Antagonistic Actions in Vivo of (23S)-25-Dehydro-1α-Hydroxyvitamin D3-26,23-Lactone on Calcium Metabolism Induced by 1α,25-Dihydroxyvitamin D3.

The vitamin D analog, (23S)-25-dehydro-1a-hydroxyvitamin D326,23-lactone (TEI-9647), is an antagonist of the 1a,25-dihydroxyvitamin D3 [1a,25(OH)2D3] nuclear receptor (VDR)-mediated differentiation of human leukemia (HL-60) cells. To clarify whether TEI-9647 could function as an antagonist of 1a,25(OH)2D3 in vivo, we investigated in vitamin D-deficient (-D) rats the effects of single doses of TEI-9647 on several parameters of calcium metabolism modulated by 1a,25(OH)2D3. TEI-9647 (50 mg/kg iv) acting alone slightly, but significantly, stimulated intestinal calcium transport (ICA) and bone calcium mobilization (BCM) only at 8 h, but not at 24 h. In contrast, TEI-9647 dose-dependently inhibited ICA and BCM stimulated by an iv dose of 0.25 mg/kg 1a,25(OH)2D3 after 24 h, but not after 8 h. With respect to serum PTH levels, the administration of either TEI-9647, 50 mg/kg, or 1a,25(OH)2D3, 0.25 mg/kg, began to decrease the circulating levels by 4 h, which reached a nadir 24 h after administration. But, when TEI-9647 and 1a,25(OH)2D3 were simultaneously administered to 2D rats, the TEI-9647 dose-dependently reversed the inhibition of PTH secretion caused by 1a,25(OH)2D3, 0.25 mg/kg, at 8 and 24 h after the treatment. In separate experiments, the daily iv administration of 20 mg/kg of TEI-9647 alone to 1D rats for 2 weeks resulted in no significant changes in the prevailing serum Ca concentration. But doses of 1–20 mg/kg of TEI-9647 in combination with 0.5 mg/kg of 1a,25(OH)2D3, for 2 weeks, dose-dependently and significantly suppressed the serum calcium concentration increase caused by the 1a,25(OH)2D3. Collectively, these results show that TEI-9647 acting alone displays in vivo weak agonistic actions, but when administered in combination with 1a,25(OH)2D3, is a potent antagonist of three genomic-mediated calcium metabolism parameters. We conclude that TEI-9647 can also function as an antagonist of 1a,25(OH)2D3 in vivo in the rat. (Endocrinology 142: 59–67, 2001) V D is known to undergo a sequential two-step metabolism, in the liver and kidney, to form 1a,25dihydroxyvitamin D3 [1a,25(OH)2D3] or 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3], respectively (1). To date, 1a,25(OH)2D3 is considered to be the most potent metabolite of vitamin D3 particularly with respect to three key calcium metabolism parameters; it stimulates intestinal absorption of calcium and bone calcium mobilization and inhibits the secretion of PTH. Each of these effects has been shown to be mediated via 1a,25(OH)2D3 acting on a target organ [intestine (2), bone (2), and parathyroid gland (3, 4)] nuclear vitamin D receptor (VDR)-mediated genomic responses (5, 6). In recent years, however, many new biological functions different from those mentioned above have been reported (7); these include inhibition of cell proliferation and induction of cell differentiation (8), modulation of immunological responses (9–11), stimulation of insulin secretion (12, 13), and neurobiological functions (14, 15). 1a,25(OH)2D3 is believed to mediate biological responses as a consequence of its interaction both with a nuclear VDR to regulate gene transcription (16, 17) and with a putative cell membrane VDR (18) to generate rapid nongenomic actions (19), including opening of voltage-gated calcium (9) and chloride channels (20), and activation of mitogen-activated protein kinase (MAP kinase) (21). (23S,25R)-1a,25-Dihydroxyvitamin D3-26,23-lactone [(23S,25R)-1a,25(OH)2D3-26,23-lactone] has been isolated and identified as a major metabolite of 1a,25(OH)2D3 in the serum both of animals given pharmacological doses of 1a,25(OH)2D3 (22, 23) and of beagle dogs and normal adult humans under physiological conditions (24, 25). This (23S,25R)-1a,25(OH)2D3-26,23-lactone slightly stimulates intestinal calcium absorption but significantly decreases serum calcium concentrations in 2D rats (26, 27). The (23S,25R)1a,25(OH)2D3-26,23-lactone increases alkaline phosphatase activity and collagen synthesis in the osteoblastic MC3T3-El cells in vitro (28) and stimulates collagen synthesis and mineralization in vivo (29). Also, in a rat experimental model of osteoporosis induced by ovariectomy, the (23S,25R)1a,25(OH)2D3-26,23-lactone significantly increased the bone formation rate in a dynamic histomorphometric study (30). Collectively, these results indicate that the naturally occurring (23S,25R)-1a,25(OH)2D3-26,23-lactone metabolite has unique biological functions quite different from those of 1a,25(OH)2D3 in osteoblasts and osteoclasts. Recently, we have synthesized various analogs of 1a,25(OH)2D3-26,23-lactone to investigate which structural Received February 4, 2000. Address all correspondence and requests for reprints to: Anthony W. Norman, Ph.D., Department of Biochemistry, University of CaliforniaRiverside, Riverside, California 92521. E-mail: [email protected]. 0013-7227/01/$03.00/0 Vol. 142, No. 1 Endocrinology Printed in U.S.A. Copyright © 2001 by The Endocrine Society