1149 Expression of genes involved in the initial steps of steroidogenesis in adipose tissue depots of dairy cows during the dry period and early lactation

In view of the significant changes in body fat content related to parturition and lactation in dairy cows, together with the role of adipose tissue (AT) not only as a store of steroid hormones but also as a potential site of steroidogenesis, mobilization of body reserves may also alter steroid hormone secretion and eventually reproduction. With this background, our research objectives were (1) to assess the expression of two rate-limiting factors of steroidogenesis, i.e., steroidogenic acute regulatory protein (StAR) and Cytochrome P450scc (CYP11A1) in bovine AT, (2) to characterize the time course of their mRNA abundance during late pregnancy and early lactation, and (3) to compare this time course in a subcutaneous versus a visceral fat depot. StAR triggers cholesterol delivery to the inner mitochondrial membrane (IMM) where CYP11A1 then initiates steroidogenesis by converting cholesterol to pregnenolone, the precursor of all other endogenous steroids. Biopsies were collected from 20 Holstein cows from the subcutaneous (sc) and the retroperitoneal fat depot (rp) on d −42 and d 1, 21, and 100 relative to calving. The mRNA abundance of StAR and CYP11A1 was assessed in the tissue samples by qPCR and normalized using the 4 most stable reference genes. Data were analyzed using the MIXED procedure of SAS. In scAT, the StAR mRNA abundance was lowest on d −42, increased on d 1 (P < 0.05) until d 21 to remain at this level at d 100 (∼3-fold greater than on d −42; P < 0.001). Expression of StAR mRNA in rpAT increased with time of lactation, but differences between sampling days were limited to d −42 versus d 100 (3.3-fold increase; P = 0.01). Expression of CYP11A1 mRNA was not detectable in both AT with the protocol used herein (suitability of the protocol was confirmed with ovary as positive control). The increased expression of StAR mRNA abundance after calving in rpAT and scAT indicate an increased capacity for cholesterol uptake to the IMM. Given the apparently absent expression of CYP11A1, de novo synthesis of steroid hormones in bovine AT seems implausible and AT steroid metabolism likely depends on the uptake of preformed steroid precursors. Cholesterol reaching the IMM may thus rather be metabolized to oxysterols which were suggested to have regulatory functions in adipocytes.