Receptor-Mediated Mitogenic Action of Prolactin in a Rat Lymphoma Cell Line*

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Abstract
PRL [prolactin] and other lactogenic hormones are potent mitogens in a lymphoma cell line derived from a lymph node of an estrogenized Noble (Nb) rat. These cells (designated Nb2 node) possess receptors that bind only lactogenic hormones. There are .apprx. 12,000 receptor sites per cell. The kinetics of binding of [125I]iodo-PRL exhibited by Nb2 lymphoma cells is unusual in that PRL binding follows a biphasic pattern. Binding of [125I]iodo-PRL reaches a maximum in 1 h at 37.degree. C, followed by a rapid decline. This pattern was not observed if binding was carried out in the presence of chloroquine, a lyososomotropic agent, or if cell homogenate was used for binding. The relationship between receptor occupancy and the magnitude of PRL response in these cells was examined. Maximal growth stimulation by PRL occurs when only 35% of the maximal binding of PRL is achieved, suggesting that a majority of the PRL binding sites may be spare receptors. The Kd of the PRL receptors in Nb2 cells is 75 pM, approximately 20-fold higher than that of the receptors in other cell types. PRL at 6 pM produces a half-maximal growth response in the Nb2 cells. Antibodies against the PRL receptors are able to abolish the PRL-induced proliferation of Nb2 cells. In the absence of PRL, these antibodies alone can induce proliferation of N32 cells, mimicking the action of PRL. Divalent (Fab)2 fragments, but not monovalent Fab, were also effective. Apparently, antibodies initiate a biological response possibly by cross-linking PRL receptors on the cell membrane, and the entry of the PRL molecule, or fragments of it, into the intracellular compartment is not necessary for the biological action of PRL.