Identification of 1,24(R)-dihydroxyvitamin D3-like bone-resorbing lipid in a patient with cancer-associated hypercalcemia.

A lipid indistinguishable from 1,24(R)-dihydroxyvitamin D3 [1,24(R)-(OH)2D3] was found in serum and tumor extracts from a hypercalcemic patient with a small cell carcinoma of the lung. The lipid comigrated with authentic 1,24(R)-(OH)2D3 on high performance liquid chromatography using both straight and reverse phase columns and competed with tritiated 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3)] for binding to intestinal 1,25-(OH)2D3 receptor. Increasing doses of the lipid factor from tumor and authentic 1,24(R)-(OH)2D3 gave parallel responses in a bone resorption assay, as assessed by 45Ca release from prelabeled mouse calvaria. The lipid factor from the patient's serum and authentic 1,24(R)-(OH)2D3 had identical biological activities in the receptor binding and bone resorption assays. In addition, the mechanisms of action of this lipid factor and 1,24(R)-(OH)2D3 were indistinguishable. Bone resorption by both was inhibited by calcitonin, and neither the lipid factor nor authentic 1,24(R)-(OH)2D3 affected cAMP content in osteoblast-like bone cells derived from mouse calvaria. The estimated concentrations of the 1,24(R)-(OH)2D3-like lipid, expressed as 1,24(R)-(OH)2D3 were 11 ng/g tumor wet wt by the receptor binding assay and 9.2 ng/g tumor wet wt by the bone resorption assay. The mean serum concentration was 1.4 +/- 0.3 (+/- SD) ng/ml (n = 3) by the receptor binding assay. No activity was detected in either bioassay when extracts of nontumor tissues from this patient or tumor extracts and sera from one hypercalcemic and four normocalcemic cancer patients were tested. The mean serum 1,25-(OH)2D level was low (6.4 +/- 0.5 pg/ml; n = 2), and serum 1,24(R),25-(OH)3D in this patient was high (103 pg/ml) compared to normocalcemic cancer patients, in whom the mean serum 1,25-(OH)2D level was 27 +/- 12 pg/ml (n = 4) and the 1,24(R),25(OH)3D level was 28 +/- 1.3 pg/ml (n = 4). Thus, the 1,24(R)-(OH)2D3-like lipid may be a substrate for metabolic conversion to 1,24(R),25-(OH)3D in vivo. These results provide evidence for the presence of a novel metabolite of vitamin D3, 1,24(R)-(OH)2D3. Detection of this bone-resorbing lipid in both tumor and serum suggests, but does not prove, that the tumor secreted this bioactive lipid into the circulation and that the high level of circulating bone-resorbing lipid was related to the hypercalcemia in this patient.