Expression of chorionic gonadotropin in the pituitary of the common marmoset (Callithrix jacchus)

Background: Hypophyseal luteinizing hormone (LH) and placental chorionic gonadotropin (CG) are glycoprotein hormones, essential for reproduction and sexual differentiation in primate species. Both are heterodimers and share a common α-subunit but are distinguished by a hormone-specific β-subunit. Both hormones bind to the luteinizing hormone receptor (LHR). Recent investigations in the common marmoset (Callithrix jacchus), a New World monkey, revealed that CG is expressed in both, the pituitary and the placenta, while LH could not be detected in the pituitary. Therefore the question arose whether CG is downregulated during pregnancy in the pituitary as LH is in the human.

Material and Methods: Organs of 4 female marmoset monkeys were dissected and snap-frozen when their serum CG level during pregnancy reached the highest concentration. A bioassay was performed to measure the CG concentration in native protein extracts of pituitaries. Furthermore absolute quantification of hypophyseal CGβ mRNA was performed by real time PCR.

Results and Discussion: At the protein level, the hypophyseal CG concentrations of two pregnant females were between the two values of hypophyseal CG-concentrations observed in non-pregnant control females and no clear trend was evident. The absolute quantification of CGβ mRNA showed a 14-fold decreased transcription in the pituitaries of pregnant females (n=2) in comparison to non-pregnant controls (n=2). This downregulation of CGβ-transcription in the marmoset is analogous to the downregulation of hypophyseal LH in humans and Old World monkeys. We conclude that expression of CG in the pituitary and placenta are differently regulated in the marmoset monkey. In the pituitary the negative feedback mechanism for CG-downregulation could be exerted via steroids, which obviously do not suppress placental CG production. In the pituitary the transcription is suppressed to a basal, constitutional level, while protein levels are maintained and CG is accumulated and stored intracellularly in hypophyseal cells during pregnancy.