Luteal expression of factors involved in the metabolism and sensitivity to oestrogens in the dog during pregnancy and in non-pregnant cycle

Abstract

The canine corpus luteum (CL) is the main source of reproductive steroids during dioestrus in the dog and remains active even in the absence of pregnancy (non-pregnant dioestrus, physiological pseudopregnancy). Whereas the biological effects of 17β-oestradiol (E2) in the canine CL remain unclear, the transcriptional availability of oestrogen receptors, ESR1 and ESR2, as well as other modulators of local availability of E2, for example, HSD17B7 (converts oestrone into oestradiol), SULT1E1 (inactivates E2 binding capacity to its own receptors through sulphonation) and STS (reverts E2 sulphonation), were previously detected in the CL of non-pregnant bitches. The aim of the present work was to evaluate the mRNA amounts of these factors involved in luteal sensitivity and metabolism of E2 in the canine CL during the course of non-pregnant dioestrus (days 10, 20, 30, 40, 50 and 60 post-ovulation, n = 5/group) and at different stages of pregnancy (n = 4-6/group): pre-implantation (days 8-12), post-implantation (days 18-25), mid-gestation (days 35-40) and prepartum luteolysis. During pregnancy, the availability of ESR1, HSD17B7, SULT1E1 and STS decreased from mid-pregnancy to prepartum luteolysis. The main findings during non-pregnant dioestrus were as follows: increased ESR2:ESR1 ratio on days 40 and 50 after ovulation, decreasing during luteal regression (day 60); increased STS at day 30 when SULT1E1 levels decreased; increased availability of SULT1E1 transcripts during luteal regression; and decreased amounts of HSD17B7 mRNA in early dioestrus, increasing towards later stages. These results suggest that E2 signalling and biologically active local concentrations could diverge in response to time and pregnancy status of the bitch.

Keywords: Corpus luteum; 17β-oestradiol; dog (Canis lupus familiaris); non-pregnant dioestrus; pregnancy.

FIGURE 1

Relative mRNA abundance of ESR1, ESR2 and the ESR2/ESR1 mRNA ratio determined by semi‐quantitative real‐time (TaqMan) PCR in the canine CL at selected stages of pregnancy and pseudopregnancy. Samples from pregnant animals were collected pre‐implantation (between days 8–12), post‐implantation (days 18–25) and at mid‐gestation (days 35–40) of pregnancy, or at the time of prepartum luteolysis. Time‐points from non‐pregnant dioestrus refer to days post‐ovulation. Data are presented as geometric mean ± geometrical standard deviation. In the case of the one‐way ANOVA reporting p < .05, analysis was followed by a Tukey–Kramer multiple comparisons post‐test. Bars with asterisks differ at: * = p < .05, ** = p < .01, *** = p < .001

FIGURE 2

Relative abundance and localization of HSD17B7, SULT1E1 and STS mRNA in the canine CL. (a‐f) Relative mRNA abundance was determined by semi‐quantitative real‐time (TaqMan) PCR. Samples from pregnant animals were collected pre‐implantation (between days 8–12), post‐implantation (days 18–25) and at mid‐gestation (days 35–40) of pregnancy, or at the time of prepartum luteolysis. Time‐points from non‐pregnant dioestrus refer to days post‐ovulation. Data are presented as geometric mean ± geometrical standard deviation. In the case of the one‐way ANOVA reporting p < .05, analysis was followed by a Tukey–Kramer multiple comparisons post‐test. Bars with asterisks differ at: * = p < .05, ** = p < .01, *** = p < .001. (g–i) The luteal localization of transcripts encoding for HSD17B7, SULT1E1 and STS was performed in mid‐pregnant dogs by in situ hybridization (ISH). Positive signals for all factors were mainly observed in luteal cells (closed arrows). mRNA encoding for HSD17B7 was further detected in endothelial cells (open arrowheads). No staining was observed in the negative controls (sense probe; insets in figures, at the same magnification)