Tumorigenesis driven by the BRAFV600E oncoprotein requires secondary mutations that overcome its feedback inhibition of migration and invasion.

BRAFV600E mutation occurs in 46% of melanomas and drives high levels of ERK activity and ERK-dependent proliferation. However, BRAFV600E is insufficient to drive melanoma in GEMM models, and 82% of human benign nevi harbor BRAFV600E mutations. We show here that BRAFV600E inhibits mesenchymal migration by causing feedback inhibition of RAC1 activity. ERK pathway inhibition induces RAC1 activation and restores migration and invasion. In cells with BRAFV600E, mutant RAC1, overexpression of PREX1, PREX2, or PTEN inactivation restore RAC1 activity and cell motility. Together, these lesions occur in 48% of BRAFV600E melanomas. Thus, although BRAFV600E activation of ERK deregulates cell proliferation, it prevents full malignant transformation by causing feedback inhibition of cell migration. Secondary mutations are, therefore, required for tumorigenesis. One mechanism underlying tumor evolution may be the selection of lesions that rescue the deleterious effects of oncogenic drivers.

STIM1-mediated calcium influx controls antifungal immunity and the metabolic function of non-pathogenic Th17 cells.

Immunity to fungal infections is mediated by cells of the innate and adaptive immune system including Th17 cells. Ca2+ influx in immune cells is regulated by stromal interaction molecule 1 (STIM1) and its activation of the Ca2+ channel ORAI1. We here identify patients with a novel mutation in STIM1 (p.L374P) that abolished Ca2+ influx and resulted in increased susceptibility to fungal and other infections. In mice, deletion of STIM1 in all immune cells enhanced susceptibility to mucosal C. albicans infection, whereas T cell-specific deletion of STIM1 impaired immunity to systemic C. albicans infection. STIM1 deletion impaired the production of Th17 cytokines essential for antifungal immunity and compromised the expression of genes in several metabolic pathways including Foxo and HIF1α signaling that regulate glycolysis and oxidative phosphorylation (OXPHOS). Our study further revealed distinct roles of STIM1 in regulating transcription and metabolic programs in non-pathogenic Th17 cells compared to pathogenic, proinflammatory Th17 cells, a finding that may potentially be exploited for the treatment of Th17 cell-mediated inflammatory diseases.

Inactivation of Fbxw7 Impairs dsRNA Sensing and Confers Resistance to PD-1 Blockade.

The molecular mechanisms leading to resistance to PD-1 blockade are largely unknown. Here, we characterize tumor biopsies from a patient with melanoma who displayed heterogeneous responses to anti-PD-1 therapy. We observe that a resistant tumor exhibited a loss-of-function mutation in the tumor suppressor gene FBXW7, whereas a sensitive tumor from the same patient did not. Consistent with a functional role in immunotherapy response, inactivation of Fbxw7 in murine tumor cell lines caused resistance to anti-PD-1 in immunocompetent animals. Loss of Fbxw7 was associated with altered immune microenvironment, decreased tumor-intrinsic expression of the double-stranded RNA (dsRNA) sensors MDA5 and RIG1, and diminished induction of type I IFN and MHC-I expression. In contrast, restoration of dsRNA sensing in Fbxw7-deficient cells was sufficient to sensitize them to anti-PD-1. Our results thus establish a new role for the commonly inactivated tumor suppressor FBXW7 in viral sensing and sensitivity to immunotherapy. SIGNIFICANCE: Our findings establish a role of the commonly inactivated tumor suppressor FBXW7 as a genomic driver of response to anti-PD-1 therapy. Fbxw7 loss promotes resistance to anti-PD-1 through the downregulation of viral sensing pathways, suggesting that therapeutic reactivation of these pathways could improve clinical responses to checkpoint inhibitors in genomically defined cancer patient populations.This article is highlighted in the In This Issue feature, p. 1241.

Omega-3 Fatty Acid Grafted PAMAM-Paclitaxel Conjugate Exhibits Enhanced Anticancer Activity in Upper Gastrointestinal Cancer Cells.

Upper Gastrointestinal Cancers (UGCs) are a leading cause of cancer-related deaths worldwide. Paclitaxel (PTX) is frequently used for the treatment of UGCs; however, low bioavailability, reduced solubility, and dose-dependent toxicity impede its therapeutic use. PAMAMG4.0 -NH2 -DHA is synthesized by linking amine-terminated fourth-generation poly(amidoamine) (PAMAMG4.0 -NH2 ) dendrimers with omega-3 fatty acid docosahexaenoic acid (DHA). Next, PAMAMG4.0 -NH2 -DHA-PTX (DHATX) and PAMAMG4.0 -NH2 -PTX (PAX) conjugates are synthesized by subsequent covalent binding of PTX with PAMAMG4.0 -NH2 -DHA and PAMAMG4.0 -NH2 , respectively. 1 H-NMR and MALDI-TOF analyses are performed to confirm conjugation of DHA to PAMAMG4.0 -NH2 and PTX to PAMAMG4.0 -NH2 -DHA. The cell viability, clonogenic cell survival, and flow cytometry analyses are used to determine the anticancer activity of PTX, PAX, and DHATX in UGC cell lines. The in vitro data indicate that treatment with DHATX is significantly more potent than PTX or PAX at inhibiting cellular proliferation, suppressing long-term survival, and inducing cell death in UGC cells.