Disruption of steroidogenesis by thermal stress in avian granulosa cells: Effects on 3β-HSD

Heat stress (HS) is one of the major factors that disrupt reproduction, at least in part through reduction of circulating levels of reproductive hormones. One of the key enzymes in the steroidogenic pathway is 3β-hydroxysteroid dehydrogenase (3β-HSD), which produces progesterone in females and androgens in males. Because progesterone is the main ovulatory/oviposition hormones in female birds, it is important for its activity to be normal. Four studies with laying hens were conducted to investigate the effect of HS on steroidogenesis. In female layers, HS reduced 3β-HSD activity in granulosa cells, and because the reduction was differential by strain, it was thought to be a good candidate for the genetic marker; therefore, the rest of the three studies with hens were focused on investigation of the mechanism behind the reduced 3β-HSD activity in granulosa cells under HS. The production of cAMP was not affected by HS, suggesting that the effect of HS on 3β-HSD is not via the adenylyl cyclase/cAMP pathway. Incubation with cAMP did not prevent disruption of 3β-HSD activity by HS, suggesting that it may not be essential for normal activity. Susceptibility to apoptosis of chicken granulosa cells was increased by HS, and incubation with gonadotropins may prevent apoptosis during HS. However, apoptosis seems to be not the main factor that interferes with steroidogenesis. The mRNA expressions of StAR, steroidogenic enzymes (cytochrome P450 side-chain cleavage and 3β-HSD), and gonadotropin (LH and FSH) receptors were not affected by HS, suggesting that these genes were not disrupted at the transcriptional level during HS. It is more likely that HS interferes with translation and/or that translated proteins remain in the inactive form. The mechanism behind the reduction of 3β-HSD activity in granulosa cells from HS hens is not clear from this study and 3β-HSD did not show clear strain differences at the genomic level; however, the activity of 3β-HSD is likely a good candidate for selecting heat resistant hens