Bone morphogenetic protein 2 increases insulin-like growth factor I and II transcripts and polypeptide levels in bone cell cultures

Abstract

Insulin-like growth factors (IGF) I and II are among the most prevalent growth factors secreted by bone cells and are presumed to act as autocrine regulators of bone formation. Certain growth factors, synthesized by skeletal cells and known to stimulate the replication but not the differentiated function of cells of the osteoblastic lineage, have been shown to inhibit skeletal IGF-I and II synthesis. We postulated that growth factors with limited mitogenic activity and with differentiation-inducing properties, such as bone morphogenetic protein (BMP) 2, have the opposite effect and enhance IGF-I and II synthesis. We tested the effects of BMP-2 on IGF-I and II mRNA expression and polypeptide concentrations in cultures of osteoblast-enriched (OB) cells from 22 day fetal rat calvariae. Steady-state IGF-I and II mRNA levels were determined by northern blot analysis, and IGF-I and II concentrations were determined in acidified and fractionated culture medium by a specific radioimmunoassay. After 24-48 h of treatment, BMP-2 at 3.3 nM increased IGF-I and II transcripts by up to twofold and polypeptide levels by up to fourfold. BMP-2 was a more potent stimulator of IGF-II synthesis, and it was active at doses as low as 0.03 nM for IGF-II mRNA and 0.3 nM for IGF-II protein, whereas a dose of 3.3 nM was required to observe the effect on IGF-I synthesis. The effects of BMP-2 on IGF-I and II transcripts and polypeptide levels were dependent on protein synthesis and decreased in the presence of cycloheximide at 3.6 μM. In conclusion, BMP-2 increases skeletal IGF-I and II synthesis by increasing IGF-I and II transcript levels, and this effect may contribute to its actions on selected aspects of OB cell differentiated function.