PTHrP and Bcl-2: essential regulatory molecules in chondrocyte differentiation and chondrogenic tumors

Human chondrosarcomas (CS) are a frequent form of malignant bone tumors. The accurate distinction between benign solitary enchondroma and conventional CS of bones is a major diagnostic goal. Although the histological characteristics of chondrogenic tumors and the grading of CS (G1 to G3) have been defined by several authors, immunohistochemical markers for the different entities and grades are still missing and the mechanisms of tumorigenesis remain poorly understood. In addition to the emerging evidence that parathyroid hormone-related peptide (PTHrP) plays a critical role in endochondral bone formation we have recently reported that Bcl-2 lies downstream of PTHrP in the regulation of chondrocyte differentiation. To further characterize chondrogenic tumors and to determine whether PTHrP and the regulation of Bcl-2-expression is of relevance to tumorigenesis, we analyzed the expression of both PTHrP and Bcl-2 on a series of 23 cases of solitary enchondroma (9 cases) and primary CS (14 cases) using light and confocal microscopy. While all 9 enchondromas exhibited a detectable level of PTHrP-expression only, 2 showed low levels of immunoreactivity for Bcl-2. In sharp contrast, strong coexpression of Bcl-2 and PTHrP was found in 11 (composed of 3 CS G3, 7 CS G2, and one dedifferentiated CS) out of 14 CS, while the expression level of these proteins was below the detection limit in two CS G1 and one dedifferentiated CS. To verify this data 3 cases each of enchondroma, CS G2, and CS G3 respectively, were subjected to quantitative confocal analysis, after double labeling for PTHrP and Bcl-2. The results showed a significant increase in the expression of both PTHrP and Bcl-2, in malignant CS versus the benign enchondromas. Most interestingly, the levels of expression of both PTHrP and Bcl-2 correlated with the degree of malignancy of the chondrogenic tumors. These results therefore suggest that both PTHrP and Bcl-2 play a role in the tumorigenesis of chondrogenic tumors and further indicate that both proteins may participate in the same pathway regulating chondrocyte differentiation.