The importance to chondrocyte differentiation of changes in expression of the multiple inositol polyphosphate phosphatase

It is important to both physiological and pathological osteogenesis to understand the significance of changes in gene expression in growth-plate chondrocytes that transit between the proliferative and hypertrophic states. MINPP is one such gene of interest. The Minpp protein dephosphorylates highly phosphorylated inositol signaling molecules InsP₅ and InsP ₆. We show here that the ATDC5 chondrocyte progenitor cell line can recapitulate developmentally specific changes in MINPP expression previously only seen in longitudinal bone growth plates - both an initial 2-3-fold increase and a subsequent decrease back to initial levels during transition to hypertrophy. The increase in MINPP expression was accompanied by a 40% decrease in InsP₆ levels in ATDC5 cells. However, InsP₅ levels were not modified. Furthermore, throughout the hypertrophic phase, during which MINPP expression decreased, there were no alterations in InsP₅ and InsP₆ levels. We also created an ATDC5 line that stably overexpressed Minpp at 2-fold higher levels than in wild-type cells. This had no significant effect upon cellular levels of InsP₅ and InsP ₆. Thus, substantial changes in MINPP expression can occur without a net effect upon InsP₅ and InsP₆ turnover in vivo. On the other hand, Minpp-overexpressing cells showed impaired chondrogenesis. We noted that the expression of alkaline phosphatase activity was inversely correlated with the expression of MINPP. The ATDC5 cells that overexpress Minpp failed to show an insulin-dependent increase in alkaline phosphatase levels, which presumably affects phosphate balance [J. Biol. Chem. 276 (2001) 33995], and may be the reason cellular differentiation was impaired. In any case, we conclude that Minpp is important to chondrocyte differentiation, but in a manner that is, surprisingly, independent of inositol polyphosphate turnover.