Effect of 1,25‐dihydroxyvitamin D₃ on mouse mammary tumor (GR) cells: Evidence for receptors, cellular uptake, inhibition of growth and alteration in morphology at physiologic concentrations of hormone

Mammary glands are target tissues for 1,25‐dihydroxyvitamin D₃ (1,25(OH)₂D₃). We have examined a mouse mammary tumor cell line (GR) for receptors of 1,25(OH)₂D₃ and have examined alterations in the growth and morphology of these cells in response to 1,25(OH)₂D₃. GR cells contain a high affinity (Kd ∼ 10⁻¹¹), lowcapacity receptor with a high specificity for 1,25(OH)₂D₃. The 1,25(OH)₂D₃ receptor in GR cells has a sedimentation coefficient of 3.5 and elutes from DEAE cellulose columns with ∼ 0.15 M KCI. These properties of the receptor are similar to those reported for other 1,25(OH)₂D₃ receptors. 1,25(OH)₂D₃ is internalized by GR cells in situ and specifically bound 1,25(OH)₂D₃ is found predominantly, if not entirely, in the nucleus as determined by cell fractionation and autoradiographic techniques. The incubation of GR cells in culture for 7 days with 1,25(OH)₂D, markedly alters cell growth. Cell growth is retarded in a dose‐dependent manner; physiologic concentrations (10⁻¹⁰ M) of l,25(OH)₂D₃ retard cell growth by approximately 50%. In addition, GR cells incubated with 10⁻⁹ to 10⁻⁸ M 1,25(OH)₂D₃ undergo marked morphological changes. The incubation of GR cells with other vitamin D metabolites such as 25‐hydroxyvitamin D₃ (25(OH)D₃) at a concentration of 10⁻⁹ M does not significantly alter cell growth or morphology. The presence of high affinity receptors for 1,25(OH)₂D₃, the specific internalization of 1,25(OH)₂D₃ predominantly into the nuclei, and the significant effects of physiological concentrations of 1,25(OH)₂D₃ on cell growth suggest a direct, specific, nuclear effect of 1,25(OH)₂D₃ on GR cells. The mouse mammary tumor model might be useful in examining the effect of 1,25(OH)₂D₃ on tumor formation.