Impaired cell cycle regulation of the osteoblast‐related heterodimeric transcription factor Runx2‐Cbfβ in osteosarcoma cells

Abstract

In mammals, bone differentiation requires the functional expression of the Runx2/Cbfβ heterodimeric complex. Our previous results indicate that Runx2 is also a suppressor of preosteoblast proliferation by affecting cell cycle progression at G 1 . Runx2 levels are cell cycle regulated, oscillating from a maximum during early G 1 to a minimum during late G 1 , S and mitosis phases in proliferating pre-osteoblasts Nevertheless, there is no information concerning Cbfβ gene expression during the cell cycle nor on Runx2 cell cycle expression in bone cancer cells. We analyzed Runx2 and Cbfβ gene expression during cell cycle progression in the preosteoblast MC3T3 and osteosarcoma ROS and SaOS cell lines. The expected reduction of Runx2 protein level was observed in MC3T3 cells arrested in late G 1 or M phase using mimosine or nocodazole, respectively. However, this reduction was not observed in the cell cycle arrested osteosarcoma cells. Cbfβ protein levels were not regulated during the cell cycle in pre-osteoblasts and osteosarcoma cells. Using cells synchronized in late G 1 and mitosis we found that Runx2 levels, but not Cbfβ levels, were cell cycle regulated in MC3T3 osteoblasts. Interestingly, both factors showed a constitutively elevated expression throughout the cell cycle in osteosarcoma cells. Proteasome inhibition by MG132 prevented cell cycle-dependent downregulation of Runx2 protein levels in osteoblasts, but not in osteosarcoma. We propose that Runx2 is involved in tumoral osteosarcoma progression. Altogether, deregulated Runx2 expression throughout the cell cycle seems to constitute a central mechanism in the pathogenesis of osteosarcoma.

MicroRNA-34c Inversely Couples the Biological Functions of the Runt-related Transcription Factor RUNX2 and the Tumor Suppressor p53 in Osteosarcoma

“…The latter observations represent 57% of these samples and corroborate previous findings with a larger patient cohort showing that RUNX2 is overexpressed in osteosarcomas that respond poorly to chemotherapy (4,30) as well as our initial findings with established OS cell lines (supplemental Fig. 1, A and B) (16,17,29). Together with other results reviewed elsewhere (1,4), it is apparent that RUNX2 and cell proliferation are not inversely coupled in a subset of osteosarcoma cells.…”

“…Two types of studies have produced the opposing findings that RUNX2 gene expression is either silenced (29,44) or amplified (4,12,(15)(16)(17) in OS. Our present data provide a plausible mechanism for the "Runx2 paradox" in which many OS cell lines and tumor biopsies express robust and sometimes elevated levels of RUNX2 as a cell growth suppressor.…”

“…Yet RUNX2 levels are frequently elevated in many but not all established OS cell lines in culture (16,17,29). Previous studies have shown that RUNX2 expression is selectively increased in subset of osteosarcomas and particularly in chemo-resistant tumors (1,4).…”

“…These results together clearly indicate that RUNX2 functions as a suppressor of cell proliferation in non-tumorigenic osteogenic cells. Therefore, it is necessary to resolve the apparent contradiction in the molecular etiology of bone-related cancers that the levels of RUNX2 are enhanced in a subset of OS (4,12,(15)(16)(17). Understanding the molecular basis for this RUNX2 paradox will not only provide insight into the pathophysiology of osteosarcomas but also that of non-osseous cancer cell types in which RUNX2 is ectopically expressed (18).…”

Characterization of canine osteosarcoma by array comparative genomic hybridization and RT‐qPCR: Signatures of genomic imbalance in canine osteosarcoma parallel the human counterpart

“…Of specific interest is the gene RUNX2, which resides within the orthologous regions defined by HSA 6p21.1 and CFA 12q13. Several recent studies have focused on investigating the dysregulation of RUNX2 in human OS Sadikovic et al, 2009Sadikovic et al, , 2010San Martin et al, 2009;Shapovalov et al, 2010), and the comparability of our data suggests that RUNX2 merits further investigation in both species.…”