Changes in osteocalcin response to 1,25-dihydroxyvitamin D3 stimulation and basal vitamin D receptor expression in human osteoblastic cells according to donor age and skeletal origin

Age-related osteopenia is known to occur differently throughout the skeleton. In the present study, we examine the influence of donor age (<50 and >50 years), and bone structure (cortical vs. trabecular) on osteocalcin and vitamin D receptor (VDR) expression in primary cultures of human osteoblastic cells (hOB) cells. Cells were isolated from trabecular bone samples obtained from donors undergoing either knee (mainly trabecular) (n = 22; 4 <50 years, 18 >50 years) or hip (mainly cortical) (n = 16; 6 <50 years, 10 >50 years) arthroplasty. Pooling the results from knee and hip hOB cell cultures, we found that secreted osteocalcin was higher in hOB cells from the younger donors, compared with that in older donors, and peaked after stimulation with 10−6–10−8 mol/L 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. In cells from the latter donors, this secretion was maximal after 10−6 mol/L 1,25(OH)2D3 treatment. On the other hand, using reverse transcription followed by polymerase chain reaction, baseline osteocalcin mRNA was found to be lower in hOB cells from the older donors than in those from younger donors. After treatment with 10−6–10−8 mol/L 1,25(OH)2D3, osteocalcin mRNA increased over baseline in all groups of hOB cells studied. In age-matched cultures, both basal and 10−6–10−8 mol/L 1,25(OH)2D3-stimulated osteocalcin mRNA showed similar values in hOB cells from both skeletal sites in younger donors. However, in the older donors, baseline as well as 10−8 mol/L 1,25(OH)2D3-stimulated osteocalcin mRNA were higher in knee hOB cells than in hip hOB cells. Furthermore, baseline VDR mRNA expression was also higher in the former cells than in the latter cells in the older group. Considering the influence of donor age at each skeletal site of origin, we found lower baseline osteocalcin and VDR mRNA levels in hip hOB cells in the older group than in the younger group. Our findings indicate that the response of osteocalcin secretion and its mRNA to physiological doses of 1,25(OH)2D3 decreases with aging and is associated with decreased VDR mRNA expression in hOB cells from mainly cortical bone.