Abstract. Estrogen (E) exerts its function by binding to two intracellular estrogen receptors, ERα and ERβ. Although ERs have been reported to be expressed in the bovine corpus luteum (CL), the mechanisms that control ER expression in the bovine CL are not fully understood. To determine the possible regulatory mechanisms of ERα and ERβ that meditate distinct E functions, we examined 1) the changes in the protein expressions of ERs in the CL throughout the luteal phase and 2) the effects of prostaglandin (PG) F2α, tumor necrosis factor-α (TNFα) and interferon-γ (IFNγ) on the expressions of ERs in cultured bovine luteal cells. Western blot analyses revealed that ERα and ERβ proteins were expressed throughout the luteal phase. The ERα protein level was high at the early luteal (Days 2-3 after ovulation) and mid-luteal stages (Days 8-12) and was extremely low at the regressed luteal stage (Days 19-21). The ERβ protein level increased from the early to developing luteal stage, remained at the same level at the mid-luteal stage and decreased thereafter. The ratio of ERβ to ERα was higher in the regressed stage than in the other stages. Luteal cells obtained from mid-stage CLs were incubated with PGF2α (0.01-1 µM), TNFα (0.0145-1.45 nM) or IFNγ (0.0125-1.25 nM) for 24 h.PGF2α and TNFα inhibited ERα and ERβ mRNA expressions. IFNγ suppressed ERβ mRNA expression but did not affect the expression of ERα mRNA. However, the ERα and ERβ protein levels were not affected by any of the above treatments. These data indicate that PGF2α, TNFα and IFNγ regulate ERα and ERβ mRNA expressions in bovine luteal cells. Moreover, the changes in the ERβ/ ERα ratio throughout the luteal phase suggest that ERα is associated with luteal maintenance. Therefore, a dramatic decrease in ERα at the regressed luteal stage could result in progression of structural luteolysis in the bovine CL.

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