TORC1 Suppression Predicts Responsiveness to RAF and MEK Inhibition in BRAF- Mutant Melanoma

Abstract

RAF and MEK (mitogen-activated or extracellular signal–regulated protein kinase kinase) inhibitors are effective in treating patients with BRAF-mutant melanoma. However, most responses are partial and short-lived, and many patients fail to respond at all. We found that suppression of TORC1 activity in response to RAF or MEK inhibitors, as measured by decreased phosphorylation of ribosomal protein S6 (P-S6), effectively predicted induction of cell death by the inhibitor in BRAF-mutant melanoma cell lines. In resistant melanomas, TORC1 activity was maintained after treatment with RAF or MEK inhibitors, in some cases despite robust suppression of mitogen-activated protein kinase (MAPK) signaling. In in vivo mouse models, suppression of TORC1 after MAPK inhibition was necessary for induction of apoptosis and tumor response. Finally, in paired biopsies obtained from patients with BRAF-mutant melanoma before treatment and after initiation of RAF inhibitor therapy, P-S6 suppression predicted significantly improved progression-free survival. Such a change in P-S6 could be readily monitored in real time by serial fine-needle aspiration biopsies, making quantitation of P-S6 a valuable biomarker to guide treatment in BRAF-mutant melanoma.

Treatment of Vemurafenib-Resistant SKMEL-28 Melanoma Cells with Paclitaxel

“…While levels of pERK were almost inhibited in A375P R and A375M R cells, its level was still quite high in SKMEL-28 R cells (Figure 3a). This result is somehow different from the previous reports (Corcoran et al, 2013;Girotti and Marais, 2013) in which expression levels of pERK in vemurafenib resistant cell lines were quite high. The difference in these patterns might be caused by different in exposure time to vemurafenib and the concentration of vemurafenib used for making acquired resistant cells.…”

Co-clinical assessment identifies patterns of BRAF inhibitor resistance in melanoma

“…None of these classified the resistant samples in the same way as did the full RPPA dataset (Supplemental Figure 5). Indeed, both the ERK signature and p-MEK were similar between the RTK and MAPK-rebound groups; instead, Figure 3C indicates that the main MAPK pathway difference between the 2 groups was downstream outputs including p-4EBP1, p-EIF4E, p-S6K, and p-S6, which have recently been shown to be prominent markers and mediators of BRAFi resistance (27,28). Furthermore, unsupervised clustering of the most variable 2.5% of the RNA-seq genes also did not generate the same 3 clusters (Supplemental Figure 5).…”

Identification of a Dual Inhibitor of Janus Kinase 2 (JAK2) and p70 Ribosomal S6 Kinase1 (S6K1) Pathways

“…Additionally, PUMA is known to antagonize all known anti-apoptotic Bcl-2 family members to induce mitochondrial dysfunction and apoptosis (56). Bim and PUMA in particular have been recognized for their pivotal role in oncogene inhibition-induced apoptosis (57)(58)(59)(60). The marked induction of all isoforms of Bim and PUMA is likely to be at least partially responsible for the strong anti-cancer effect observed after JAK2 and S6K1 inhibition.…”