COORDINATE DEVELOPMENTAL EXPRESSION OF GENES REGULATING STEROL ECONOMY AND CHOLESTEROL SIDE-CHAIN CLEAVAGE IN THE PORCINE OVARY

To investigate the coordinate developmental expression of low-density lipoprotein (LDL) receptor, 3-hydroxy-3-methylglutaryl-coenzyme A (HMG -CoA) reductase, sterol carrier protein 2 (SCP2), steroidogenic acute regulatory protein (StAR), and cytochrome P450 side-chain cleavage (P4 50(scc)) enzyme messages throughout the pig estrous cycle, RNase prote ction analysis was performed using homologous (partially cloned) porci ne sequences. Total RNA was isolated from ovarian tissues from unstimu lated prepubertal gilts and gilts stimulated with eCG (Day -3) and hCG (Day 0) to induce follicular growth and ovulation. Specific transcrip ts (relative to 18S rRNA) were quantified in immature ovaries, preovul atory follicles (greater than or equal to 5 mm), corpora lutea (CL), a nd corpora albicantia. As an index of steroidogenesis, tissue progeste rone content (per microgram protein) was low in the unstimulated ovary and preovulatory follicles, and it began to increase 4 days post-hCG, peaked at 12 days, and returned to preovulatory concentrations by 20 days post-hCG. HMG-CoA reductase mRNA was expressed at low levels and did not change significantly throughout the estrous cycle. The amount of LDL receptor mRNA increased approximately 6-fold after eCG stimulat ion and was expressed at similar concentrations in both preovulatory f ollicles and functional CL. Expression of SCP2 mRNA did not differ amo ng the four tissue types but tended to be highest in midcycle (Day 12) CL compared other stages of CL (p = 0.007). StAR mRNA expression was minimal in unstimulated ovaries, was higher in preovulatory follicles (p = 0.014), and then rose again in CL (p = 0.009 compared with unstim ulated ovary). P450(scc) mRNA concentrations were low in unstimulated ovaries, increased in preovulatory follicles (p = 0.044), and increase d further in CL (p = 0.001 compared with preovulatory follicles). P450 (scc) and StAR mRNA levels correlated with progesterone levels (r = +0 .37, p = 0.025, and r = +0.71, p < 0.001, respectively). The expressio n of LDL receptor, StAR, and P450(scc) messages showed a dramatic decl ine by Day 20 post-hCG (p = 0.002, p = 0.003, p = 0.006, respectively, compared with CL) corresponding with functional regression of the CL. In summary, P450(scc) and StAR message expression are coordinately am plified during the pig follicular and luteal phase, whereas LDL recept or message after an initial increase is expressed at constitutively hi gh levels, thus indicating a differential regulation of ovarian sterol -metabolizing genes during the steroidogenic life of the follicle and CL.