Hidaka Kiyoshi, Kanematsu Takashi, Caffrey James J, Takeuchi Hiroshi, Shears Stephen B, Hirata Masato
Laboratory of Molecular and Cellular Biochemistry, Faculty of Dental Science, Kyushu University, Fukuoka 812-8582, Japan.
Exp Cell Res. 2003 Nov 1;290(2):254-64. doi: 10.1016/s0014-4827(03)00337-9.
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 InsP(5) 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 InsP(6) levels in ATDC5 cells. However, InsP(5) levels were not modified. Furthermore, throughout the hypertrophic phase, during which MINPP expression decreased, there were no alterations in InsP(5) and InsP(6) 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 InsP(5) and InsP(6). Thus, substantial changes in MINPP expression can occur without a net effect upon InsP(5) and InsP(6) 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.
了解生长板软骨细胞在增殖和肥大状态之间转变时基因表达变化的意义，对于生理性和病理性成骨过程均至关重要。MINPP就是这样一个令人感兴趣的基因。Minpp蛋白可使高度磷酸化的肌醇信号分子InsP(5)和InsP(6)去磷酸化。我们在此表明，ATDC5软骨细胞祖细胞系能够重现先前仅在纵向骨生长板中观察到的MINPP表达的发育特异性变化——在向肥大转变过程中，先是最初增加2 - 3倍，随后又降至初始水平。MINPP表达的增加伴随着ATDC5细胞中InsP(6)水平降低40%。然而，InsP(5)水平未发生改变。此外，在整个肥大期，MINPP表达降低，但InsP(5)和InsP(6)水平没有变化。我们还构建了一个ATDC5细胞系，其Minpp的稳定过表达水平比野生型细胞高2倍。这对InsP(5)和InsP(6)的细胞水平没有显著影响。因此，MINPP表达发生显著变化时，体内InsP(5)和InsP(6)的周转并无净效应。另一方面，过表达Minpp的细胞软骨形成受损。我们注意到碱性磷酸酶活性的表达与MINPP的表达呈负相关。过表达Minpp的ATDC5细胞未能显示出碱性磷酸酶水平的胰岛素依赖性增加，这可能影响磷酸盐平衡[《生物化学杂志》276 (2001) 33995]，可能是细胞分化受损的原因。无论如何，我们得出结论，Minpp对软骨细胞分化很重要，但其方式出人意料地独立于肌醇多磷酸的周转。
