Influence of 1,25-hydroxyvitamin D3 on cytosolic free calcium concentrations

Decrease in bone level of 1,25-dihydroxyvitamin D in women over 45 years old

The most active metabolite of vitamin D is 1,25-dihydroxyvitamin D [1,25(OH)2D]. Its level in the bone may play a role in the pathogenesis of metabolic bone diseases such as osteoporosis. To assess this, and to see whether there is correlation between serum and bone levels, we studied serum and bone samples taken from 43 patients (18 men and 25 women) undergoing different orthopedic procedures. Patients were studied according to sex and age groups (less than 45 years, 46-60 years, greater than 61 years). Serum level of 1,25(OH)2D was found to be 29.7 +/- 2.61 pg/ml (mean +/- SEM) for women, 32.2 +/- 3.86 pg/ml for men, and 30.7 +/- 2.18 pg/ml for the group as a whole. No significant statistical differences were found among age subgroups in either sex or between sexes. Bone level of 1,25(OH)2D was found to be 31.5 +/- 4.46 pg/g for women, 26.5 +/- 3.06 pg/g for men, and 29.4 +/- 2.81 pg/g for the entire group. No significant statistical difference was found between the age subgroups for men. However, the level of 1,25(OH)2D was found to be higher in the group of younger women (less than 45 years) compared with the older women (46-60 years and greater than 61 years) (P less than 0.005).

20-epi-vitamin D3 analogues: a novel class of potent regulators of cell growth and immune responses

The 20-epi-vitamin D3 analogues are a novel class of vitamin D3 derivatives, structurally related to 1 alpha,25-dihydroxycholecalciferol (1 alpha,25(OH)2D3). They are characterized by an altered stereochemistry at carbon 20 in the side-chain. In vitro, these new analogues were found to be considerably more potent as regulators of growth and differentiation in the human histiocytic lymphoma cell line U 937 than 1 alpha,25(OH)2D3, despite a practically unchanged calcemic activity in vivo. The most potent analogue, KH 1060, inhibited cell proliferation by 50% at 10(-12) M (14,000 times more active than 1 alpha,25(OH)2D3). At the same time, KH 1060 induced cell differentiation at concentrations as low as 10(-14)M. In addition, the 20-epi-vitamin D3 analogues were found to be very potent inhibitors of T-lymphocyte proliferation induced by interleukin-1 or alloantigen. In this respect, they were several orders of magnitude more active than the potent immunosuppressive agent cyclosporin A (CyA). KH 1060, the most potent analogue, inhibited interleukin-1-induced mouse thymocyte proliferation by 50% at 3 x 10(-16) M and allogeneic stimulation of mouse spleen lymphocytes at 5 x 10(15) M. These effects were considered to be mediated by inhibition of interleukin-2 release from activated T-lymphocytes. The new analogues are of potential interest in the prevention of graft rejection and in the treatment of psoriasis, cancer and auto-immune diseases.

Role of intracellular-free calcium in the cornified envelope formation of keratinocytes: differences in the mode of action of extracellular calcium and 1,25 dihydroxyvitamin D3

Extracellular calcium (Cao) and the steroid hormone 1,25(OH)2D, induce the differentiation of human epidermal cells in culture. Recent studies suggest that increases in intracellular free calcium (Cai) levels may be an initial signal that triggers keratinocyte differentiation. In the present study, we evaluated cornified envelope formation, the terminal event during keratinocyte differentiation, and correlated it with changes in the Cai levels during differentiation of keratinocytes in culture induced by Cao or 1,25(OH)2D. Keratinocytes were grown in different Cao concentrations (0.1 or 1.2 mM) or in the presence of 1,25(OH)2D (10(-11) to 10(-7) M), and the Cai levels were measured using the fluorescent probe Indo-1. Our results suggest that the induction of cornified envelope formation is associated with an increase in Cai level during calcium-induced differentiation. Cao and the calcium ionophore ionomycin acutely increased Cai and cornified envelope formation. In contrast, the effect of 1,25(OH)2D on increasing Cai levels and stimulating cornified envelope formation was long-term, requiring days of treatment with 1,25(OH)2D. Our data are consistent with other recent studies and support the hypothesis that Cao regulates keratinocyte differentiation primarily by acutely increasing their Cai levels. The role of calcium in the mechanism of action of 1,25(OH)2D on keratinocyte differentiation is less clear. The increase in Cai of keratinocytes during 1,25(OH)2D induced differentiation may be essential for or subsequent to its prodifferentiation effects.

