Fibronectin (FN) is a major component of the tumor microenvironment, but its role in promoting metastasis is incompletely understood. Here, we show that FN gradients elicit directional movement of breast cancer cells, in vitro and in vivo. Haptotaxis on FN gradients requires direct interaction between α5β1 integrin and MENA, an actin regulator, and involves increases in focal complex signaling and tumor cell–mediated extracellular matrix (ECM) remodeling. Compared with MENA, higher levels of the prometastatic MENA^INV isoform associate with α5, which enables 3-D haptotaxis of tumor cells toward the high FN concentrations typically present in perivascular space and in the periphery of breast tumor tissue. MENA^INV and FN levels were correlated in two breast cancer cohorts, and high levels of MENA^INV were significantly associated with increased tumor recurrence as well as decreased patient survival. Our results identify a novel tumor cell–intrinsic mechanism that promotes metastasis through ECM remodeling and ECM-guided directional migration. Significance: Here, we provide new insight into how tumor cell:ECM interactions generate signals and structures that promote directed tumor cell migration, a critical component of metastasis. Our results identify a tumor cell–intrinsic mechanism driven by the actin regulatory protein MENA that promotes ECM remodeling and haptotaxis along FN gradients. Cancer Discov; 6(5); 516–31. ©2016 AACR. See related commentary by Santiago-Medina and Yang, p. 474. This article is highlighted in the In This Issue feature, p. 461

Cancer metastasis requires the invasion of tumor cells into the stroma and the directed migration of tumor cells through the stroma toward the vasculature and lymphatics where they can disseminate and colonize secondary organs. Physical and biochemical gradients that form within the primary tumor tissue promote tumor cell invasion and drive persistent migration toward blood vessels and the lymphatics to facilitate tumor cell dissemination. These microenvironment cues include hypoxia and pH gradients, gradients of soluble cues that induce chemotaxis, and ions that facilitate galvanotaxis, as well as modifications to the concentration, organization, and stiffness of the extracellular matrix that produce haptotactic, alignotactic, and durotactic gradients. These gradients form through dynamic interactions between the tumor cells and the resident fibroblasts, adipocytes, nerves, endothelial cells, infiltrating immune cells, and mesenchymal stem cells. Malignant progression results from the integrated response of the tumor to these extrinsic physical and chemical cues. Here, we first describe how these physical and chemical gradients develop, and we discuss their role in tumor progression. We then review assays to study these gradients. We conclude with a discussion of clinical strategies used to detect and inhibit these gradients in tumors and of new intervention opportunities. Clarifying the role of these gradients in tumor evolution offers a unique approach to target metastasis.

Collagen VI, an ECM protein up-regulated in triple-negative breast cancer and obese individuals, drives breast cancer metastasis.

Kinase inhibitor resistance often involves upregulation of poorly understood “bypass” signaling pathways. Here, we show that extracellular proteomic adaptation is one path to bypass signaling and drug resistance. Proteolytic shedding of surface receptors, which can provide negative feedback on signaling activity, is blocked by kinase inhibitor treatment and enhances bypass signaling. In particular, MEK inhibition broadly decreases shedding of multiple receptor tyrosine kinases (RTK), including HER4, MET, and most prominently AXL, an ADAM10 and ADAM17 substrate, thus increasing surface RTK levels and mitogenic signaling. Progression-free survival of patients with melanoma treated with clinical BRAF/MEK inhibitors inversely correlates with RTK shedding reduction following treatment, as measured noninvasively in blood plasma. Disrupting protease inhibition by neutralizing TIMP1 improves MAPK inhibitor efficacy, and combined MAPK/AXL inhibition synergistically reduces tumor growth and metastasis in xenograft models. Altogether, extracellular proteomic rewiring through reduced RTK shedding represents a surprising mechanism for bypass signaling in cancer drug resistance.Significance: Genetic, epigenetic, and gene expression alterations often fail to explain adaptive drug resistance in cancer. This work presents a novel post-translational mechanism of such resistance: Kinase inhibitors, particularly targeting MAPK signaling, increase tumor cell surface receptor levels due to widely reduced proteolysis, allowing tumor signaling to circumvent intended drug action. Cancer Discov; 6(4); 382–99. ©2016 AACR.This article is highlighted in the In This Issue feature, p. 331

In the adult brain, neural stem cells proliferate within the subventricular zone before differentiating into migratory neuroblasts that travel along the rostral migratory stream (RMS) to populate the olfactory bulb with new neurons. Because neuroblasts have been shown to migrate to areas of brain injury, understanding the cues regulating this migration could be important for brain repair. Recent studies have highlighted an important role for endocannabinoid (eCB) signaling in the proliferation of the stem cell population, but it remained to be determined whether this pathway also played a role in cell migration. We now show that mouse migratory neuroblasts express cannabinoid receptors, diacylglycerol lipase α (DAGLα), the enzyme that synthesizes the endocannabinoid 2-arachidonoylglycerol (2-AG), and monoacylglycerol lipase, the enzyme responsible for its degradation. Using a scratch wound assay for a neural stem cell line and RMS explant cultures, we show that inhibition of DAGL activity or CB₁/CB₂receptors substantially decreases migration. In contrast, direct activation of cannabinoid receptors or preventing the breakdown of 2-AG increases migration. Detailed analysis of primary neuroblast migration by time-lapse imaging reveals that nucleokinesis, as well as the length and branching of the migratory processes are under dynamic control of the eCB system. Finally, similar effects are observedin vivoby analyzing the morphology of green fluorescent protein-labeled neuroblasts in brain slices from mice treated with CB₁or CB₂antagonists. These results describe a novel role for the endocannabinoid system in neuroblast migrationin vivo, highlighting its importance in regulating an additional essential step in adult neurogenesis.