Mouse mammary tumor virus production stimulated by hormones and polyamines in cells grown in semi‐synthetic in vitro conditions

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Abstract
Hormonal regulation of mouse mammary tumor virus (MMTV) synthesis was studied in the CCL‐51‐SF cell subline derived from the Sykes' mammary tumor cell line CCL‐51 and adapted to grow in semisynthetic in vitro conditions. The virus was quantitated by measuring the supernatant reverse transcriptase activity in exogenous reaction using poly (rA) · oligo (dT) and poly (rC) · oligo (dG) as template/primers. The cells produced a low but significant amount of virus in the absence of any hormones and serum proteins. The synthetic glucocorticoid dexamethasone increased production considerably, up to 100‐fold. Pretreatment of CCL‐51‐SF cells with serum or 5‐bromodeoxyuridine, BrdUrd, partly reduced the stimulation by dexamethasone of MMTV production. Insulin and prolactin alone or in combination had no stimulating effect on spontaneous MMTV synthesis and cell growth. Prolactin, and more efficiently the prolactin‐insulin combination, enhanced the MMTV production stimulated with dexamethasone. Insulin alone remained without effect. The polyamine spermidine, but not spermine, increased the MMTV production over the control by a factor of 2. Polyamines did not influence cell replication at the concentrations used.