Difference in physiological roles of phosphorylated and non-phosphorylated rat fetuin.

在本项目中，我们取得了如下成果：1.由肝细胞合成和分泌的大鼠胎球蛋白是磷酸化形式，并且这种磷酸化胎球蛋白（磷酸胎球蛋白）的生物合成速率被炎性细胞因子如IL-1（α和β）和TNF-α抑制，但它被肝细胞生长因子（HGF）、表皮生长因子和胰岛素等肝细胞生长因子刺激。这种对磷酸胎球蛋白生物合成的调节在蛋白质和mRNA水平上都得到了证实.磷酸胎球蛋白抑制HGF的生物活性，已知HGF是肝细胞的有效促分裂原。然而，非磷酸化胎球蛋白没有显示出抑制HGF活性的活性。对于磷酸胎球蛋白如何抑制HGF活性的机制，我们发现磷酸胎球蛋白与结合的HGF具有亲和力，因此磷酸胎球蛋白干扰HGF与其特异性受体c-Met的结合。这些观察结果表明，肝细胞合成的磷酸胎球蛋白可能是HGF的天然调节剂，作为7-chalone。非磷酸化大鼠胎球蛋白增强低浓度（0.1-0.5nM）甲状旁腺激素（PTH）诱导的骨吸收，通过<45>从^<45>Ca-预标记的大鼠桡骨释放^ Ca来测量，但在没有PTH的情况下，它对骨吸收没有活性。我们还发现，大鼠非磷酸化胎球蛋白增强组织蛋白酶L消化I型胶原。牛胎球蛋白和人胎球蛋白α_2-HS糖蛋白作为一种血浆唾液蛋白早已被人们所知，但其生理功能尚不清楚。从以上对大鼠胎球蛋白的观察中，我们发现了磷酸化和非磷酸化胎球蛋白/α_2-HS糖蛋白的生理作用之一。

In this project, we obtained following results.1. Rat fetuin synthesized and secreted by hepatocytes was phosphorylated form and the rate of biosynthesis of this phosphorylated fetuin (phosphofetuin) was suppressed by inflammatory cytokines such as IL-1 (alpha and beta) and TNF-alpha, but it was stimulated by growth factors to hepatocytes such as hepatocyte growth factor (HGF), epidermal growth factor and insulin. This regulation of biosynthesis of phosphofetuin was confirmed both by the levels of protein and mRNA.2. Phosphofetuin suppressed the biological activity of HGF which is known to be a potent mitogen to hepatocytes. However, non-phosphorylated fetuin did not show the activity to suppress the HGF activity. For the mechanism how phosphofetuin suppress the HGF activity, we found that phosphofetuin has affinity to bound HGF and thus phosphofetuin interfered with HGF binding to its specific receptor, c-Met. These observation suggest that phosphofetuin synthesized by hepatocytes may be a natural modulator of HGF as a 7chalone'.3. Non-phosphorylated rat fetuin enhanced bone resorption induced by low concentrations (0.1-0.5nM) of parathyroid hormone (PTH), as measured by release of ^<45>Ca from ^<45>Ca-prelabeled rat radius, but it had no activity on bone resorption without PTH.We also found that rat non-phosphorylated fetuin enhanced the digestion of type I collagen by cathepsin L.4. Although bovine fetuin and its human counterpart of alpha_2-HS glycoprotein is known to be a plasma sialoprotein for long time, its physiological role is not yet clear. From above observations using rat fetuin, we found one of physiological roles of phosphorylated and non-phosphorylated fetuin/alpha_2-HS glycoprotein.