Internalization of urinary trypsin inhibitor in human uterine fibroblasts

Abstract

 We have characterized the molecular species and internalization of urinary trypsin inhibitor (UTI) in human uterine fibroblasts. Link protein (LP) has previously been identified as one of the cell-associated UTI binding proteins. The truncated forms of UTI were readily detectable in the cells after incubating the cells with purified UTI. Immunoblotting analysis with a panel of domain-specific antibodies revealed that the UTI species lacked the amino-terminal domain of UTI, but contained the carboxyl-terminal domain. We have examined whether LP is involved in the UTI internalization in the cells. Internalization of ¹²⁵I-labelled UTI was blocked by the intact UTI, but not by the carboxyl-terminal domain of UTI. Treatment with a polyclonal antibody to the UTI binding domain of LP partially inhibited UTI binding to the cells, but did not significantly prevent UTI internalization. In addition, preincubation of the cells with hyaluronidase reduced the UTI binding to the cells, but had no effect on the rate with which UTI was internalized. These data allow us to conclude that there are at least two different mechanisms for internalization of UTI. The major one is via unknown UTI receptors in a Ca²⁺, Mg²⁺-sensitive manner and another is via LP.