AbstractThe functional role of INSL3 and its receptor RXFP2 in carcinogenesis is largely unknown. We have previously demonstrated (pro‐)cathepsin‐L as a target of INSL3 in human thyroid cancer cells facilitating penetration of tumor cells through elastin matrices. We demonstrate the expression of RXFP2 in human thyroid tissues and in mouse follicular thyroid epithelial cells using Cre‐recombinase transgene driven by Rxfp2 promoter. Recombinant and secreted INSL3 increased the motility of thyroid carcinoma (TC) cells in an autocrine/paracrine manner. This effect required the presence of RXFP2. We identified S100A4 as a novel INSL3 target molecule and showed that S100A4 facilitated INSL3‐induced enhanced motility. Stable transfectants of the human follicular TC cell line FTC‐133 expressing and secreting bioactive human INSL3 displayed enhanced anchorage‐independent growth in soft agar assays. Xenotransplant experiments in nude mice showed that INSL3, but not EGFP‐mock transfectants, developed fast‐growing and highly vascularized xenografts. We used human umbilical vein endothelial cells in capillary tube formation assays to demonstrate increased 2‐dimensional tube formations induced by recombinant human INSL3 and human S100A4 comparable to the effect of vascular endothelial growth factor used as positive control. We conclude that INSL3 is a powerful and multifunctional promoter of tumor growth and angiogenesis in human thyroid cancer cell xenografts. INSL3 actions involve RXFP2 activation and the secretion of S100A4 and (pro‐)cathepsin‐L.