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Am J Physiol Endocrinol Metab 276: E536-E542, 1999;
0193-1849/99 $5.00
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Vol. 276, Issue 3, E536-E542, March 1999

HaCaT human keratinocytes express IGF-II, IGFBP-6, and an acid-activated protease with activity against IGFBP-6

Joe A. Marinaro, Elke C. Hendrich, Kerri S. Leeding, and Leon A. Bach

Department of Medicine, University of Melbourne, Heidelberg, Victoria 3084, Australia

The insulin-like growth factor (IGF) system plays an important role in skin. HaCaT human keratinocytes proliferate in response to IGFs and synthesize IGF-binding protein-3 (IGFBP-3). Recently, IGFBP-6 was also identified by NH2-terminal sequencing, but it has not been identified by Western ligand blotting. In the present study, IGFBP-6 was detected in HaCaT-conditioned medium by use of immunoblotting and Western ligand blotting with 125I-labeled IGF-II. Proteolytic activity against IGFBPs, an important mechanism for regulation of their activity, was then studied. An acid-activated, cathepsin D-like protease that cleaved both IGFBP-6 and IGFBP-3 was detected. Although proteolysis did not substantially reduce the size of immunoreactive IGFBP-6, it greatly reduced the ability of IGFBP-6 to bind 125I-IGF-II as determined by Western ligand blotting and solution assay. HaCaT keratinocytes do not express IGF-I mRNA, but IGF-II mRNA and protein expression was detected. These observations suggest the possibility of an autocrine IGF-II loop that is regulated by the relative expression of IGF-II, IGFBP-3, and IGFBP-6, and IGFBP proteases in these keratinocytes, although demonstration of this loop requires further study.

insulin-like growth factor; binding protein; cathepsin D; autocrine growth factor


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