Regulation of IGFBP-4 levels in human intestinal muscle by an IGF-I-activated, confluence-dependent protease.
Human intestinal smooth muscle cells in culture produce insulin-like growth factor-I (IGF-I), IGF binding protein-3 (IGFBP-3), IGFBP-4, and IGFBP-5, which can modulate the effects of IGF-I on growth. This study examined the role of IGFBP-4 on IGF-I-induced growth and the mechanisms regulating IGFBP-4 levels. IGFBP-4 inhibited IGF-I-induced [(3)H]thymidine incorporation. IGFBP-4 mRNA levels were not altered by IGF-I. IGF-I caused a concentration-dependent activation of an endogenous IGFBP-4 protease, resulting in time-dependent degradation of intact IGFBP-4 into inactive fragments. Protease activity was measured in a cell-free assay using smooth muscle cell conditioned medium containing the IGFBP-4 protease. The protease was inhibited by EDTA and benzamidine. Protease activity was highest in proliferating cells and lowest in postconfluent cells. The role of endogenous IGF-I in regulating IGFBP-4 degradation was confirmed by the ability of an IGF-I antagonist to inhibit IGF-I-activated IGFBP-4 proteolysis in intact cells. We conclude that in human intestinal smooth muscle cells levels of secreted IGFBP-4 are determined by the confluence-dependent production of a cation-dependent serine protease that is activated by endogenous IGF-I.
Kuemmerle
J F
JF
Department of Medicine, Medical College of Virginia Campus, Virginia Commonwealth University, Richmond, Virginia 23298, USA. jkuemmerle@hsc.vcu.edu
Teng
B
B
D002478:Cells, Cultured; D004789:Enzyme Activation / Q000187:drug effects / Q000502:physiology; D015870:Gene Expression / Q000187:drug effects / Q000502:physiology; D006801:Humans; D018974:Insulin-Like Growth Factor Binding Protein 4 / Q000235:genetics / Q000378:metabolism*; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D007583:Jejunum / Q000166:cytology / Q000201:enzymology*; D018485:Muscle Fibers, Skeletal / Q000166:cytology / Q000201:enzymology; D009130:Muscle, Smooth / Q000166:cytology / Q000201:enzymology*; D012333:RNA, Messenger / Q000032:analysis; D012697:Serine Endopeptidases / Q000378:metabolism*
D018974:Insulin-Like Growth Factor Binding Protein 4; D012333:RNA, Messenger; D007334:Insulin-Like Growth Factor I; D012697:Serine Endopeptidases