Molecular characterization of TRIB1 gene and its role in regulation of steroidogenesis in bos grunniens granulosa cells

To explore the expression pattern of the TRIB1 gene in yak follicles and its effect on the steroidogenesis of granulosa cells (GCs). Here, 4–5 years old female yaks were treated as the subjects. Immunohistochemically assay found that TRIB1 protein was expressed in different developmental follicles. Among different cell types of follicles, including cumulus cells (CCs), granulosa cells (GCs) and theca cells (TCs), the TRIB1 protein was most abundant in GCs (P < 0.0001). In addition, we cloned the coding sequence (CDS) of the yak TRIB1 gene, which is 1119 bp, encoding 372 amino acids (AA). The amino acid sequence homology of TRIB1 is >80% to those of other species, except for zebrafish. To further explore the function of TRIB1 in steroidogenesis, the pcDNA3.1(+)-TRIB1 eukaryotic expression vector was constructed and then transfected into GCs. The data showed that overexpression of TRIB1 significantly reduced the progesterone (P4) secretion of granulosa cells measured by ELISA assay (P < 0.05), but not Estradiol (E2) secretion. Consistently, TRIB1 gain-of-function downregulated the mRNA levels of steroidogenesis related genes steroidogenic acute regulatory protein (StAR), cytochrome P450 family 11 subfamily A member 1 (CYP11A1) and 3β-hydroxysteroid dehydrogenase (3β-HSD) (P < 0.01), while cytochrome P450 family 17 subfamily A member 1 (CYP17A1) and cytochrome P450 family 19 subfamily A member 1 (CYP19A1) had no significant difference (P > 0.05). Interestingly, mito-tracker staining showed that mitochondrial number significantly decreased in TRIB1 overexpressed GCs (P < 0.01). Further, overexpression of TRIB1 inhibited mRNA levels of mitochondrial biogenesis related genes, including Mitochondrial transcription factor (TFAM) and Peroxisome proliferator-activated receptor alpha co-activator (PPARGC1A) (P < 0.05). Conclusively, this work indicates that TRIB1 inhibited progesterone synthesis of GCs might be involved in the reduction of the mitochondria number.