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 porcine conceptus is transient, coincident with morphological development (trophoblast elongation), and temporally associated with maximal uterine release of insulin-like growth factor-I (IGF-I) into the luminal fluid. We investigated the possible linkages of conceptus steroidogenesis and IGF-I by characterizing the developmental expression of the pig conceptus cytochrome P450 aromatase (P450(arom)) and cytochrome P450 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 derived from porcine P450(arom) and P450(17 alpha) DNA sequences were used in reverse transcription-polymerase chain reaction analysis of conceptuses of differing morphologies and developmental stages. RNA transcripts for P450(arom) and P450(17 alpha) were readily detected only in conceptuses obtained at day 12 of pregnancy, with RNA levels varying according to morphological stage. P450(17 alpha) gene expression was maximal 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) spherical (large >> small), as well as day 12, filamentous conceptuses. Using primers derived from the human IGF-I receptor (IGF-IR) DNA sequence, constitutive expression of IGF-IR mRNA in porcine conceptuses throughout early pregnancy was demonstrated. Constitutively low expression of conceptus RNA transcripts for IGF-I ligand was similarly observed. However, uterine luminal fluid concentrations of IGF-I on day 12 were highly 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 distinct effects on P450(arom) mRNA abundance. Filamentous conceptuses had increased amounts of P450(arom) mRNA when treated with IGF-I. In contrast, IGF-I decreased the levels of P450(arom) mRNA in spherical conceptuses. IGF-I treatment did not alter mRNA expression of the IGF-IR gene in conceptuses of either morphology. These results demonstrate the transient expression of P450(arom) and P450(17 alpha) genes in preimplantation porcine conceptuses, document the constitutive expression of the conceptus IGF-IR gene, and support the involvement of endometrial IGF-I in modulation of embryonic P450(arom) mRNA abundance.