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

在这个项目中，我们得到了以下结果：肝细胞合成和分泌的大鼠胎蛋白以磷酸化形式存在，磷酸化后的胎蛋白（phosphofetuin）的生物合成速率受到IL-1 （α和β）和tnf - α等炎性细胞因子的抑制，但受到肝细胞生长因子（HGF）、表皮生长因子和胰岛素等生长因子的刺激。蛋白和mrna水平证实了这种对磷蛋白生物合成的调节。磷蛋白抑制HGF的生物活性，HGF是肝细胞的一种有效的丝裂原。而非磷酸化胎蛋白对HGF活性无抑制作用。对于磷蛋白抑制HGF活性的机制，我们发现磷蛋白与HGF具有亲和力，从而干扰了HGF与其特异性受体c-Met的结合。这些观察结果表明，肝细胞合成的卵黄蛋白可能是HGF的天然调节剂。非磷酸化大鼠胎儿素增强低浓度（0.1-0.5nM）甲状旁腺激素（PTH）诱导的骨吸收，通过^<45>Ca预标记的大鼠桡骨释放^<45>Ca来测量，但对没有PTH的骨吸收没有活性。我们还发现，大鼠非磷酸化的胎儿素增强了组织蛋白酶L.4对I型胶原的消化。虽然很早以前就知道牛胎蛋白和人α _2- hs糖蛋白是一种血浆唾液蛋白，但其生理作用尚不清楚。通过对大鼠胎蛋白的上述观察，我们发现了磷酸化和非磷酸化的胎蛋白/alpha_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.