Expression of Steroid Receptors and Steroidogenic Enzymes in the Baboon (Papio anubis) Corpus Luteum during the Menstrual Cycle and Early Pregnancy1

As estrogen and progesterone are proposed regulators of luteal function, this study was undertaken to correlate the presence of receptors for these steroids with luteal function during early pregnancy. Corpora lutea (CL) were obtained from nonpregnant baboons during the midluteal [ML; days 7–8 postovulation (PO)] and late luteal (LL; days 11–12 PO) phases of the menstrual cycle or from pregnant baboons on days 18, 25, 29, or 31–33 PO. Estrogen and progestin receptors (ER and PR, respectively) and 3β-hydroxysteroid dehydrogenase (3βHSD) were detected by immunocytochemistry using specific monoclonal (H222 for ER; JZB39 for PR) or polyclonal (S683 for 3βHSD) antibodies. In addition, ribonucleic acid (RNA) was extracted from CL, processed for Northern blot analysis, and probed with complementary DNAs to human PR, human 3βHSD, and rat aromatase. Levels of messenger RNA (mRNA) for 3βHSD were quantified by laser densitometric scanning, and the data were normalized to the expression of a housekeeping gene (glyceraldehyde-3-phosphate dehydrogenase) to correct for loading differences. CL did not demonstrate specific nuclear stain for ER at any stage of the menstrual cycle or pregnancy. In contrast, PR-positive cells were present during the ML phase, but decreased during the LL phase (P < 0.05). PR-positive cells were maintained during early pregnancy at levels comparable to the ML phase (P > 0.05). Staining for 3βHSD was present at all stages of the cycle and pregnancy. Although the percent of 3βHSD-positive cells appeared to decrease as pregnancy proceeded, this was not statistically different (P > 0.05). The complementary DNA to PR hybridized to multiple transcripts (∼4.4, 3.1, 1.6, and 0.95 kilobases) in CL of the cycle. A single transcript (∼1.8 kilobases) for 3βHSD was present in CL at all stages of the cycle and pregnancy. The level of 3βHSD mRNA was highest during the ML phase and declined significantly (P < 0.05) during the LL phase and early pregnancy. Three transcripts (∼3.6, 3.0, and 1.7 kilobases) for aromatase were detected in CL of the cycle and pregnancy. Aromatase mRNA increased during early pregnancy. These results support the concept of PR-mediated events, but not ER-regulated processes in the primate CL. Furthermore, the data suggest that the steroidogenic enzymes 3βHSD and aromatase are differentially regulated during early pregnancy.