CD99 drives terminal differentiation of osteosarcoma cells by acting as a spatial regulator of ERK 1/2.

Differentiation therapy is an attractive treatment for osteosarcoma (OS). CD99 is a cell surface molecule expressed in mesenchymalstem cells and osteoblasts that is maintained during osteoblast differentiation while lost in OS. Herein, we show that whenever OScells regain CD99, they become prone to reactivate the terminal differentiation program. In differentiating conditions, CD99‐transfected OS cells express osteocyte markers, halt proliferation, and largely die by apoptosis, resembling the fate of matureosteoblasts. CD99 induces ERK activation, increasing its membrane‐bound/cytoplasmic form rather than affecting its nuclearlocalization. Through cytoplasmic ERK, CD99 promotes activity of the main osteogenic transcriptional factors AP1 and RUNX2, whichin turn enhance osteocalcin and p21WAF1/CIP1, leading to G0/G1arrest. These data underscore the alternative positions of active ERKinto distinct subcellular compartments as key events for determining OS fate. © 2014 The Authors. Journal of Bone and MineralResearch published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research. This is an open accessarticle under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution inany medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made.