Increased matrix metalloproteinase (MMP) expression and activities help to mediate tissue involution through increasing extracellular matrix remodeling and promoting dedifferentiation and, ultimately, apoptosis. Therefore, we hypothesized that prostaglandin (PG) F2α administration would decrease expression of the tissue inhibitor of metalloproteinase (TIMP)-1, -2, and -3 and effectively increase the MMP:TIMP ratio, leading to glandular involution. In experiment 1, we tested the effects of PGF2α administration (Day 10 postestrus; Day 0 = estrus) on luteal TIMP-1, -2, and -3 mRNA and protein expression. Corpora lutea were collected at 0, 15, or 30 min or at 1, 2, 4, 6, 12, 24, and 48 h following PGF2α administration (n = 3–9 animals/time point). Following PGF2α administration, TIMP-1 mRNA levels decreased (P < 0.05) at 1 and 2 h relative to 0 h (controls), then increased to levels greater than controls at 4 and 6 h. In contrast, TIMP-2 and -3 mRNA levels did not decrease following PGF2α administration. The TIMP-1, -2, and -3 proteins were localized to large luteal cells (LLCs) within control (untreated) tissues. However, histodepletion of TIMP-1 within LLCs was evident within 30 min (earliest time point collected) following PGF2α injection and continued through 48 h. Luteal concentration of TIMP-1, as determined by RIA, was decreased (P < 0.05) by 15 min (earliest time point collected) following PGF2α administration and remained low through 48 h. In contrast, TIMP-2 and -3 immunolocalization was not altered by PGF2α administration. Experiment 2 was conducted to determine if PGF2α could initiate the preceding changes in TIMP-1 in early (Day 3) corpora lutea that can bind PGF2α but are refractory to its luteolytic effects. Serum concentrations of progesterone and luteal concentrations of TIMP-1 mRNA and protein were similar at 0 and 6 h after PGF2α injection on Day 3 postestrus. These data suggest that an early and sustained effect of PGF2α is the specific depletion of TIMP-1 within LLCs that are capable of responding to the luteolytic action of PGF2α. This action may increase the MMP:TIMP-1 ratio, creating an environment that favors extracellular matrix degradation and, thereby, facilitates both functional and structural regression.
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