Prostaglandin synthesis and binding is increased in regressing NMU mammary carcinoma

Mammary tumors induced in Buffalo rats by treatment with nitrosomethyl urea will regress after oophorectomy. Their ability to synthesize and bind prostaglandins E and F2 alpha was studied in the growing and regressing states. Prostaglandins present in suspension of 100,000 xg tumor membranes after 2 hr incubation at 37 degrees C +/- 5x10(-4)M indomethacin were partially purified by silica gel column chromatography before assay by specific PG RIA. The amounts of PGE and F2 alpha synthesized rose from 0.13 and 10.5 ng/mg protein in the growing tumors to a maximum of 1.2 and 26.5 ng/mg protein 5 days after oophorectomy. Specific binding of 3H-PGE2 ad 3H-PGF2 alpha to 15,000 xg tumor membranes was achieved during a 45 min incubation at 23 degrees C +/- excess unlabelled PG. Free and bound prostaglandins were separated by filtration. Binding reached equilibrium after 30 min, was saturable and reversible. Scatchard analysis revealed high affinity binding of PGF2 alpha but only low affinity PGE2 binding in membranes obtained from growing tumors. A 2-3-fold increase in specific binding of PGE2 and PGF2 alpha was noted at 4 days after oophorectomy which represented an increase in the number of PGF2alpha receptors. PGE2 binding retained a low affinity character. The elevated PGF2 alpha synthesis rates observed in the regressing tumors coupled with a regression-associated increase in receptor number suggests that PGF2 alpha plays a significant role in hormone-dependent mammary tumor regression.