All-trans retinoic acid antagonizes the action of calciferol and its active metabolite, 1,25-dihydroxycholecalciferol, in rats.

An antagonistic interaction between retinol and calciferol has been established. However, the mechanism by which this antagonism occurs is unclear. One possibility is that retinol affects the metabolism of calciferol. To investigate this hypothesis, retinol- and calciferol-depleted rats were given various amounts of ergocalciferol, cholecalciferol, 1alpha,25-dihydroxycholecalciferol [1,25(OH)2D3], or 24,24-difluoro-1alpha,25-dihydroxycholecalciferol [24-F2-1,25(OH)2D3] in combination with various amounts of retinyl acetate or all-trans retinoic acid (ATRA) in a series of studies. Rats administered 1720 or 3440 microg retinyl acetate once every 3 d for 33 d in combination with 25.8 ng ergocalciferol or 25 ng cholecalciferol every 3 d had lower serum calcium and greater serum phosphorus concentrations than rats fed 0 or 11.4 mug retinyl acetate every 3 d. In addition, rats fed 400 microg ATRA/d in combination with 25.8 ng ergocalciferol every 3 d, 25 ng cholecalciferol every 3 d, 2-5 ng 1,25(OH)2D3/d, or 0.5-1 ng 24-F2-1,25(OH)2D3/d had significantly lower serum calcium and higher serum phosphorus concentrations than rats not given ATRA in the diet. Therefore, both retinyl acetate and ATRA are able to antagonize the action of ergocalciferol and cholecalciferol in vivo. Additionally, ATRA antagonizes the in vivo action of 1,25(OH)2D3 and an analog, 24-F2-1,25(OH)2D3, that cannot be 24-hydroxylated. Together, these results suggest that retinol does not antagonize the action of calciferol by altering the metabolism of calciferol or 1,25(OH)2D3, but does so by another mechanism.

Ergocalciferol and cycloheximide in vivo stimulate protein kinase C of intestinal crypt cells.

Previous reports have suggested that 1,25-dihydroxychole-calciferol regulated cellular differentiation via its effects on protein kinase C activity. This study examined the in vivo effects of ergocalciferol on the activity of protein kinase C, and whether the differentiation of crypt intestinal cells is dependent on the activation of this enzyme. Ergocalciferol in saline was injected intramuscularly into rats and the animals sacrificed 24 hr after fasting. Protein kinase C specific activity was determined from the rate of incorporation of 32p-ATP into protamine. Injections of 60 micrograms ergocalciferol/200 g of body wt, raised protein kinase C specific activity to 59818 +/- 4010 (SEM, n = 5) cpm 32p-protamine/min/mg cell protein, compared with a control of 46173 +/- 4612 (P < 0.0005). Optimal specific activities were seen within 72 hr of injection. Administration of 120 micrograms ergocalciferol/200 g of body wt, raised the concentrations of serum calcium to 9.8 and 10.4 mg/dl following the intramuscular injection by 24 and 72 hr, respectively, compared with a control of 7.7 mg/dl. Actinomycin D (intramuscular, 100 micrograms/200 g of body wt) together with ergocalciferol (120 micrograms/200 g of body wt) reduced protein kinase C activity by 51% 24 hr after injection. Cycloheximide blocked the activation, but when injected alone stimulated endogenous protein kinase C activity by 34% 24 hr injection. The study shows activation of crypt protein kinase C by ergocalciferol. The inhibition of activation by actinomycin D and cycloheximide suggests the involvement of both transcriptional and translational processes in this activation.