Mast cells are an essential hematopoietic component for polyp development.

It is generally agreed that most colon cancers develop from adenomatous polyps, and it is this fact on which screening strategies are based. Although there is overwhelming evidence to link intrinsic genetic lesions with the formation of these preneoplastic lesions, recent data suggest that the tumor stromal environment also plays an essential role in this disease. In particular, it has been suggested that CD34(+) immature myeloid precursor cells are required for tumor development and invasion. Here we have used mice conditional for the stabilization of beta-catenin or defective for the adenomatous polyposis coli (APC) gene to reinvestigated the identity and importance of tumor-infiltrating hematopoietic cells in polyposis. We show that, from the onset, polyps are infiltrated with proinflammatory mast cells (MC) and their precursors. Depletion of MC either pharmacologically or through the generation of chimeric mice with genetic lesions in MC development leads to a profound remission of existing polyps. Our data suggest that MC are an essential hematopoietic component for preneoplastic polyp development and are a novel target for therapeutic intervention.

Dissecting Therapeutic Resistance to ERK Inhibition.

The MAPK pathway is frequently activated in many human cancers, particularly melanomas. A single-nucleotide mutation in BRAF resulting in the substitution of glutamic acid for valine (V(600E)) causes constitutive activation of the downstream MAPK pathway. Selective BRAF and MEK inhibitor therapies have demonstrated remarkable antitumor responses in BRAF(V600) (E)-mutant melanoma patients. However, initial tumor shrinkage is transient and the vast majority of patients develop resistance. We previously reported that SCH772984, an ERK 1/2 inhibitor, effectively suppressed MAPK pathway signaling and cell proliferation in BRAF, MEK, and concurrent BRAF/MEK inhibitor-resistant tumor models. ERK inhibitors are currently being evaluated in clinical trials and, in anticipation of the likelihood of clinical resistance, we sought to prospectively model acquired resistance to SCH772984. Our data show that long-term exposure of cells to SCH772984 leads to acquired resistance, attributable to a mutation of glycine to aspartic acid (G(186D)) in the DFG motif of ERK1. Structural and biophysical studies demonstrated specific defects in SCH772984 binding to mutant ERK. Taken together, these studies describe the interaction of SCH772984 with ERK and identify a novel mechanism of ERK inhibitor resistance through mutation of a single residue within the DFG motif. Mol Cancer Ther; 15(4); 548-59. ©2016 AACR.

Discovery of a novel ERK inhibitor with activity in models of acquired resistance to BRAF and MEK inhibitors.

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 efficacy 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 identification 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.

Live imaging of cysteine-cathepsin activity reveals dynamics of focal inflammation, angiogenesis, and polyp growth.

It has been estimated that up to 30% of detectable polyps in patients regress spontaneously. One major challenge in the evaluation of effective therapy of cancer is the readout for tumor regression and favorable biological response to therapy. Inducible near infra-red (NIR) fluorescent probes were utilized to visualize intestinal polyps of mice hemizygous for a novel truncation of the Adenomatous Polyposis coli (APC) gene. Laser Scanning Confocal Microscopy in live mice allowed visualization of cathepsin activity in richly vascularized benign dysplastic lesions. Using biotinylated suicide inhibitors we quantified increased activities of the Cathepsin B & Z in the polyps. More than (3/4) of the probe signal was localized in CD11b(+)Gr1(+) myeloid derived suppressor cells (MDSC) and CD11b(+)F4/80(+) macrophages infiltrating the lesions. Polyposis was attenuated through genetic ablation of cathepsin B, and suppressed by neutralization of TNFalpha in mice. In both cases, diminished probe signal was accounted for by loss of MDSC. Thus, in vivo NIR imaging of focal cathepsin activity reveals inflammatory reactions etiologically linked with cancer progression and is a suitable approach for monitoring response to therapy.