Cells respond to physical stimuli, such as stiffness¹, fluid shear stress²and hydraulic pressure^3,4. Extracellular fluid viscosity is a key physical cue that varies under physiological and pathological conditions, such as cancer⁵. However, its influence on cancer biology and the mechanism by which cells sense and respond to changes in viscosity are unknown. Here we demonstrate that elevated viscosity counterintuitively increases the motility of various cell types on two-dimensional surfaces and in confinement, and increases cell dissemination from three-dimensional tumour spheroids. Increased mechanical loading imposed by elevated viscosity induces an actin-related protein 2/3 (ARP2/3)-complex-dependent dense actin network, which enhances Na⁺/H⁺exchanger 1 (NHE1) polarization through its actin-binding partner ezrin. NHE1 promotes cell swelling and increased membrane tension, which, in turn, activates transient receptor potential cation vanilloid 4 (TRPV4) and mediates calcium influx, leading to increased RHOA-dependent cell contractility. The coordinated action of actin remodelling/dynamics, NHE1-mediated swelling and RHOA-based contractility facilitates enhanced motility at elevated viscosities. Breast cancer cells pre-exposed to elevated viscosity acquire TRPV4-dependent mechanical memory through transcriptional control of the Hippo pathway, leading to increased migration in zebrafish, extravasation in chick embryos and lung colonization in mice. Cumulatively, extracellular viscosity is a physical cue that regulates both short- and long-term cellular processes with pathophysiological relevance to cancer biology.

Cell migration on planar surfaces is driven by cycles of actin protrusion, integrin‐mediated adhesion, and myosin‐mediated contraction; however, this mechanism may not accurately describe movement in 3‐dimensional (3D) space. By subjecting cells to restrictive 3D environments, we demonstrate that physical confinement constitutes a biophysical stimulus that alters cell morphology and suppresses mesenchymal motility in human breast carcinoma (MDA‐MB‐231). Dorsoventral polarity, stress fibers, and focal adhesions are markedly attenuated by confinement. Inhibitors of myosin, Rho/ROCK, or β1‐integrins do not impair migration through 3‐μm‐wide channels (confinement), even though these treatments repress motility in 50‐μm‐wide channels (unconfined migration) by ≥50%. Strikingly, confined migration persists even when F‐actin is disrupted, but depends largely on microtubule (MT) dynamics. Interfering with MT polymerization/depolymerization causes confined cells to undergo frequent directional changes, thereby reducing the average net displacement by ≥80% relative to vehicle controls. Live‐cell EB1‐GFP imaging reveals that confinement redirects MT polymerization toward the leading edge, where MTs continuously impact during advancement of the cell front. These results demonstrate that physical confinement can induce cytoskeletal alterations that reduce the dependence of migrating cells on adhesion‐contraction force coupling. This mechanism may explain why integrins can exhibit reduced or altered function during migration in 3D environments.—Balzer, E. M., Tong, Z., Paul, C. D., Hung, W.‐C., Stroka, K. M., Boggs, A. E., Martin, S. S., Konstantopoulos, K. Physical confinement alters tumor cell adhesion and migration phenotypes. FASEB J. 26, 4045–4056 (2012). www.fasebj.org

The existence of distinct breast microbiota has been recently established, but their biological impact in breast cancer remains elusive. Focusing on the shift in microbial community composition in diseased breast compared with normal breast, we identified the presence of Bacteroides fragilis in cancerous breast. Mammary gland as well as gut colonization with enterotoxigenic Bacteroides fragilis (ETBF), which secretes B. fragilis toxin (BFT), rapidly induces epithelial hyperplasia in the mammary gland. Breast cancer cells exposed to BFT exhibit “BFT memory” from the initial exposure. Intriguingly, gut or breast duct colonization with ETBF strongly induces growth and metastatic progression of tumor cells implanted in mammary ducts, in contrast to nontoxigenic Bacteroides fragilis. This work sheds light on the oncogenic impact of a procarcinogenic colon bacterium ETBF on breast cancer progression, implicates the β-catenin and Notch1 axis as its functional mediators, and proposes the concept of “BFT memory” that can have far-reaching biological implications after initial exposure to ETBF. Significance: B. fragilis is an inhabitant of breast tissue, and gut or mammary duct colonization with ETBF triggers epithelial hyperplasia and augments breast cancer growth and metastasis. Short-term exposure to BFT elicits a “BFT memory” with long-term implications, functionally mediated by the β-catenin and Notch1 pathways. This article is highlighted in the In This Issue feature, p. 995

Mesothelin (MSLN) and cancer antigen125/mucin 16 (CA125/MUC16) are potential biomarkers for pancreatic cancer (PC) that are co-overexpressed at the invading edges of PC tissues, and their expression correlates with poor survival rates. However, the role of MSLN-MUC16 molecular interaction in PC cell motility and invasion has yet to be elucidated. Using sophisticated bioengineering and molecular biology tools, we report that the binding of MSLN to MUC16 markedly enhances PC cell motility and invasion via the selective induction of matrix metalloproteinase (MMP)-7. MSLN-mediated MMP-7 upregulation in MUC16-expressing PC cells occurs via a p38 MAPK-dependent pathway. Depletion of MMP-7 or inhibition of p38 activity abolishes MSLN-mediated PC motility and invasion. These findings provide a novel perspective on the enhanced invasive potential associated with MSLN and MUC16 co-overexpression, and the mechanism underlying MMP-7 activation in PC invasion and metastasis.