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Am J Physiol Gastrointest Liver Physiol 279: G975-G982, 2000;
0193-1857/00 $5.00
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Vol. 279, Issue 5, G975-G982, November 2000

Regulation of IGFBP-4 levels in human intestinal muscle by an IGF-I-activated, confluence-dependent protease

John F. Kuemmerle1,2 and Baiqin Teng2

Departments of 1 Medicine and 2 Physiology, Medical College of Virginia Campus, Virginia Commonwealth University, Richmond, Virginia 23298

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 [3H]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.

cell culture; growth regulation


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