Membrane progesterone receptors mediate progesterone-stimulated glycogenolysis in the bovine uterine epithelium

In livestock, the amount of glucose needed by the endometrium and embryo increases during early pregnancy. Yet, how glucose concentrations in the endometrium are regulated remains unclear. The bovine uterine epithelium can store glucose as glycogen, and glycogen content decreases in the luteal phase. Our objective was to elucidate the role of progesterone in glycogen breakdown in immortalized bovine uterine epithelial (BUTE) cells. After 48 h of treatment, progesterone decreased glycogen abundance in BUTE cells (P < 0.001) but did not alter glycogen phosphorylase levels. RU486, a nuclear progesterone receptor (nPR; part of the PAQR family) antagonist, did not block progesterone's effect, suggesting that progesterone acted through membrane progesterone receptors (mPRs). RT-PCR confirmed that BUTE cells express all five mPRs, and immunohistochemistry showed that the bovine uterine epithelium expresses mPRs in vivo. An mPRα agonist (Org OD 02-0) reduced glycogen abundance in BUTE cells (P < 0.001). Progesterone nor Org OD 02-0 affected cAMP concentrations. Progesterone increased phosphorylated AMP-activated protein kinase (pAMPK) levels (P < 0.001), indicating that progesterone increases intracellular AMP concentrations. However, AMPK did not mediate the effect of progesterone. AMP allosterically activates glycogen phosphorylase, and D942 (which increases intracellular AMP concentrations) decreased glycogen abundance in BUTE cells. A glycogen phosphorylase inhibitor partially blocked the effect of progesterone (P < 0.05). Progesterone and Org OD 02-0 had similar effects in Ishikawa cells (P < 0.01), a human cell line that lacks nPRs. In conclusion, progesterone stimulates glycogen breakdown in the uterine epithelium via mPR/AMP signaling. Glucose released from glycogen could support embryonic development or be metabolized by the uterine epithelium.