Human chorionic gonadotropin induces an inverse regulation of steroidogenic acute regulatory protein messenger ribonucleic acid in theca interna and granulosa cells of equine preovulatory follicles.

The research article highlights the regulation of Steroidogenic Acute Regulatory Protein (StAR) in preovulatory follicles of horses, brought on by the administration of Human Chorionic Gonadotropin (hCG). It details the cloning of equine StAR and the interesting inverse regulation it plays in the theca interna and granulosa cells of equine follicles prior to ovulation.

Cloning of Equine StAR

• The study cloned equine StAR by screening an equine follicle cDNA library with a mouse StAR cDNA probe.
• This revealed two types of equine StAR, identified by the length of their 3′-untranslated region (3′-UTR); one long form consisted of 2918 base pairs while the shorter form contained 1599 base pairs.
• The long form cDNA had a 5′-UTR of 117 base pairs, an open reading frame (ORF) of 855 base pairs, and a 3′-UTR of 1946 base pairs.
• Primer extension analysis indicated that the cDNA clone was missing the first 10 base pairs of the primary transcript, giving a total of 127 base pairs for the full StAR 5′-UTR.
• The ORF encoded a 285-amino acid protein that is 86-90% identical to StAR of other species studied to date.

Regulation of StAR mRNA

• The regulation of StAR mRNA was studied within equine preovulatory follicles taken during estrus at varying times following an ovulatory dose of hCG.
• However, when the analysis was performed on an intact follicle wall, there were no significant changes in StAR mRNA levels after hCG treatment.
• Examination of isolated follicle cells revealed unanticipated regulation of StAR mRNA.
• In granulosa cells, there were no detectable StAR transcripts at the initiation of the experiment. However, by 30 hours post-hCG, the levels rose significantly. This increase was associated with an increase in follicular fluid concentrations of progesterone.
• In contrast, theca interna cells had high StAR mRNA levels at the start, which remained constant up until 33 hours post-hCG, after which they dropped dramatically.

Inferences and Conclusions

• The study outlined the primary structure of equine StAR and presented a documentation of the regulation of StAR mRNA in vivo in preovulatory follicles of a large monoovulatory species, such as the horse.
• The unique inverse regulation of StAR transcripts in the theca interna and granulosa cells of equine follicles before ovulation was also established, contributing to a novel understanding in the field.