Hormone synergism in the in vitro production of the mouse mammary tumor virus.

The production of mouse mammary tumor virus (MMTV) in primary cell cultures of BALB/cfC3H mammary tumor cells was measured by radioimmune assay and RNA=dependent DNA polymerase activity. Maximum virus production was dependent on cell density, nutritional milieu, and hormone supplementation. The addition of insulin (u), estradiol-17 beta (E2), progesterone (P), prolactin (PRL), or thyroxine (T3) alone had little or no effect on MMTV production. Hydrocortisone (F) had a primary stimulatory effect. The combination of I, F, and T3 increased MMTV levels. The combinations containing I, F, and E2 had the greatest stimulatory effect. The stimulation of MMTV production was dose dependent. These experiments demonstrate that a variety of hormones act in a synergistic manner to stimulate MMTV production.