Normal human and murine fibroblasts can inhibit proliferation of tumor cells when cocultured in vitro. The inhibitory capacity varies depending on the donor and the site of origin of the fibroblast. We showed previously that effective inhibition requires formation of a morphologically intact fibroblast monolayer before seeding of the tumor cells. Here we show that inhibition is extended to motility of tumor cells and we dissect the factors responsible for these inhibitory functions. We find that inhibition is due to two different sets of molecules: (i) the extracellular matrix (ECM) and other surface proteins of the fibroblasts, which are responsible for contact-dependent inhibition of tumor cell proliferation; and (ii) soluble factors secreted by fibroblasts when confronted with tumor cells (confronted conditioned media, CCM) contribute to inhibition of tumor cell proliferation and motility. However, conditioned media (CM) obtained from fibroblasts alone (nonconfronted conditioned media, NCM) did not inhibit tumor cell proliferation and motility. In addition, quantitative PCR (Q-PCR) data show upregulation of proinflammatory genes. Moreover, comparison of CCM and NCM with an antibody array for 507 different soluble human proteins revealed differential expression of growth differentiation factor 15, dickkopf-related protein 1, endothelial-monocyteactivating polypeptide II, ectodysplasin A2, Galectin-3, chemokine (C-X-C motif) ligand 2, Nidogen1, urokinase, and matrix metalloproteinase 3.tumor microenvironment | cancer-associated fibroblast | motility | extracellular matrix | soluble factors T he normal balance between epithelial cells and the surrounding stroma is disrupted during tumor development. Developing preneoplastic cells in the process of escaping from their intrinsic checkpoints that prevent illegitimate cell proliferation also have to overcome the microenvironmental forces that maintain the integrity of the normal tissue architecture. It is becoming increasingly clear that the normal microenvironment can restrict cancer development and progression (1-3). Inhibition of tumor cell growth by normal fibroblasts is one measurable manifestation of this multicomponential control. Part of this process is reflected by the ability of the tumor cell to corrupt the surrounding stroma and turn it from restrictive to supportive. The generation of cancer-associated fibroblasts (CAFs) that enhance angiogenesis and support tumor growth and spreading through the release of growth factors and cytokines is a case in point (1-5).We have departed from the observation of Stoker et al. that normal fibroblasts can inhibit the growth of admixed tumor cells upon contact (6). Having confirmed their findings, we have extended such findings into a high throughput microwell system and showed that the strength of the inhibition differs depending on the source of the fibroblasts. Moreover, such inhibition is contact dependent as well as requires an intact fibroblast monolayer (7, 8).Here we report the surprising finding that the inhi...