The high frequency of activating RAS or BRAF mutations in cancer provides strong rationale for targeting the mitogen-activated protein kinase (MAPK) pathway. Selective BRAF and MAP-ERK kinase (MEK) inhibitors have shown clinical effi cacy in patients with melanoma. However, the majority of responses are transient, and resistance is often associated with pathway reactivation of the extracellular signal-regulated kinase (ERK) signaling pathway. Here, we describe the identifi cation and characterization of SCH772984, a novel and selective inhibitor of ERK1/2 that displays behaviors of both type I and type II kinase inhibitors. SCH772984 has nanomolar cellular potency in tumor cells with mutations in BRAF , NRAS , or KRAS and induces tumor regressions in xenograft models at tolerated doses. Importantly, SCH772984 effectively inhibited MAPK signaling and cell proliferation in BRAF or MEK inhibitor-resistant models as well as in tumor cells resistant to concurrent treatment with BRAF and MEK inhibitors. These data support the clinical development of ERK inhibitors for tumors refractory to MAPK inhibitors. SIGNIFICANCE: BRAF and MEK inhibitors have activity in MAPK-dependent cancers with BRAF or RAS mutations. However, resistance is associated with pathway alterations resulting in phospho-ERK reactivation. Here, we describe a novel ERK1/2 kinase inhibitor that has antitumor activity in MAPK inhibitor-naïve and MAPK inhibitor-resistant cells containing BRAF or RAS mutations. Cancer Discov; 3(7); 742-50.

The E2F1 transcription factor can promote proliferation or apoptosis when activated, and is a key downstream target of the retinoblastoma tumor suppressor protein (pRB). Here we show that E2F1 is a potent and specific inhibitor of β-catenin/T-cell factor (TCF)-dependent transcription, and that this function contributes to E2F1-induced apoptosis. E2F1 deregulation suppresses β- catenin activity in an adenomatous polyposis coli (APC)/glycogen synthase kinase-3 (GSK3)-independent manner, reducing the expression of key β-catenin targets including c-MYC. This interaction explains why colorectal tumors, which depend on β-catenin transcription for their abnormal proliferation, keep RB1 intact. Remarkably, E2F1 activity is also repressed by cyclin-dependent kinase-8 (CDK8), a colorectal oncoprotein1. Elevated levels of CDK8 protect β-catenin/TCF-dependent transcription from inhibition by E2F1. Thus, by retaining RB1 and amplifying CDK8, colorectal tumor cells select conditions that collectively suppress E2F1 and enhance the activity of β-catenin.

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