Prolactin is an autocrine growth factor for the Jurkat human T-leukemic cell line

Despite convincing evidence of cooperation between IL-2 and endogenous prolactin (PRL) during T cell activation, the individual role of PRL as a T-cell lineage cytokine remains to be defined. We have examined the production and function of PRL on the Jurkat human T-leukemic cell line, which does not constitutively produce IL-2. The majority of Jurkat cells expressed PRL receptor (R) under standard culture conditions, whereas appearance of the alpha chain of the IL-2-R required PHA-PMA stimulation, as did IL-2 synthesis. Western blotting revealed a predominant band at 23.5 kDa and a weaker band at 25.5 kDa in both Jurkat cell lysates and human (h) pituitary PRL. Metabolic labeling of the cell lysates with 35S-methionine and immunoprecipitation with an antiserum against hPRL showed that both forms of PRL are actively synthesized by the Jurkat cell line. PRL released in the medium was biologically active in the rat Nb2 lymphoma mitogenic assay. Depletion of medium PRL with two polyclonal anti-hPRL antisera inhibited the growth of Jurkat cells in a dose-dependent manner, as evaluated by cell number and 3H-TdR uptake. Purified pituitary or recombinant hPRL at a wide range of concentrations had no significant effect on their growth, but reversed the blocking activity of the anti-hPRL antibody. Recombinant IL-2 had no effect on the antibody-induced growth inhibition. Taken as a whole, these results demonstrate that PRL can act as an autocrine T cell growth factor independently of IL-2 and are the first evidence of its involvement in human leukemic growth and possibly in leukemic transformation.