Decrease in the basal levels of cytosolic free calcium in chondrocytes during aging in culture: possible role as differentiation-signal

Cell- and matrix-related parameters, which characterize the aging and differentiation process of cartilage in vivo, were measured in cultured chick epiphyseal chondrocytes during maintenance in a suspension culture for 34 days. A gradual decrease in the rates of proliferation and an increase in the size of the cells were observed. Ultrastructural examination revealed increased vacuolization and appearance of glycogen-storing pools. The rate of proteoglycan synthesis gradually increased. Age-related changes in the composition of the proteoglycan consisted of an increase in the ratio of keratan sulfate/chondroitin sulfate. The results indicate that the process of aging in culture resembles maturation and differentiation of cartilage tissue in vivo. The levels of cytosolic free calcium ions ([Ca2+]i) were measured in fura-2-loaded cells during the course of aging in culture. A gradual decrease in [Ca2+]i was observed. In 5-day cultures, a value of 184 nM [Ca2+]i was measured; this value decreased to 61 nM in 34-day cultures. On the basis of the present data and the previous results, which showed that cartilage-derived growth factors caused a decrease in [Ca2+]i, concomitantly with enhancing differentiation, whereas factors which elevated [Ca2+]i caused an increase in proliferation and a decrease in proteoglycan synthesis, we suggest a model for control of chondrocyte differentiation and aging. The model suggests that the rate of differentiation may be paced by changes in steady-state levels of [Ca2+]i.

Relationship between the uptake of calcium or phosphorus and alkaline phosphatase activity induced by certain modulators in rat organs

The effects of phorbol ester or calmodulin on the calcium and phosphorus uptakes by rat tissues and their relationship to the alkaline phosphatase activity (ALP) were investigated in vivo. In rat tissues, ALP activity and calcium uptake in the duodenum and liver were clearly induced by phorbol ester treatment, whereas in the calvarium and ileum they were decreased. Phosphorus uptake was increased by the administration of phorbol ester only in the calvarium. In rats pretreated with an injection of indomethacin as an inhibitor of prostaglandin-synthesizing enzyme, the selective uptake of calcium by phorbol ester was eliminated in the duodenum and liver, as was the ALP activity. In contrast, rats showed a marked increase in ALP activity in the ileum after calmodulin treatment. Moreover, the increased uptake of calcium after calmodulin treatment was clearly seen in the ileum, calvarium, and kidney, and an increased uptake in phosphorus was seen in the duodenum, ileum, and calvarium, but not in kidney. Furthermore, prior injection of W-7 or calmidazolium as an antagonist of calmodulin, reduced the increased ALP activities and the uptake of calcium in all organs tested, but did not reduce the increased phosphorus uptake by the calvarium. Consequently, it is suggested that calcium uptake under the above conditions correlated well with changes in the ALP activity; however, phosphorus uptake seemed to be less in accord with ALP activity. The amount of tested other mineral metabolic markers suggests that the Ca uptake and ALP activity induced by certain effectors regulate 1,25(OH)2D3 level by the modulation of 25(OH)D3-1 alpha-hydroxylase activity.

Calcitriol increases Ca2+-ATPase activity

Treatment with calcitriol of isolated cartilage cells derived from epiphyseal growth plates of rachitic chicks results in reduced intracellular calcium concentrations. The reduction in calcium was found to correlate with increased activity of Ca2+-ATPase. The activities of Na+-K+-ATPase and of Mg2+-ATPase did not change in response to the treatment with calcitriol. It is suggested that calcitriol regulates intracellular calcium by modulating the activity of the Ca2+-pumping ATPase.