MDCK cells secrete neutral proteases cleaving insulin-like growth factor-binding protein-2 to-6

Proteolysis of insulin-like growth factor-binding proteins (IGFBPs) may be an important mechanism to regulate IGF availability and IGF-independent fun ctions of IGFBPs. We analyzed the secretion of IGFBP proteases in Madin-Dar by canine kidney (MDCK) cells. The results showed that several specific pro teases were secreted, cleaving IGFBP-2 to -6 at neutral pH. The proteolytic activity against IGFBP-6 differed at least from IGFBP-5 protease activity in its sensitivity both to IGF-II and to the hydroxamic acid-based disinteg rin metalloprotease inhibitor, as well as serine protease inhibitors. Durin g partial purification steps, the serine protease inhibitor-sensitive fract ion with IGFBP-6 protease activity was separated from fractions characteriz ed by the presence of a 30-kDa disintegrin immunoreactive band. Whereas the IGFBP-4 and -6 proteases are predominantly secreted across the basolateral membrane, the majority of IGFBPs are sorted to the apical medium from filt er-grown cells. These studies indicate that the side-specific secretion of several distinct IGFBP proteases with partially overlapping IGFBP specifici ties may be another level in the regulation of IGF-dependent epithelial fun ctions.