Regulation of Ras signaling and oncogenesis by plasma membrane microdomains

Abstract

In this study, we assessed the contributions of plasma membrane (PM) microdomain targeting to the functions of H-Ras and R-Ras. These paralogues have identical effector-binding regions, but variant C-terminal targeting domains (tDs) which are responsible for lateral microdomain distribution: activated H-Ras targets to lipid ordered/disordered (Lo/Ld) domain borders, and R-Ras to Lo domains (rafts). We hypothesized that PM distribution regulates Ras effector interactions and downstream signaling. We used tD swap mutants, and assessed effects on signal transduction, cell proliferation, transformation, and tumorigenesis. R-Ras harboring the H-Ras tD (R-Ras-tH) interacted with Raf, and induced Raf and ERK phosphorylation similar to H-Ras. R-Ras-tH stimulated proliferation and transformation in vitro, and these effects were blocked by both MEK and PI3K inhibition. Conversely, the R-Ras tD suppressed H-Ras-mediated Raf activation and ERK phosphorylation, proliferation, and transformation. Thus, Ras access to Raf at the PM is sufficient for MAPK activation and is a principal component of Ras mitogenesis and transformation. Fusion of the R-Ras extended N-terminal domain to H-Ras had no effect on proliferation, but inhibited transformation and tumor progression, indicating that the R-Ras N-terminus also contributes negative regulation to these Ras functions. PI3K activation was tD-independent; however, H-Ras was a stronger activator of PI3K than R-Ras, with either tD. PI3K inhibition nearly ablated transformation by R-Ras-tH, H-Ras, and H-Ras-tR, whereas MEK inhibition had a modest effect on Ras-tH-driven transformation but no effect on H-Ras-tR transformation. R-Ras-tH supported tumor initiation, but not tumor progression. Whereas H-Ras-tR-induced transformation was reduced relative to H-Ras, tumor progression was robust and similar to H-Ras. H-Ras tumor growth was moderately suppressed by MEK inhibition, which had no effect on H-Ras-tR tumor growth. In contrast, PI3K inhibition markedly suppressed tumor growth by H-Ras and H-Ras-tR, indicating that sustained PI3K signaling is a critical pathway for H-Ras-driven tumor progression, independent of microdomains. In the second phase of the study, we investigated the combinatorial use of two drugs currently either in active use as anti-cancer agents (Rapamycin) or in clinical trials (OTX008), as a novel strategy to inhibit H-Ras-driven tumor progression. H-Ras anchored to the plasma membrane shuttles from the lipid ordered (Lo) domain to the lipid ordered/lipid disordered border upon activation, and retention of H-Ras at these sites requires Galectin-1 (Gal-1). We have previously found that genetically-mediated Lo sequestration of H-Ras inhibited MAPK signaling but not PI3K activation. Here we show that inhibition of Gal-1 with OTX008 sequestered H-Ras in the Lo domain, blocked H-Ras-mediated MAPK signaling, and attenuated H-Ras-driven tumor progression in mice. H-Ras-driven tumor growth was also attenuated by treatment with mTOR inhibitor Rapamycin, and this effect was further enhanced in tumors driven by Lo-sequestered H-Ras. These drugs also revealed bidirectional cross-talk in H-Ras pathways. Moreover, dual pathway inhibition with OTX008 and Rapamycin resulted in nearly complete ablation of H-Ras-driven tumor growth. These findings indicate that membrane microdomain sequestration of H-Ras with OTX008, coupled with mTOR inhibition, may support a novel therapeutic approach to treat H-Ras mutant cancers.