Alterations in insulin-like growth factor (IGF)-dependent IGF-binding protein-4 proteolysis in transformed osteoblastic cells.

Abstract

Insulin-like growth factor (IGF)-binding protein-4 (IGFBP-4) is secreted by a variety of osteoblastic cells and appears to be an integral component of bone cell physiology. We have previously reported that normal human osteoblast-like (hOB) cells secrete IGFBP-4 as well as a novel IGFBP-4 protease, which requires IGF for functional activity. In this study we assessed the IGFBP-4/IGFBP-4 protease system in transformed osteoblastic cells by Western ligand blotting and cell-free IGFBP-4 protease assays. Simian virus-40-immortalized hOB cells (HOBIT), human osteosarcoma cells (TE-85), and rat osteosarcoma cells (UMR 106-01, ROS 17/2.8) secrete IGFBP-4. In contrast to the rapid and dramatic proteolysis in hOB medium, medium conditioned by these cells had no apparent IGFBP-4 protease activity when assayed with exogenous IGF-II in culture or under cell-free conditions. Assayed in the presence of exogenous protease. HOBIT cells, but not the osteosarcoma cell lines, appeared to produce a cycloheximide-sensitive inhibitor of the IGFBP-4 proteolytic reaction. Transient cell transformation induced by incubating human osteoblasts transfected with a temperature-sensitive mutant of simian virus-40 T-antigen at the permissive temperature or by treating hOB cells with phorbol ester tumor promoters also resulted in inhibition of IGF-dependent IGFBP-4 proteolysis. Inhibition was observed if phorbol ester was added to the cultures at the time of medium change or after the protease had been expressed and secreted. Differences in IGFBP-4 proteolysis could not be accounted for by changes in IGFBP-4 messenger RNA expression or substrate levels. These data suggest that transformation is associated with alterations in the IGFBP-4/IGFBP-4 protease system in osteoblastic cells. Normal human osteoblasts secrete an IGF-dependent IGFBP-4 protease. The induction of an inhibitor of the IGF-dependent IGFBP-4 proteolytic reaction may be associated with early transformation processes. Fully tumorigenic bone cells expressed neither IGFBP-4 protease nor protease inhibitor activity.

• Authors: S K Durham, B L Riggs, S A Harris, C A Conover
• Journal: Endocrinology (Endocrinology) Vol. 136 Issue 4 Pg. 1374-80 (Apr 1995) ISSN: 0013-7227 [Print] United States
• PMID: 7534697 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't, Research Support, U.S. Gov't, P.H.S.)

Chemical References

• Carrier Proteins
• Insulin-Like Growth Factor Binding Protein 4
• Phorbol Esters
• Somatomedins
• Insulin-Like Growth Factor II
• Endopeptidases

Topics

• Animals
• Carrier Proteins (metabolism)
• Cell Line, Transformed
• Cells, Cultured
• Endopeptidases (metabolism)
• Humans
• Insulin-Like Growth Factor Binding Protein 4
• Insulin-Like Growth Factor II (pharmacology)
• Osteoblasts (enzymology)
• Osteosarcoma (metabolism)
• Phorbol Esters (pharmacology)
• Rats
• Simian virus 40
• Somatomedins (pharmacology)
• Temperature
• Tumor Cells, Cultured