Highlights d CDK4/6 inhibitors should be applied after and not before cytotoxic chemotherapy d CDK4/6 inhibitors prevent DNA repair after mitotic poisons and DNA-damaging agents d Inhibiting CDK4/6 results in an RB1-dependent repression of the DNA-repair machinery d CDK4/6 and PARP inhibitors cooperate in preventing tumor cell proliferation

No abstract

Summary As we age, our tissues are repeatedly challenged by mutational insult, yet cancer occurrence is a relatively rare event. Cells carrying cancer-causing genetic mutations compete with normal neighbors for space and survival in tissues. However, the mechanisms underlying mutant-normal competition in adult tissues and the relevance of this process to cancer remain incompletely understood. Here, we investigate how the adult pancreas maintains tissue health in vivo following sporadic expression of oncogenic Kras ( KrasG12D ), the key driver mutation in human pancreatic cancer. We find that when present in tissues in low numbers, KrasG12D mutant cells are outcompeted and cleared from exocrine and endocrine compartments in vivo . Using quantitative 3D tissue imaging, we show that before being cleared, KrasG12D cells lose cell volume, pack into round clusters, and E-cadherin-based cell-cell adhesions decrease at boundaries with normal neighbors. We identify EphA2 receptor as an essential signal in the clearance of KrasG12D cells from exocrine and endocrine tissues in vivo . In the absence of functional EphA2, KrasG12D cells do not alter cell volume or shape, E-cadherin-based cell-cell adhesions increase and KrasG12D cells are retained in tissues. The retention of KRasG12D cells leads to the early appearance of premalignant pancreatic intraepithelial neoplasia (PanINs) in tissues. Our data show that adult pancreas tissues remodel to clear KrasG12D cells and maintain tissue health. This study provides evidence to support a conserved functional role of EphA2 in Ras-driven cell competition in epithelial tissues and suggests that EphA2 is a novel tumor suppressor in pancreatic cancer.

The AKT‐mTOR pathway is a central regulator of cell growth and metabolism. Upon sustained mTOR activity, AKT activity is attenuated by a feedback loop that restrains upstream signaling. However, how cells control the signals that limit AKT activity is not fully understood. Here, we show that MASTL/Greatwall, a cell cycle kinase that supports mitosis by phosphorylating the PP2A/B55 inhibitors ENSA/ARPP19, inhibits PI3K‐AKT activity by sustaining mTORC1‐ and S6K1‐dependent phosphorylation of IRS1 and GRB10. Genetic depletion of MASTL results in an inefficient feedback loop and AKT hyperactivity. These defects are rescued by the expression of phosphomimetic ENSA/ARPP19 or inhibition of PP2A/B55 phosphatases. MASTL is directly phosphorylated by mTORC1, thereby limiting the PP2A/B55‐dependent dephosphorylation of IRS1 and GRB10 downstream of mTORC1. Downregulation of MASTL results in increased glucose uptake in vitro and increased glucose tolerance in adult mice, suggesting the relevance of the MASTL‐PP2A/B55 kinase‐phosphatase module in controlling AKT and maintaining metabolic homeostasis.

Purpose: To assess the preclinical efficacy, clinical safety and efficacy, and maximum tolerated dose (MTD) of palbociclib plus nab-paclitaxel in patients with advanced pancreatic ductal adenocarcinoma (PDAC). Experimental Design: Preclinical activity was tested in patient-derived xenograft (PDX) models of PDAC. In the open-label, phase 1 clinical study, the dose-escalation cohort received oral palbociclib initially at 75 mg/day (range 50‒125mg/day; modified 3+3 design; 3/1 schedule); intravenous nab-paclitaxel was administered weekly for 3 weeks/28 day cycle at 100‒125mg/m2. The modified dose–regimen cohorts received palbociclib 75mg/day (3/1 schedule or continuously) plus nab-paclitaxel (biweekly 125 or 100mg/m2, respectively). The prespecified efficacy threshold was 12-month survival probability of ≥65% at the MTD. Results: Palbociclib plus nab-paclitaxel was more effective than gemcitabine plus nab-paclitaxel in 3 of 4 PDX models tested; the combination was not inferior to paclitaxel plus gemcitabine. In the clinical trial, 76 patients (80% received prior treatment for advanced disease) were enrolled. Four dose-limiting toxicities were observed (mucositis [n=1], neutropenia [n=2], febrile neutropenia [n=1]). The MTD was palbociclib 100mg for 21 of every 28 days and nab-paclitaxel 125mg/m2 weekly for 3 weeks in a 28-day cycle. Among all patients, the most common all-causality any-grade adverse events were neutropenia (76.3%), asthenia/fatigue (52.6%), nausea (42.1%), and anemia (40.8%). At the MTD (n=27), the 12-month survival probability was 50% (95% CI, 29.9%–67.2%). Conclusions: This study showed the tolerability and antitumor activity of palbociclib plus nab-paclitaxel treatment in patients with PDAC; however, the prespecified efficacy threshold was not met.