Yasuho Nishii

Cooperative effect of 1 α,25-dihydroxyvitamin D3 and dexamethasone in inducing differentiation of mouse myeloid leukemia cells

Murine myeloid leukemia cells (MI) are induced to differentiate into macrophages by the metabolically active form of vitamin D3,1 alpha,25-dihydroxyvitamin D3[1 alpha,25(OH)2D3] (E. Abe et al., (1981) Proc. Natl. Acad. Sci. USA 78, 4990-4994). At 0.12-120 nM, 1 alpha,25(OH)2D3 suppressed cell growth in a dose-dependent manner and markedly induced phagocytic activity, lysozyme activity, and C3-receptor formation. The potency of 1 alpha,25(OH)2D3, at 0.12-120 nM, in inducing differentiation was nearly equivalent to that of 10-10,000 nM of dexamethasone, one of the most potent stimulators of Ml cells. Simultaneous treatment with low physiological plasma concentrations of 1 alpha,25(OH)2D3 (0.12 nM) and dexamethasone (10 nM) induced differentiation of Ml cells equivalent to the responses obtained only by using much higher concentrations of the respective steroids when used separately. In addition, two variant clones of Ml cells resistant to either 1 alpha,25(OH)2D3 or dexamethasone were isolated. One was resistant to 120 nM of 1 alpha,25(OH)2D3 but sensitive to 10-1000 nM of dexamethasone. The other was resistant to 1000 nM of dexamethasone but sensitive to 12 nM of 1 alpha,25(OH)2D3. This suggests that the mechanism of action of 1 alpha,25(OH)2D3 in inducing differentiation of Ml cells is different at least in part from that of dexamethasone, and that combination therapy by both steroids may be useful in reducing leukemogenicity of Ml cells in vivo.

History of Vitamin D Treatment of Renal Osteodystrophy

Vitamin D treatment was tried when renal osteodystrophy was first recognized in the early 20th century, using vitamin D2, D3, or dihydrotachysterol. Large doses of vitamin D2 or D3 (150,000-500,000 IU) were prescribed by monitoring serum calcium, phosphate, and alkaline phosphatase. After the discovery of 1,25-dihydroxycholecalciferol, this compound or 1 alpha-hydroxycholecalciferol was applied to the treatment of renal osteodystrophy. In a preclinical study, especially of 1 alpha-hydroxycholecalciferol, nephritogenoside nephritis was the most responsive condition. These active vitamin D preparations are now widely used in patients with chronic renal failure under hemodialysis. Other active vitamin D compounds, such as hexafluoro-1,25-dihydroxycholecalciferol and 22-oxacalcitriol, are also under investigation.

Metabolites of 24,25-dihydroxyvitamin D3 made in the kidney of chicks supplemented with vitamin D3.

Metabolism of 25-hydroxyvitamin D3 (25-OH-D3) was examined in chicks supplemented with vitamin D3. Kidney homogenates metabolized in vitro [3H]-25-OH-D3 to 3 new metabolites (peaks A, C and E) by way of 24,25-dihydroxyvitamin D3. The enzymes responsible for the synthesis of these metabolites appeared to be induced by 1 alpha,25-dihydroxyvitamin D3. Production of these metabolites was increased in parallel with the increase of the supplemented levels of vitamin D3, while recovery of the radioactivity in the chloroform phase was sharply decreased. The production of peak C was considered to be closely related to the transfer of the radioactive metabolites to the water-soluble phase. These results may indicate that 24-hydroxylation is a degradation step in the 25-OH-D3 metabolism.

The Future of New Vitamin D Analogs in Uremic Bone Disease

The spectrum of activities promoted by calcitriol has been found to extend far beyond a role in calcium homeostasis. Calcitriol has been shown to have important immunomodulating properties. The major limitation of calcitriol therapy is its accompanying calcemic activity. The recent development of analogs of vitamin D, that retain the properties of calcitriol with less calcemic activity, may offer a potential therapeutic tool in the treatment of uremic bone disease and other clinical conditions such as psoriasis and leukemia. Among these analogs 22-oxacalcitriol (OCT), 1,25-(OH)2-24-homo-D3, 1,25-(OH)2-22-ene-24-homo-D3, formerly MC 903 (now Calcipotriol), and 1,25-(OH)2-16-ene-23-yne-D3 have been shown to have important biological effects.

Parathyroid Hormone-Action and Degradation

Peptide hormone hydrolysis has two main purposes. One is processing, and the other is degradation. In most of the hormones, these two processes are clearly distinguished, because the former takes place in the secretory gland and the latter in the target organ. The former is a metabolically controlled limited hydrolysis yielding certain active fragment, whereas the latter is a random hydrolysis to generate small, in-active fragments with no other purpose but getting rid of the biological effect of the hormone. While the processing and generation of active fragment is directly connected with the action of the hormone, degradation into small fragments may be involved only indirectly in the control of hormone action, by controlling its rate of disappearance and consequently, the half-life of the hormone in blood. Parathyroid hormone is unique in its inhibition of secretion by calcium ion along with renin, and this was explained by calcium stimulation of its degradation already in the secretory gland, from which only inactive fragments are secreted during hypercalcemia, because of the augmentation of PTH degradation in the parathyroid glands. It is thus difficult to separate processing and degradation of PTH. In the target cells of PTH, both processing and degradation may take place.

Chronological changes of3H-1,25(OH)2D3 in tissue and serum after consecutive oral doses of active vitamin D in rats : Comparative study of 1αOHD3 and 1,25(OH)2D3

To determine the effect of consecutive oral administration of 1αOHD3 or 1,25(OH)2D3 on the metabolism of 1,25(OH)2D3, seven-month-old female rats were given 1αOHD3 (0.4 µg/kg/day) or 1,25(OH)2D3 (0.2 µg/kg/day) for 14 days. After the oral administration of 2 μCi of3H-1αOHD3 or3H-1,25(OH)2D3, the rats were sacrificed at 2,6 or 24 h, and the distribution of these tracers and their metabolites in the serum, intestines, liver, kidneys and bone were studied. The consecutive treatment with 1,25(OH)2D3 or 1αOHD3 did not basically alter the elution patterns of3H labeled metabolites on HPLC. The tissue levels of3H-1,25(OH)2D3, administered or converted from3H-1αOHD3, were lower in the treated rats than those in the controls at 24 h, indicating the accelerated disappearance of 3H-1,25 (OH)2D3 following the treatment with 1αOHD3 or 1,25(OH)2D3. The degree of acceleration, however, was less following the treatment with 1αOHD3 than that after treatment with 1,25(OH)2D3. The degree ofacceleration, however, was less following the treatment with 1αOHD3 than that after treatment with 1,25(OH)2D3. This finding might indicate that, when 1αOHD3 or 1,25(OH)2D3 is consecutively administered, the 1,25(OH)2D3 converted from 1αOHD3 by the liver remains longer in the tissues including bone than 1,25(OH)2D3 absorbed directly from the intestine.