Miyakoshi N, Qin X, Kasukawa Y, Richman C, Srivastava A K, Baylink D J, Mohan S
Musculoskeletal Disease Center, J. L. Pettis Veterans Administration Medical Center, Loma Linda, California 92357, USA.
Endocrinology. 2001 Jun;142(6):2641-8. doi: 10.1210/endo.142.6.8192.
Insulin-like growth factor (IGF)-binding protein-4 (IGFBP-4) is a potent inhibitor of IGF actions in vitro. However, we found that systemic administration of IGFBP-4 at pharmacological doses caused a significant increase in bone formation parameters in mice by a mechanism that may involve increased IGF bioavailability via proteolysis of IGFBP-4. To evaluate the hypothesis that proteolysis of IGFBP-4 is essential for the stimulatory effects of systemically administered IGFBP-4, we produced wild-type, protease-resistant, and IGFBP-4 proteolytic fragments and evaluated their effects using biochemical markers. Protease-resistant IGFBP-4 was more potent than wild-type IGFBP-4 in inhibiting IGF-I-induced mouse osteoblast cell proliferation in vitro and in inhibiting IGF-I-induced increase in alkaline phosphatase (ALP) activity in bone extract after local administration in vivo. Systemic administration of wild-type IGFBP-4, but not protease-resistant IGFBP-4, increased serum osteocalcin, serum ALP, and ALP in skeletal extracts in a dose-dependent manner, with a maximal effect of 40% (P < 0.05) at 1.25 nmol/mouse. Systemic administration of wild-type, but not protease-resistant, IGFBP-4 increased free IGF-I levels in serum in normal mice. IGF-I, but not wild-type IGFBP-4, increased bone formation parameters in IGF-I-deficient mice. This study demonstrates that systemic administration of IGFBP-4 increases bone formation parameters in mice by increasing IGF bioavailability in the circulation via an IGFBP-4 protease-dependent mechanism.
胰岛素样生长因子（IGF）结合蛋白4（IGFBP - 4）在体外是IGF作用的有效抑制剂。然而，我们发现以药理剂量全身给予IGFBP - 4会使小鼠的骨形成参数显著增加，其机制可能涉及通过IGFBP - 4的蛋白水解作用提高IGF的生物利用度。为了评估IGFBP - 4的蛋白水解作用对于全身给予IGFBP - 4的刺激作用至关重要这一假说，我们制备了野生型、抗蛋白酶型和IGFBP - 4蛋白水解片段，并使用生化标志物评估它们的作用。在体外抑制IGF - I诱导的小鼠成骨细胞增殖以及在体内局部给药后抑制骨提取物中IGF - I诱导的碱性磷酸酶（ALP）活性增加方面，抗蛋白酶型IGFBP - 4比野生型IGFBP - 4更有效。全身给予野生型IGFBP - 4（而非抗蛋白酶型IGFBP - 4）可使血清骨钙素、血清ALP以及骨骼提取物中的ALP呈剂量依赖性增加，在剂量为1.25 nmol/小鼠时最大效应为40%（P < 0.05）。全身给予野生型（而非抗蛋白酶型）IGFBP - 4可使正常小鼠血清中的游离IGF - I水平升高。IGF - I（而非野生型IGFBP - 4）可增加IGF - I缺乏小鼠的骨形成参数。本研究表明，全身给予IGFBP - 4通过一种依赖于IGFBP - 4蛋白酶的机制增加循环中的IGF生物利用度，从而增加小鼠的骨形成参数。
