Abstract 1870: Inhibition of Galectin-1 sensitizes oncogenic H-Ras to Rapamycin treatment

Abstract

Abstract Specific plasma membrane (PM) localization is essential for H-Ras signaling, and relies on post-translational modifications on the C-terminal targeting domain. H-Ras shuttles from the lipid ordered (Lo) domain to the lipid ordered/lipid disordered (Lo/Ld) border upon activation, which is dependent on Galectin-1. We have previously found that H-Ras, which is sequestered in the Lo domain by swapping the C-terminal targeting domain with the Lo-sequestering targeting domain of R-Ras, is deficient in MAPK signal propagation, while having no effect on PI3K activation, nor on H-Ras-driven tumor progression. We have further found that inhibition of PI3K with LY294002 inhibited tumor progression by H-Ras with or without Lo sequestration. Here we show that Lo sequestration of H-Ras attenuated, but did not completely block, H-Ras-induced mTOR signaling (S6kinase phosphorylation) despite similar activation of PI3K as H-Ras. Interestingly, MEK inhibition with U0126 diminished S6kinase phosphorylation by H-Ras, as well as by Lo-sequestered H-Ras. Here we demonstrate that H-Ras-driven allograft tumor growth in mice was substantially blunted upon treatment with mTOR inhibitor Rapamycin, and this effect of Rapamycin was further enhanced in tumors driven by Lo-sequestered H-Ras. Moreover, Rapamycin treatment ablated ERK phosphorylation in H-Ras tumors as well as in tumors with Lo-sequestered H-Ras (in which ERK phosphorylation was already greatly reduced). Together these findings indicate that Lo sequestration of H-Ras inhibits MAPK pathway activation, and that the MAPK pathway engages in crosstalk with mTOR pathways by regulating S6kinase phosphorylation downstream of H-Ras, whereas mTOR activity is required for H-Ras-induced MAPK signaling. These data further indicate that mTOR activation downstream of H-Ras is sufficient to drive tumorigenesis, but this pathway also requires H-Ras-MAPK activation. To recapitulate H-Ras Lo sequestration by the C-terminal targeting domain swap genetic model, we used a Galectin-1 inhibitor, OTX008, to disrupt H-Ras from transitioning between the Lo and Ld domains. OTX008 treatment alone inhibited H-Ras-driven allograft tumor growth to a similar extent as Rapamycin. However, a combination of OTX008 and Rapamycin resulted in nearly complete ablation of H-Ras-dependent tumor growth. These findings indicate that blockade of H-Ras targeting to the lipid ordered/disordered plasma membrane microdomain border, coupled with blockade of mTOR signaling, could provide a novel therapeutic approach to treat H-Ras-associated cancers. Citation Format: James V. Michael, Jeremy G T Wurtzel, Lawrence E. Goldfinger. Inhibition of Galectin-1 sensitizes oncogenic H-Ras to Rapamycin treatment. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1870.