The proteolysis of insulin-like growth factor binding proteins in ovine uterine luminal fluid

Abstract

During days 12-15 after oestrus (day 0), the uterine luminal fluid (ULF)of both pregnant and non-pregnant ewes contains only two prominentinsulin-like growth factor binding proteins (IGFBPs) of 16-18 kDa and22-24 kDa which preferentially bind IGF-2. Immunoblotting with anIGFBP-3 antibody revealed these to be proteolytic fragments of IGFBP-3. Incontrast, the ULF from anoestrus and ovariec-tomized ewes contained intactIGFBP-3 (40-44 kDa) and IGFBP-2 (34 kDa). Co-incubation of ULF from ananoestrus ewe with that from a day 12 cycling ewe cleaved the IGFBP-3 presentinto the two lower molec-ular weight IGFBPs characteristic of ewes in the lateluteal phase of the oestrous cycle. The variation in pro-teolytic activityboth during the year and during the cycle suggested an influence ofprogesterone. Supplementation of progesterone to long-term ovariectomized ewesvia a CIDR-G™ breeding device for 5, 10 or 15 days induced markedproteolytic activity in all 10-day treated sheep. The ULF from the 15-daytreated ewes showed reduced activity and could inhibit the activity present in10-day ULF, suggesting the induction of an inhibitor after prolonged exposureto progesterone treatment. A possible role of IGFBP-3 proteolysis in the ovineULF may be to selectively increase the bioavailability of IGF-1 in the uterinemicroenvironment, which may be crucial for the rapid elongation of trophoblastthat begins during days 12-15 after mating.