Characterization of the canine 3β-hydroxysteroid dehydrogenase and its expression in the corpus luteum during diestrus

3β-Hydroxysteroid dehydrogenase (3βHSD) is a key enzyme in the synthesis of bioactive steroid hormones. Objectives of the present study were to clone canine 3βHSD and to investigate its expression in dog corpora lutea (CL) covering the periods of their formation, early and late regression (days 5, 15, 25, 35, 45, 65 after ovulation). Complete complementary DNA sequence was amplified by RACE PCR. Subsequent cloning revealed that the canine ovarian 3βHSD transcript was composed of a 5′-untranslated region (5′-UTR) of 126 nucleotides, an open reading frame (ORF) of 1122 nucleotides and a 3′-UTR of 441 nucleotides. The putative ORF encoded a 374 amino acid protein which remains highly conserved (79–85% identity) between species. The transient expression of the cloned canine 3βHSD in a mammalian heterologous cell expression system (HEK293T cells) identified the 3βHSD activity as the only activity of this canine enzyme (absence of any detectable 17-hydroxysteroid dehydrogenase activity).Qualitative RT-PCR revealed expression of 3βHSD on all days investigated and the signals were strongest on days 5 and 15, with day 25 intensity tending to decrease. However, variability between individual animals was high. The significant decrease in the expression of 3βHSD towards the end of diestrus as indicated by Real Time PCR (p < 0.01) and immunhistochemistry may indicate that the provision of progesterone is controlled by availability of the enzyme rather than the substrate.