Molecular cloning and expression of StAR protein in the canine corpus luteum during dioestrus.

Expression of the steroidogenic acute regulatory protein (StAR) in the canine corpus luteum (CL) was examined throughout the luteal phase covering the periods of CL formation, early and late regression. Following a homology cloning and characterization of a cDNA fragment of the canine StAR spanning the sequence coding for the open reading frame (ORF), which encodes a 286 amino acid protein being highly conserved (86-91%) between species, quantitative RT-PCR was performed with the respective primers. Expression of StAR was demonstrated on all days examined; mRNA-levels increased gradually in developing CL from day 5 until 25; a steep, 4-fold downregulation was observed on day 35 with a further gradual decrease thereafter. Similar data were obtained by immunohistochemistry and the effect of time was highly significant (p<0.0001). These data suggest that the progesterone production during the canine CL-phase is controlled by the provision of substrate at the level of cholesterol transfer to the inner mitochondrial membrane, a step which involves the StAR-protein activity.