54 IMMUNOLOCALIZATION OF STEROID SULFATASE AND ESTROGEN-SPECIFIC SULFOTRANSFERASE IN BOVINE FOLLICLES

Abstract

Steroid hormones are regulators in the fine-tuned mechanism of follicular development in cattle. Their concentration and property can be modulated via different processes. Sulfoconjugation via sulfotransferases (SULT) changes them from being hydrophobic to hydrophilic molecules, thereby preventing them from diffusing freely across the lipid bilayer and necessitating a transport system like the sodium-dependent organic anion transporter (SOAT; SLC10A6). In addition, sulfated steroids can no longer bind to their nuclear receptors, rendering them biologically inactive. Steroid sulfatase (STS) removes the sulfate moiety from conjugated steroids, transforming them to the free active forms. Data about the concentration of sulfated steroids in follicular fluid and the expression of the related enzymes are limited in horses and cattle. Recently, transcripts of the corresponding enzymes have been detected in cumulus cells of immature bovine cumulus-oocyte complex. Furthermore, it has been shown that small amounts of sulphated steroids are present in bovine follicular fluid. The objective of the study was to investigate the steroid metabolizing enzymes at the protein level via immunohistochemistry (IHC). Bovine ovaries collected at a slaughterhouse were categorized according to their oestrus cycle stage. Three pairs of ovaries of each cycle stage (proestrus, oestrus, postestrus, interestrus) were collected. Morphological criteria (ovaries: presence/absence of follicles and/or CL, size and number of follicles; closure of the cervix, amount of mucus) were employed to categorize them. Samples for IHC were fixed in formalin. After washing and dehydration, the samples were finally embedded in paraffin and mounted onto blocks. Indirect immunoperoxidase staining methods were applied using the streptavidin-biotin technique for signal enhancement following standard procedures. Tissue sections of 3 µm were mounted onto glass slides, and a polyclonal rabbit anti-human STS and a polyclonal rabbit anti-bovine SULT1E1 primary antibody were used to detect STS or SULT1E1, respectively. Negative controls were included using nonspecific rabbit IgG. The evaluation of the staining was descriptive in relation to a positive control (bovine placenta). Immunostaining for STS was detected in granulosa cells of antral and secondary follicles and in the endothelium of blood vessels, irrespective of the stage of the oestrus cycle. Moreover, staining for SULT1E1 was restricted to granulosa cells in antral follicles, again irrespective of the stage of the oestrus cycle. However, staining intensity for STS and SULT1E1 was only weak. These data indicate for the first time the presence of the steroid metabolising enzymes STS and SULT1E1 in bovine ovaries at the protein level. However, due to the low expression level, further studies are needed to clarify the function. We gratefully acknowledge the financial support of the German Research Foundation (DFG; FOR 1369, WR 154/3–1).