Reproductive epidemiology of glial tumors may reveal novel treatments: high-dose progestins or progesterone antagonists as endocrino-immune modifiers against glioma.

Female gender, contraceptives, and menopausal hormone replacement treatments containing progesterone analogues associate with higher risk of meningiomas yet with lower risk of gliomas. Progesterone receptor (PR) expression and mifepristone treatment was highly discussed for meningiomas. However, much less is known in regard to progesterone actions in gliomas despite PR expression strongly correlates with their grade. Meningiomas and gliomas may grow faster during gestation; but paradoxically, parousity reduces lifetime risk of gliomas which can be explained with dichotomous cell growth-stimulating and inhibitory actions of progesterone at low versus high levels. Progesterone levels gradually increase in gestation up to 200-fold and the incidence of highly angiogenic brain tumors decreases in the last trimester. Indeed, progesterone stimulates glial tumor cell growth at low doses (10 nM) while induces cell kill at higher doses. During gestation, some immune pathways are activated to protect the mother and the fetus against microbial pathogens. In parallel, high-dose medroxyprogesterone acetate (MPA) used in treatment of endometrial carcinoma decreases tumoral expression of PR-B and increases infiltration of cytotoxic T lymphocytes and natural killer cells. MPA also synergies with IL-2 in clinical treatment of renal cancer. In both glioma and meningioma, the dominant cytosolic PR is PR-B which increases cell growth, while PR-A limits cell growth. This seems also paradoxical at the first glance due to opposite behavior of these tumors in diverse endocrine conditions. High-dose progestins may inhibit brain tumor growth by downregulating PR-B, yet the dosage thresholds may differ between glial and meningeal tumors due to higher total PR expression in meningiomas. Supporting this proposal, certain progestins were reported to stimulate meningioma growth in anecdotal reports, but same agents at much higher doses reduced meningioma cell proliferation in pilot clinical studies. PR antagonist mifepristone reduced meningioma growth in some clinical studies, but lacked efficacy in others. In fact, mifepristone also has partial PR agonist efficacy and acts in synergy with MPA to block EC growth. Hence, a similar mechanism of receptor downregulation may also account for mifepristone. Both MPA and mifepristone also harbor myeloprotective features against chemotherapy. Ulipristal is another contraceptive PR antagonist and exerts promising anticancer activity on drug-resistant ovarian cancer and BRCA1-mutant breast cancer cells, which can be tested in animal glioblastoma models. We propose that progestins strongly deserve to be investigated in experimental models of glioblastoma alone and in combination with immunostimulating agents.