CD99 Acts as an Oncosuppressor in Osteosarcoma

Abstract

CD99 was recently reported to be under control of the osteoblast-specific transcription factor Cbfa1 (RUNX2) in osteoblasts, suggesting a role in the phato-physiology of these cells. No extensive information is available on the role(s) of this molecule in malignant phenotype, and osteosarcoma, in particular, has never been studied. We report that in 11 different cell lines and 17 clinical samples CD99 expression is either undetectable or very low. Being expressed in the normal counterpart, we tested the hypothesis that CD99 down-regulation may have a role in osteosarcoma development and progression. CD99-forced expression in two osteosarcoma cell lines significantly reduced resistance to anoikis, inhibited growth in anchorage independence as well as cell migration, and led to abrogation of tumorigenic and metastatic ability. Therefore, the molecule acts as a potent suppressor of malignancy in osteosarcoma. CD99 gene transfection induces caveolin-1 up-regulation and the two molecules were found to colocalize on the cell surface. Treatment with antisense oligonucleotides to caveolin-1 abrogates the effects of CD99 on migration. The findings point to an antioncogenic role for CD99 in osteosarcoma, likely through the regulation of caveolin-1 and inhibition of c-Src kinase activity.

CD99 isoforms dictate opposite functions in tumour malignancy and metastases by activating or repressing c-Src kinase activity

“…We have previously reported a significant reduction in local tumour and lung metastasis formation following forced expression of the CD99wt isoform in U-2 OS cells (Manara et al, 2006). Herein, we extend the study to prostate cancer, showing a wider role for CD99wt in the abrogation of metastasis.…”

“…Stable transfectants were selected in G418 and chosen for CD99 isoform expression on cell surface (Figure 1a and b). U-2 OS clones transfected with the long CD99wt isoform had already been described (Manara et al, 2006). Here we report the data of U-2/ CD99wt57 or U-2/CD99wt136 as additional controls to highlight the effects of the truncated CD99 variant in osteosarcoma cells.…”

“…c-src kinase activity is involved in CD99 functions and oppositely affected by the two isoforms Recently, we identified at least two molecules interacting with CD99 in U-2 OS cells, caveolin-1 and c-Src (Manara et al, 2006). C-Src is largely involved in pathophysiology of osteoblasts (Marzia et al, 2000) and CD99 isoforms differently affect tumour metastasis K Scotlandi et al prostate cancer (Jee et al, 2003;Recchia et al, 2003) along with the significant role that it plays in the regulation of crucial processes implicated in the pathogenesis and progression of tumours, such as cell cycle, migration, resistance to anoikis and anchorage independence (Irby and Yeatman, 2000).…”

“…In pathological conditions, the triggering of CD99 results in the induction of cell-cell adhesion and apoptosis of Ewing's sarcoma cells (Sohn et al, 1998;Scotlandi et al, 2000;Cerisano et al, 2004). Moreover, it has recently been found that CD99 downregulation is essential in osteosarcoma to express full malignancy (Manara et al, 2006). However, globally, the functions of CD99 in malignancies are still poorly understood.…”

“…However, it remains to be elucidated whether the CD99 expression may even display specific role/s in malignant phenotypes. Indeed, we recently found that the forced expression of the long CD99wt form acts as an oncosuppressor in osteosarcoma (Manara et al, 2006), whereas in Ewing's sarcoma, the abrogation of CD99 expression is required for in vitro malignant reversion (Kreppel et al, 2006), pointing out a dual role of the molecule as an oncosuppressor or oncogene depending on the cellular context. Here we extended the analysis to the functional effects of the truncated variant of CD99.…”