cancers Article Mechanical Stimulation Modulates Osteocyte Regulation of Cancer Cell Phenotype Stefaan W. Verbruggen 1,2,3, * , Clare L. Thompson 2,4 , Michael P. Duffy 1 , Sophia Lunetto 2 , Joanne Nolan 2,4,5 , Oliver M. T. Pearce 5 , Christopher R. Jacobs 1 and Martin M. Knight 2,4   Citation: Verbruggen, S.W.; Thompson, C.L.; Duffy,M.P.; Lunetto, S.; Nolan, J.; Pearce, O.M.T.; Jacobs, C.R.; Knight, M.M. Mechanical Stimulation Modulates Osteocyte Regulation of Cancer Cell Phenotype. Cancers 2021, 13, 2906. https:// doi.org/10.3390/cancers13122906 Academic Editor: Delphine Merino Received: 14 April 2021 Accepted: 7 June 2021 Published: 10 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Biomedical Engineering, Columbia University in the City of New York, New York, NY 10027, USA; michael.p.duffy@columbia.edu (M.P.D.); christopher.jacobs@columbia.edu (C.R.J.) 2 Centre for Predictive in vitro Models, School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, UK; clare.l.thompson@qmul.ac.uk (C.L.T.); s.lunetto@qmul.ac.uk (S.L.); joanne.nolan@qmul.ac.uk (J.N.); m.m.knight@qmul.ac.uk (M.M.K.) 3 Department of Mechanical Engineering and INSIGNEO Institute for in silico Medicine, University of Sheffield, Sheffield S1 3JD, UK 4 Queen Mary + Emulate Organs-on-Chips Centre, Queen Mary University of London, London E1 4NS, UK 5 Barts Cancer Institute, School of Medicine and Dentistry, Queen Mary University of London, London EC1M 5PZ, UK; o.pearce@qmul.ac.uk * Correspondence: s.verbruggen@sheffield.ac.uk Simple Summary: Metastatic lesions in bone tissue are a common complication in breast and prostate cancer patients, accounting for the larger part of pain and suffering in late-stage cancer. The metastatic cancer cells that form these lesions must travel from the primary tumour to a distant bone and enter a mechanically active environment that is largely regulated in response to physical exercise by bone cells known as osteocytes. This study used cell culture techniques to investigate if osteocytes can regulate breast and prostate cancer cells, and how mechanical stimulation of these sensitive bone cells affects cancer cell behaviour. Osteocytes signalled for decreased proliferation of cancer cells, but mechanical stimulation reversed this in breast cancer. By developing a microfluidic organ-chip model, we demonstrated the feasibility and importance of replicating the mechanical tumour microenvironment, finding increased invasion of cancer cells with mechanical stimulation. Abstract: Breast and prostate cancers preferentially metastasise to bone tissue, with metastatic lesions forming in the skeletons of most patients. On arriving in bone tissue, disseminated tumour cells enter a mechanical microenvironment that is substantially different to that of the primary tumour and is largely regulated by bone cells. Osteocytes, the most ubiquitous bone cell type, orchestrate healthy bone remodelling in response to physical exercise. However, the effects of mechanical loading of osteocytes on cancer cell behaviour is still poorly understood. The aim of this study was to characterise the effects of osteocyte mechanical stimulation on the behaviour of breast and prostate cancer cells. To replicate an osteocyte-controlled environment, this study treated breast (MDA-MB-231 and MCF-7) and prostate (PC-3 and LNCaP) cancer cell lines with conditioned media from MLO-Y4 osteocyte-like cells exposed to mechanical stimulation in the form of fluid shear stress. We found that osteocyte paracrine signalling acted to inhibit metastatic breast and prostate tumour growth, characterised by reduced proliferation and invasion and increased migration. In breast cancer cells, these effects were largely reversed by mechanical stimulation of osteocytes. In contrast, conditioned media from mechanically stimulated osteocytes had no effect on prostate cancer cells. To further investigate these interactions, we developed a microfluidic organ-chip model using the Emulate platform. This new organ-chip model enabled analysis of cancer cell migration, proliferation and invasion in the presence of mechanical stimulation of osteocytes by fluid shear stress, resulting in increased invasion of breast and prostate cancer cells. These findings demonstrate the importance of osteocytes and mechanical loading in regulating cancer cell behaviour and the need to incorporate these factors into predictive in vitro models of bone metastasis. Cancers 2021, 13, 2906. https://doi.org/10.3390/cancers13122906 https://www.mdpi.com/journal/cancers