Abstract 5340: Regulation of osteoblast differentiation: The pivotal role of CD99 molecule
Authors: Sciandra Marika , Marino Maria Teresa , Manara Maria Cristina , Lucarelli Enrico , Grano Maria , Picci Piero , Scotlandi Katia 
Published: June 2012

Abstract
Osteosarcoma (OS) is the most common primary bone malignancy in children and young adults. It displays a complex karyotype and a broad range of molecular abnormalities; however most of these alterations are not constantly detected in the majority of osteosarcoma tumors.
Although its precise origin still remains unknown, increasing findings suggest OS should be regarded as a differentiation disease caused by genetic and epigenetic changes which interrupt osteoblast differentiation from mesenchymal stem cells (MSCs).
Few sporadic data link CD99 molecule to osteoblast functions. It is a 32 kDa surface glycoprotein involved in several biological processes. We have previously demonstrated that CD99 acts as a tumor suppressor in osteosarcoma where it is either undetectable or present at very low levels. Being expressed in osteoblasts (OBs), the normal counterpart, we tested the hypothesis that CD99 may contribute to osteoblastic differentiation.
Its role was assessed, first of all, in physiological conditions. We provided evidence that CD99 is present in MSCs, considered the cells of origin for OS, and its expression increases during osteoblastogenesis and osteoblast maturation evaluated in MSCs as well as in OBs.
To study the function of CD99 in OS we took advantage of two osteosarcoma cell lines, Saos-2 and OS7, and their respective clones overexpressing the glycoprotein. Grown in osteogenic conditions for 21 days, these cells were able to differentiate and mineralize, as underlined by typical genes (collagen I, alkaline phosphatase and osteocalcin) as well as by specific stainings for matrix mineralization.
In presence of the molecule, cells were able to restore a normal phenotype, following the normal osteogenic program, thus allowing transition toward the latter step of differentiation. An effective proliferation arrest was reached in association with withdrawal from cell cycle in G1 phase, reduction of cyclin A, high expression of p21, and massive apoptosis induction.
The strong and prolonged activation of ERK in presence of CD99 suggested the involvement of MAP kinase in differentiation, as it was detected in MSCs under differentiating conditions. After PD98059 treatment, ERK inhibition really attenuated osteoblastic features in CD99 expressing cells.
In conclusion, these results give new insight into the role of the glycoprotein in osteoblastic differentiation. Its forced expression helps to overcome the impaired osteoblastogenesis that occurs in OS cells. These data can open new avenue for the understanding of mechanisms regulating OS pathogenesis and progression and they also suggest a possible new therapeutic approach to OS based on modulating tumor differentiation (grant from Italian Association for Cancer Research).
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5340. doi:10.1158/1538-7445.AM2011-5340