Arthritis-induced increase in serum levels of IGF-binding protein-3 in rats is secondary to the decrease in its proteolytic activity

Abstract

Adjuvant-induced arthritis is a chronic inflammatory illness that induces a catabolic state, with a decrease in pituitary GH and hepatic IGF-I synthesis. We have previously observed an increase in serum IGF-binding protein-3 (IGFBP-3) in arthritic rats, and found that GH administration prevents the increase in circulating IGFBP-3 in arthritic rats. The aim of this work was therefore to study IGFBP-3 synthesis in the liver as well as its proteolysis in serum as the two possible causes of the increased circulating IGFBP-3 in arthritic rats. The effect of recombinant human GH (rhGH) administration was also analysed. Adult male Wistar rats were injected with complete Freund's adjuvant or vehicle, and 14 days later they were injected s.c. daily until day 22 after adjuvant injection with rhGH (3 IU/kg) or saline. Three hours after the last GH injection, all rats were killed by decapitation. Arthritis increased serum IGFBP-3 levels (P<0.01). The increase in serum IGFBP-3 levels in arthritic rats seems to be due to decreased proteolysis (P<0.01) rather than to an increased synthesis, since liver IGFBP-3 mRNA content was not modified by arthritis. GH administration to control rats resulted in an increase in both hepatic IGFBP-3 mRNA content and in serum IGFBP-3 levels in spite of the increase in IGFBP-3 proteolysis in serum. In arthritic rats, GH treatment did not modify liver IGFBP-3 synthesis, but it increased serum proteolysis of IGFBP-3, leading to a serum concentration of IGFBP-3 similar to that of control rats. Furthermore, there was a negative correlation between circulating IGFBP-3 and its proteolytic activity in the serum of adjuvant-induced arthritic rats. These data suggest that in chronic arthritis the increase in IGFBP-3 serum concentration is secondary to a decrease in proteolytic activity, rather than to an increase in hepatic IGFBP-3 gene expression.