Different effects of bone morphogenetic proteins 2, 4, 12, and 13 on the expression of cartilage and bone markers in the MC615 chondrocyte cell line

In order to study the lineage leading to chondrocyte and osteoblast phenoty pe in vertebrate development, we examined the effect of recombinant human b one morphogenetic protein (BMP)-2, BMP-4, BMP-12 [or growth and differentia tion factor (GDF)-7], and BMP-13 (or GDF-6) on the phenotypic expression of the mouse chondrocyte cell line MC615, grown for 1 or 2 weeks in monolayer . Protein synthesis rates were monitored after incubation with [C-14]prolin e. BMP-2 and BMP-4 increased protein synthesis, in agreement with our obser vation by phase-contrast microscopy of a highly refractile matrix around MC 615 cells treated with BMP-2 and -4. Markers of the chondrocytic and osteob lastic differentiation were analyzed at mRNA level. Expression of the type II collagen gene, a marker of the cartilage phenotype, was up-regulated in the presence of low concentration of BMP-2 or -4 (50 ng/ mi) and down-regul ated at higher concentrations (100-400 ng/ml). In parallel, this expression was stable in the presence of BMP-12 or -13 in the dose range tested (50-4 00 ng/ml). Expression of the matrix Gla protein (MGP) gene, another marker of cartilage, was also reduced in the presence of 100 ng/ml BMP-2 or -4, wh ile it remained stable in the presence of BMP-12 or -13 at the same concent ration. In contrast, expression of the bone Gla protein (BGP) gene, or oste ocalcin, a marker of the bone phenotype, was induced when the cells were tr eated with BMP-2 or -4 but was not detected when the cells were treated wit h BMP-12 or -13. At the same time, BMP-2 or -4 markedly up-regulated expres sion of type X collagen mRNA, indicating that MC615 cells possess the abili ty to express traits associated with endochondral ossification, when expose d to specific BMPs. Furthermore, detailed analysis of type II collagen expr ession showed that the alternatively spliced transcript collagen IIB, speci fic for cartilage, is expressed concomitantly with BGP. Therefore, MC615 ch ondrocytes can simultaneously express chondrocytic and osteoblastic markers , in response to BMP-2 or -4, but show minimal response to BMP-12 (or GDF-7 ) or to BMP-13 (or GDF-6). These results raise the possibility that chondro cytes in vivo can express osteoblastic properties, provided they are induce d by BMP-2 or-4. (C) 1999 Academic Press.