is blocked by the preincubation of the cells with proteasome inhibitor MG-132, thereby suggesting the involvement of an ubiquitin ligase complex. A phosphodegron activated by the LATS1/2 kinases is present in YAP1 that promotes the degradation of the protein by the SCF-βTRCP ubiquitin ligase complex. Yet, the knockdown of neither the LATS1/2 kinases nor βTRCP affected the degradation of YAP1 after Rac1 inhibition. The knockdown of Elongin B and C, which are part of a different ubiquitin ligase complex controlling YAP1, also yielded similar results. A mutant of YAP1 lacking its LATS1/2 phosphorylation sites was also tested (YAP1-5SA). After Rac1 inhibition, this mutant was degraded with similar kinetics as the wild-type YAP1 protein. These findings reveal the existence of a novel ubiquitin ligase complex regulating YAP1 stability, this time in response to the inhibition of the Rac1 pathway. The screening of a siRNA library has now been undertaken to identify components of the ubiquitin ligase involved. The proposed work is expected to shed light on novel mechanisms underlying the development of PC and of other Ras-driven malignancies. Citation Format: Chitra Palanivel, Bailey Gabler, Ying Yan, Surinder K. Batra, Michel M. Ouellette. The small GTPase Rac1 controls the stability of the Yes-associated protein 1 (YAP1) independently of the LATS1/2 kinases and SCF-βTRCP ubiquitin ligase [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4366.