Cellular localization and regulation of gene expression for components of the insulin-like growth factor ternary...

Insulin-like growth factors (IGFs) are present in the circulation, largely as part of a high mol wt complex including IGF-binding protein-3 (IGFBP-3) and an acid-labile subunit (ALS). This study used in situ hybridization to investigate the cellular sites of synthesis of these factors in the rat and to evaluate changes in transcript levels during development and after hypophysectomy and GH treatment. IGFBP-3 transcripts are considerably more abundant and widely expressed than ALS at birth, but both are present in liver and kidney. Hepatic IGFBP-3 gene expression increases slightly, whereas ALS increases dramatically in the first few weeks after birth. IGFBP-3 mRNA is concentrated in portal venous and sinusoidal endothelium, but is not detected in hepatocytes, whereas ALS mRNA is diffusely expressed by hepatocytes, but is not detected in nonparenchymal cells. Both transcripts are localized in the renal cortex; however, IGFBP-3 mRNA is concentrated in interstitial cells, whereas ALS is expressed in proximal tubule epithelium. Hypophysectomy results in a 90% reduction in hepatic ALS and an approximately 50% decrease in IGFBP-3 mRNA level. ALS, but not IGFBP-3, transcripts were also reduced in the kidney. GH receptor mRNA is coexpressed with ALS in liver and kidney, suggesting that the effects of GH on ALS gene expression may be direct. In summary, the fact that IGFBP-3 gene expression is far more widespread than that of ALS in both spatial and temporal parameters suggests that IGFBP-3 has a role apart from contribution to the ternary complex. We have also shown that IGFBP-3 and ALS are synthesized by distinct hepatic cell types in an anatomical organization that may serve to ensure efficient formation of the ternary complex in the blood passing through the sinusoids. Finally, the present data suggest that regulation of ALS synthesis may be the primary site of GH regulation of ternary complex formation.