Steroid hormone receptors in MCCLX, a transplantable lactogen-dependent mammary tumor of the rat.

MCCLX is a transplantable rat mammary tumor which, for sustained growth, requires the elevated levels of circulating lactogen provided by pregnancy or the implantation of an estrogen pellet. High affinity receptors for estradiol, as well as for the glucocorticoids, dexamethasone and triamcinolone acetonide and the progestin R5020 were measured in the cytosols of these tumors. Estrogen binding capacities were significantly lower in the cytosols of tumors from estrogen pellet treated animals compared with tumors from pregnant animals. Ligand exchange assays demonstrated that nuclei of tumors from estrogen-treated rats contained 3-4 times the estrogen receptors but that there was a definite decrease in total estrogen binding capacity compared with tumors from pregnant rats. It was concluded that this lactogen-dependent tumor contains steroid receptors with molecular properties similar to those of normal target tissues, including estrogen receptors capable of nuclear translocation, the levels of which are modulated by the specific growth conditions.