Extrarenal production of calcitriol in normal and uremic humans.
We have previously reported low serum levels of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and increased 1,25-(OH)2D3 production after the administration of 25-hydryoxyvitamin D (25OHD) to anephric humans. Since normal alveolar macrophages are known to synthesize 1,25-(OH)2D3 when stimulated with gamma-interferon or lipopolysaccharide, we determined whether macrophages derived from peripheral blood monocytes could be an extrarenal source of 1,25-(OH)2D3. Our results demonstrated that macrophages from normal individuals synthesize 1,25-(OH)2D3. The apparent Km for 25OHD3 was 6.6 +/- 0.5 nM and the maximum velocity was 47.4 +/- 13.7 fmol 1,25-(OH)2D3/h.microgram DNA. The activity of this enzyme was reduced 37.2 +/- 3.1% by physiological concentrations (96 pmol/L) of 1,25-(OH)2D3 in the incubation medium. Normal macrophages further hydroxylated 1,25-(OH)2D3 to more polar metabolites, and this catabolic activity was significantly enhanced by physiological concentrations of 1,25-(OH)2D3. In chronic renal failure, peripheral macrophages exhibited an enhanced 1 alpha-hydroxylase activity (8.2 +/- 0.8 vs. 4.2 +/- 0.5 fmol 1,25-(OH)2D3/microgram DNA.h in controls) and a decreased capacity to degrade 1,25-(OH)2D3. Exogenous 1,25-(OH)2D3, in physiological concentrations, reduced 1,25-(OH)2D3 synthesis to a degree (23.6 +/- 8.5%) comparable to that observed in normal cells. 1,25-(OH)2D3 production by macrophages did not correlate with the severity of hyperparathyroidism. Moreover, human PTH-(1-34) in supraphysiological concentrations (20,000 and 100,000 ng/L) did not stimulate the 1 alpha-hydroxylase activity of macrophages from either normal or uremic subjects. These results demonstrate that 1) normal peripheral macrophages metabolize 25OHD3 and 1,25-(OH)2D3; 2) macrophages in uremia display higher rates of 1,25-(OH)2D3 synthesis and lower rates of catabolism than normal macrophages; and 3) 1,25-(OH)2D3 deficiency, but not hyperparathyroidism, may play a role in the stimulation of 1,25-(OH)2D3 production by macrophages in chronic renal failure.
Dusso
A S
AS
Renal Division, Washington University School of Medicine, St. Louis, Missouri 63110.
Finch
J
J
Brown
A
A
Ritter
C
C
Delmez
J
J
Schreiner
G
G
Slatopolsky
E
E
D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase / Q000097:blood; D002117:Calcitriol / Q000096:biosynthesis*; D065607:Cytochrome P-450 Enzyme Inhibitors; D006801:Humans; D007654:Ketoconazole / Q000494:pharmacology; D007676:Kidney Failure, Chronic / Q000097:blood; D008264:Macrophages / Q000187:drug effects / Q000378:metabolism*; D010281:Parathyroid Hormone / Q000494:pharmacology; D010446:Peptide Fragments / Q000494:pharmacology; D019379:Teriparatide; D014511:Uremia / Q000097:blood*
D065607:Cytochrome P-450 Enzyme Inhibitors; D010281:Parathyroid Hormone; D010446:Peptide Fragments; D019379:Teriparatide; D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase; D002117:Calcitriol; D007654:Ketoconazole