STARD6 is expressed in steroidogenic cells of the ovary and can enhance de novo steroidogenesis

Abstract

STARD6 is a member of the StAR-related lipid transfer (START) domain family of proteins whose function thus far remains obscure. While it recently was shown to facilitate steroidogenesis in a cell-free setting, it has not been localized to steroidogenic cells of normal reproductive tissues. In a recent microarray study, we detected STARD6 mRNA in cultured porcine ovarian granulosa cells which are steroidogenic. In the present study, we examined regulation of STARD6 mRNA in porcine granulosa cultures, and found that it was not regulated by cyclic AMP, but it was reduced by combined knockdown of the transcription factors GATA4 and GATA6. We detected both STARD6 mRNA and protein in fresh granulosa cells and whole antral follicles and different stage corpora lutea of pig. The highest levels were discovered in the mid-luteal phase corpus luteum. Immunolocalization within ovarian tissues indicated robust STARD6 immunoreactivity in steroidogenic cells of the corpus luteum. Relatively lesser amounts of STARD6 signal were found in granulosa cells, theca cells, and oocytes. To test the ability of STARD6 to facilitate de novo steroidogenesis, non-steroidogenic COS-1 cells were co-transfected with components of the P450 cholesterol side-chain cleavage system, enabling them to make pregnenolone, and STARD6. STARD6 increased pregnenolone production by two- to three-fold over the empty vector control. In summary, STARD6 is found in the pig ovary, exhibits the strongest expression in highly steroidogenic luteal cells, and significantly enhances pregnenolone production in transfected COS cells independent of cyclic AMP treatment. Collectively, these findings indicate that STARD6 may contribute to steroidogenesis in ovarian cells, but also suggests other cellular functions that require cholesterol trafficking.

STARD6 on steroids: solution structure, multiple timescale backbone dynamics and ligand binding mechanism

“…In addition, its expression in testes decreases with ageing and may be associated with decreased spermatogenesis in aged rats 61 . By analogy, in the pig ovary, STARD6 is differentially expressed during the estrous cycle, in the mid-luteal phase corpora lutea 71 .…”

Estrogen biosynthesis in cultured skeletal muscle cells (L6) induced by amino acids

“…Transcripts of all enzymes in the mevalonate pathway for the production of cholesterol were significantly upregulated in the presence of high extracellular amino acid concentrations (fold changes from 2 to 6) [10], while transcripts of enzymes involved in the final conversion to biologically active forms of sex steroids showed overall lower magnitudes, although yet significant upregulations (1.5–3 fold; the “Results” section). Similar differences in transcript levels were also displayed by genes related to intracellular cholesterol trafficking or transfer of cholesterol to inner mitochondrial membranes, where StarD6 is reported to interact with the mitochondrial membrane moving cholesterol from outer to inner mitochondrial membranes, necessary for initiation of steroid synthesis [19, 20]. It was thus appealing to suggest that amino acids may induce de novo steroid biosynthesis in muscle cells.…”

Intracellular cholesterol transport proteins: roles in health and disease

“…Both STARD1 and STARD6 unfold and refold with similar (slow) kinetic patterns, and interact equally well with mitochondrial membranes [149]. Initially STARD6 was reported to be limited to male germ-line cells [84,150,151], but recent studies have shown that this protein is expressed in steroidogenic cells of the ovary, where it lies under the control of GATA4 and GATA6 transcription factors, and facilitates pregnenolone production [150]. Further, Chang and coworkers have reported STARD6 to be present in the brain, spinal cord and dorsal root ganglia, where it may play a role in the formation of neuroprotective steroids after excitotoxic brain injury [151][152][153].…”