Inhibitory effect of 1α,25-dihydroxyvitamin D3 on the growth of the renal carcinoma cell line

Inhibitory effect of 1α,25-dihydroxyvitamin D3 on the growth of the renal carcinoma cell line. We studied the effect of vitamin D compounds on the growth of the human renal carcinoma cell line (KU-2) and discovered a receptor protein specific for the active form of vitamin D3, 1α,25-dihydroxyvitamin D3. The KU-2 cell line was established from a pulmonary metastasis of renal cell carcinoma in a patient with hyperhemoglobinemia. The cells were tumorigenic in nude mice and clonogenic in a soft agar culture.Vitamin D3 derivatives suppressed proliferation of KU-2 cells in a monolayer culture and also clonogenicity in a soft agar culture dose-dependently. Of the vitamin D3 derivatives tested, 1α,25-dihydroxyvitamin D3 was the most potent in inhibiting cell growth, followed successively by 1α,24R,25-trihydroxyvitamin D3, 25-hydroxyvitamin D3, 1α-hydroxyvitamin D3 and 24R,25-dihydroxyvitamin D3 in that order. Analysis of the cell cycle phase of treated and non-treated KU-2 cells revealed that the action of 1α,25-dihydroxyvitamin D3 was not phase–specific but simply extended the doubling time of the cells. Radioreceptor assay and sucrose density gradient analysis of the cytosol showed that KU-2 cells contained a 3.2S receptor protein to which 1α, 25-dihydroxyvitamin D3 was specifically bound (Kd = 20.8 ± 4.8 pM, Nmax = 87 ± 24 fmole/mg protein, 4000 molecules/cell). On the other hand, the equilibrium dissociation constant of internalization of 1α,25-dihydroxyvitamin D3 (Kint) by intact KU-2 cells was 1.2 nM and the internalizing capacity was 33 fmole/8 × 106 cells (2500 molecules/cell) in the 10% serum medium, which was the same as that used in the growth study. This Kint value was very close to the half-maximal dose in growth inhibition. Also the affinity of various vitamin D3 derivatives for binding to the cytosol receptor in the KU-2 cells was closely related to the ability to inhibit growth of the cells. These results indicate that the actions of vitamin D3 derivatives in inhibiting proliferation and clonogenicity of KU-2 cells are affected by a receptor-mediated mechanism, and that the active form of vitamin D3 may be one of the regulatory factors affecting the proliferation and other biological functions of renal carcinoma cells