DEVELOPMENTAL REGULATION OF STEROIDOGENIC ENZYME GENE-EXPRESSION IN THE PERIIMPLANTATION PORCINE CONCEPTUS - A PARACRINE ROLE FOR INSULIN-LIKE GROWTH-FACTOR-I

During periimplantation, the estrogen biosynthetic capability of the p orcine conceptus is transient, coincident with morphological developme nt (trophoblast elongation), and temporally associated with maximal ut erine release of insulin-like growth factor-I (IGF-I) into the luminal fluid. We investigated the possible linkages of conceptus steroidogen esis and IGF-I by characterizing the developmental expression of the p ig conceptus cytochrome P450 aromatase (P450(arom)) and cytochrome P45 0 17 alpha-hydroxylase (P450(17 alpha)) genes during periimplantation (days 8, 10, 12, 15, and 18) and the potential regulation by IGF-I of conceptus P450(arom) messenger RNA (mRNA) content. Primer pairs derive d from porcine P450(arom) and P450(17 alpha) DNA sequences were used i n reverse transcription-polymerase chain reaction analysis of conceptu ses of differing morphologies and developmental stages. RNA transcript s for P450(arom) and P450(17 alpha) were readily detected only in conc eptuses obtained at day 12 of pregnancy, with RNA levels varying accor ding to morphological stage. P450(17 alpha) gene expression was maxima l in spherical conceptuses greater than 5 mm in diameter, whereas P450 (arom) mRNA was expressed in small (3-5 mm) and large (6-10 mm) spheri cal (large >> small), as well as day 12, filamentous conceptuses. Usin g primers derived from the human IGF-I receptor (IGF-IR) DNA sequence, constitutive expression of IGF-IR mRNA in porcine conceptuses through out early pregnancy was demonstrated. Constitutively low expression of conceptus RNA transcripts for IGF-I ligand was similarly observed. Ho wever, uterine luminal fluid concentrations of IGF-I on day 12 were hi ghly correlated with conceptus morphology, with maximal IGF-I in uteri possessing filamentous conceptuses. In vitro addition of IGF-I to day 12 conceptuses of filamentous or spherical morphologies resulted in d istinct effects on P450(arom) mRNA abundance. Filamentous conceptuses had increased amounts of P450(arom) mRNA when treated with IGF-I. In c ontrast, IGF-I decreased the levels of P450(arom) mRNA in spherical co nceptuses. IGF-I treatment did not alter mRNA expression of the IGF-IR gene in conceptuses of either morphology. These results demonstrate t he transient expression of P450(arom) and P450(17 alpha) genes in prei mplantation porcine conceptuses, document the constitutive expression of the conceptus IGF-IR gene, and support the involvement of endometri al IGF-I in modulation of embryonic P450(arom) mRNA abundance.