Expression of Opioid Receptors in Osteoblast-Like MG-63 Cells, and Effects of Different Opioid Agonists on Alkaline Phosphatase and Osteocalcin Secretion by These Cells

We have previously shown that several stressful situations associated with tissue injury determine a decrease in serum osteocalcin concentration. Since reduced osteocalcin production is a marker of decreased osteoblastic activity, this finding could be related to the pathogenesis of osteoporosis secondary to some diseases. Endogenous opioids are involved in stress response. Proenkephalin-derived peptides have been shown to inhibit alkaline phosphatase activity, another marker of bone formation, in the murine cell line ROS-17/2.8. On the other hand, serum osteocalcin has been reported as being low in heroin abusers. We have therefore thought it of interest to study the presence of opioid receptors in the human osteoblast-like cell line MG-63, and to evaluate the effects of different opioid agonists on the secretion of alkaline phosphatase and osteocalcin by these cells. The presence of opioid receptors was studied by means of RT-PCR and immunohistochemistry. RT-PCR studies suggested the presence of specific mRNA for the three types of receptors, and immunohistochemistry clearly showed their occurrence. Osteocalcin synthesis was significantly inhibited by high concentrations of the mu agonists morphine and (D-Ala 2 ,N-MePhe 4 ,Gly 5 -ol)-enkephalin although no changes were seen with the delta agonist (D-Ala 2 ,D-leu 5 )-enkephalin. Morphine-induced osteocalcin inhibition was abolished when osteoblastic cells were incubated simultaneously with naloxone, whereas it was potentiated when cells were preincubated with naloxone. None of the opioid agonists modified the secretion of alkaline phosphatase. In conclusion, human osteoblast-like cells MG-63 express the three types of opioid receptors. Endogenous opioids may be involved in the reduction of osteocalcin observed in stressful situations associated with tissue injury.