In the present study, we investigated the time-dependent interactive effects of daily injections of prolactin (PRL) and corticosterone (CORT) on the activation of lymphocyte function and inhibition of tumor growth in vivo in mice. BALB/c mice were injected subcutaneously with EMT-6 fibrosarcoma cells (a murine connective tissue tumor cell derived from mammary gland), and then different groups of animals were treated with PRL (1 μg/g body weight [BW] ip) at 0h, 4h, 8h, 12h, 16h, or 20h after CRT (1 μg/g BW ip) daily for 10 days. Different control groups were vehicle treated or treated with either hormone alone. Mice were kept in constant light 1 week before and during injections and in a 14:10 light-dark cycle thereafter. Tumor progression was monitored for up to 21 days after the cessation of treatment, and thereafter spleen lymphocytes were harvested and tested for mitogen-triggered proliferation. Prolactin administration at 8h or 16-20h after corticosteroid treatment reduced tumor volume by 77% and 49%, respectively, relative to vehicle-treated controls. Other time relations of hormone treatment were ineffectual. Further studies indicated that the immunosuppressant cyclosporin A (CSA) substantially stimulated tumor growth; this effect was completely abrogated by a simultaneous 8h related hormone treatment. However, the 8h hormone treatment was ineffective in inhibiting tumor growth in T-cell- deficient nude mice. Spleen lymphocytes from tumor-bearing (TB) mice showed an elevated basal proliferative capacity stimulated by concanavalin A (ConA; a stimulus for T-cell proliferation) and lipopolysaccharide (LPS; a stimulus for B-cell proliferation) compared to non-TB mice. Spleen lymphocytes from TB mice treated with CORT and PRL at 8h intervals exhibited an increased spontaneous (as well as LPS- and ConA- triggered) proliferation (by 104%, 48%, and 70%, respectively) compared with vehicle control TB mice. Fluorescence-activated cell sorting (FACS) analysis of splenocytes from hormone-treated animals indicated a 34-100% increase in the CD4+ (e.g., T helper cell) population. Treatment of animals with either hormone alone did not inhibit tumor growth or stimulate immune function relative to vehicle controls. The daily rhythms of plasma PRL, CORT, and thyroxine were all substantially altered by the presence of tumor in these mice. These results indicate that appropriately timed daily treatment of PRL and CORT can attenuate tumor growth, in part, via activation of antitumor immune mechanisms. Collectively, these data suggest that circadian neuroendocrine activities must be temporally organized appropriately to inhibit tumor growth.
- Tumor immunology