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Prolactin Binding and Localization in Rat Mammary Tumor Mast Cells¹

Department of Biochemistry, St. Jude Children's Research Hospital, Memphis, Tennessee 38101

We found that prolactin is taken up by mast cells residing in prolactin-dependent, 7,12-dimethylbenzanthracene-induced rat mammary tumors. Light and electron microscopic immunocytochemistry showed that mast cells concentrate prolactin in their cytoplasmic granules. No prolactin was found on mast cell surface membranes or in their nuclei. In primary cultures of tumor cells, mast cells were found mainly in the periphery of dome structures and these cells concentrated prolactin. When purified rat peritoneal mast cells were incubated with ¹²⁵I-labeled prolactin, uptake was time, energy, and temperature dependent. Seventy % of accumulated prolactin was released intact from cytoplasmic granules by C48/80-induced degranulation. A mouse mastocytoma cell line also took up and released prolactin. These cells contained prolactin receptors (K_d = 4.5 nM) as determined in whole cells (3150 sites/cell) and in crude membranes (180 fmol/mg protein). We conclude that mast cells might significantly influence mammary tumor growth by accumulating and releasing prolactin within tumor tissue.

¹ This work was supported in part by Grant CA 38190 and Cancer Center Support Grant CA 21765 from the National Cancer Institute as well as by the American Lebanese Syrian Associated Charities.

² To whom requests for reprints should be addressed, at Department of Biochemistry, St. Jude Children's Research Hospital, 332 N. Lauderdale, Memphis, TN 38101.

Received 9/21/87. Revised 12/15/87. Accepted 4/ 5/88.

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