PROTEOLYSIS OF INSULIN-LIKE GROWTH FACTOR-BINDING PROTEINS (IGFBPS) WITHIN THE PIG UTERINE LUMEN ASSOCIATED WITH PERIIMPLANTATION CONCEPTUSDEVELOPMENT

Pig conceptuses undergo morphological development from spherical to fi lamentous forms during days 10 to 12 of pregnancy, coincident with a h igh content of mRNAs encoding insulin-like growth factor (IGF)-I in th e uterine endometrium and secretion of IGF-I into the uterine lumen. T he potential regulation by developing conceptuses of the bioavailabili ty of IGF-binding proteins (IGFBPs) within the uterine microenvironmen t was investigated. Uterine luminal flushings (ULFs) were obtained bet ween days 10 and 18 of pregnancy and the presence of specific IGFBPs w as detected by ligand blot analysis. ULFs collected at days 10 and II of pregnancy contained 46 and 43 kDa IGFBP-3, several IGFBPs of about 30 kDa including IGFBP-2, and an unidentified 26 kDa IGFBP; IGFBP-3 wa s the most abundant. By day 12, however, IGFBPs were substantially dim inished or undetectable. Examination of the morphology of flushed conc eptuses revealed that the loss of IGFBPs in ULF was associated with th e transition from spherical to filamentous morphology. The abundance o f IGFBP-3 mRNA in uterine endometrium, as monitored by blot-hybridizat ion, was not altered in a similar way, suggesting that lack of IGFBP-3 in 'filamentous' ULF resulted from proteolysis rather than from decre ased expression of the IGFBP-3 gene. Consistent with this, incubation of 'spherical' ULF with or without added 'filamentous' ULF at 37 degre es C resulted in the disappearance of endogenous IGFBP-3 only in 'sphe rical + filamentous' ULF. The protease activity in 'filamentous' ULF w as inhibited by EDTA, but unlike matrix metalloproteinases, was not zi nc ion-dependent or inhibited by 1,10-phenanthroline, Moreover, this a ctivity was partially inhibited by the serine protease inhibitor aprot inin, but not by 4-(2-aminoethyl)-benzenesulfonyl fluoride (AEBSF), a known inhibitor of plasmin. The IGFBP protease activity of ULF may the refore comprise a group of enzymes including an unidentified serine pr otease. The results suggest that elongating pig conceptuses induce IGF BP protease activity which may increase the intrauterine bioavailabili ty of IGF.