Abstract 237: Species-specific role of RAS in malignant transformation

Abstract Malignant transformation is a complex, multi-step process driven by the interplay between oncogene activation and tumor suppressor inhibition. The role of RAS in this process has been shown to exhibit striking species-specific differences. In mouse cells, RAS can effectively induce transformation with the inhibition of a single tumor suppressor. However, in human cells, RAS-driven transformation uniquely requires the inhibition of Protein Phosphatase 2A (PP2A). Despite over 20 years of investigation, the molecular explanation for this radical difference remains elusive. To address this fundamental gap in understanding, we performed a comprehensive phosphoproteogenomics analysis comparing mouse and human cells overexpressing oncogenic RAS.Transcriptomics, proteomics, and phosphoproteomics analyses revealed remarkably significant intra- and interspecies heterogeneity, highlighting the profound species-specific differences in RAS-induced signaling. Strikingly, while mTOR, CDK4-E2F1, and MYC signaling pathways were robustly activated by RAS in mouse cells, these activations were notably absent in human cell models. However, co-inhibition of the tumor suppressor phosphatase PP2A rendered human cells permissive to RAS-induced malignant transformation and restored pathway activities overlapping with RAS-only transformed mouse cells. Intriguingly, our findings suggest that RAS overexpression in human cells opposes phosphosignaling and pathway-targeted therapy responses, a phenomenon absent in mouse cells. This highlights a unique buffering role for RAS against transformation in human cells. Moreover, whereas RAS induced substantial signaling pathway connectivity in mouse cells, similar connectivity in human cells was observed only upon PP2A co-inhibition.Collectively, these findings reveal that, unlike in immortalized mouse cells, RAS signaling in human cells acts as a buffer against transformation unless PP2A is inhibited. This underscores the inadequacy of mouse models in representing RAS-driven signaling and transformation in human cells. Our results highlight the need for human cancer models to revolutionize the cancer therapies. Citation Format: Nikhil Gupta, Milla Majaniemi, Otto Kauko, Tero Aittokallio, Vera Gorbunova, Jukka Westermarck. Species-specific role of RAS in malignant transformation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 237.