Modulation by epidermal growth factor of the basal 1,25(OH)2D3 receptor level and the heterologous up-regulation of the 1,25(OH)2D3 receptor in clonal osteoblast-like cells

Abstract

Summary. The effects of epidermal growth factor (EGF) on basal 1,25-dihydroxyvitamin D 3 (1,25-(OH)2D3) receptor level and on parathyroid hormone (PTH)-induced 1,25-(OH)2D3 receptor up-regulation were studied in the phenotypically osteoblastic cell line UMR 106. EGF in concentrations exceeding 0.1 ng/ml reduced the number of 1,25(OH)2D 3 binding sites without changing the binding affinity. Maximal reduction was 30% at about 1 ng/ml. This reduction was independent of a change in cAMP content. EGF dose-dependently attenuated both PTH-induced 1,25(OH)2D 3 receptor up-regulation and PTH-stimulated cAMP production, without an effect on the EDso of the PTH effects. For both PTH responses the IC5o and the maximal effective dose were similar, 0.1 ng/ml and 1 ng/ml EGF, respectively. Reduction was first seen at 0.01 ng/ml EGF. At this concentration, EGF reduced PTH-stimulated 1,25-(OH)2D 3 receptor binding without an inhibition of the cAMP response. Time-course studies with 1 ng/ml EGF revealed that at 2 h preincubation EGF reduced the heterologous upregulation by PTH, and maximal inhibition was seen after 4 h. In contrast, PTH-stimulated cAMP production was just significantly inhibited only after 6 h, with 60% inhibition after 24 h preincubation. The effects of prostaglandin E 2 and forskolin on both 1,25(OH)eD3 binding and cAMP production were inhibited in a similar fashion. On the other hand, dibutyryl cAMP-and 3-isobutyl-l-methylxanthinestimulated 1,25(OH)2D3 binding were not affected by EGF. Taken together, our results demonstrate that EGF reduces both the basal number of 1,25(OH)2D3 binding sites and the heterologous up-regulation of the 1,25(OH)2D 3 receptor. The current data suggest that EGF reduces heterologous upregulation of the 1,25(OH)2D3 receptor independent of as well as dependent on the cAMP messenger system. The EGF effect is not primarily located at the PTH receptor, at cAMP phosphodiesterase, or at protein kinase A level.Key words: EGF-1,25(OH)zD 3 binding -PTH -Osteoblast cell line.1,25-Dihydroxyvitamin D 3 (1,25(OH)2D3) and parathyroid hormone (PTH) play an important role in calcium homeostasis. One of the target tissues for 1,25(OH)2D3 and PTH is bone. For both hormones the receptors in bone are located on the osteoblast [1][2][3][4]. From in vitro as well as in vivo studies, evidence has been obtained indicating that 1,25 (OH)2D 3 and PTH act in an interrelated fashion [5,6]. Also at the level of the osteoblast, interactions between 1,25(OH)2D 3 and PTH have been reported. For instance, preincubation of osteoblast-like cells with 1,25(OH)2D 3 attenuates the stimulation of cAMP production by PTH [7][8][9][10]. Furthermore, we have recently reported that PTH and PTHrelated protein cause heterologous up-regulation of the 1,25(OH)2D3 receptor [11].It has been shown that besides these two well-known calciotrophic hormones, growth factors and cytokines also affect bone cell metabolism. One of these polypeptide growth factors is epidermal growth factor (EGF) which has been shown to stimulate bone resorpti...

Calcium and vitamin-D deficiency marginally impairs fracture healing but aggravates posttraumatic bone loss in osteoporotic mice

“…It is hypothesized that vitamin D exerts its positive effects on bone regeneration mainly indirectly through its endocrine action on calcium homeostasis 2 , thus increasing the calcium supply for fracture callus mineralization. However, direct vitamin-D effects on bone are also widely discussed, because osteoblasts express the VDR and its expression on these cells is directly regulated by biologically active 1,25(OH) 2 D 3 45 , 46 . Several in-vitro studies demonstrated that 1,25(OH) 2 D 3 binding to the VDR enhanced osteoblast differentiation and mineralization, which is often indicated by the increased activity of the osteoblast differentiation marker ALP 47 , 48 .…”

Vitamin D and gene networks in human osteoblasts

“…This is supported by in vitro studies demonstrating direct effects on osteoblasts. The VDR is present in osteoblasts and its expression can be regulated by 1,25D3 itself and by other factors such as parathyroid hormone (PTH), glucocorticoids, transforming growth factor-β, and epidermal growth factor (Pols et al, 1988a,b; Reinhardt and Horst, 1990; van Leeuwen et al, 1991, 1992a,b; Godschalk et al, 1992). The expression of VDR allows 1,25D3 to directly affect osteoblast growth and differentiation.…”

Genomic mechanisms involved in the pleiotropic actions of 1,25-dihydroxyvitamin D3

“…Besides homologous up-regulation, the VDR has been reported to be regulated by a number of other factors, including glucocorticoids [53,[59][60][61][62][63] and activation of protein kinase A [64][65][66] and protein kinase C [67]. Glucocorticoids have been reported to upregulate as well as to down-regulate the VDR in a number of cells and tissues.…”