Inhibition of gap-junctional communication induces the trans-differentiation of osteoblasts to an adipocytic phenotype in vitro

Osteoblasts and adipocytes are thought to differentiate from a common strom al progenitor cell. These two phenotypically mature cell types show a high degree of plasticity, which can be observed when cells are grown under spec ific culture conditions. Gap junctions are abundant among osteoblastic cell s in vivo and in vitro, whereas they are down-regulated during adipogenesis , Gap junctional communication (GJC) modulates the expression of genes asso ciated with the mature osteoblastic phenotype. Inhibition of GJC utilizing 18-alpha -glycyrrhetinic acid (AGRA) blocks the maturation of preosteoblast ic cells in vitro, Moreover, cytoplasmic lipid droplets are detectable at t he end of the culture period, suggesting that GJC inhibition may favor an a dipocytic phenotype, We used several human osteoblastic cell lines, as well as bone-derived primary osteoblastic cells, to show that confluent culture s of human osteoblastic cells grown under osteogenic conditions developed a n adipocytic phenotype after 3 days of complete inhibition of GJC using AGR A or oleamide, two dissimilar nontoxic reversible inhibitors. Development o f an adipogenic phenotype was confirmed by the accumulation of triglyceride droplets and the increase in mRNA expression of the adipocytic markers per oxisome proliferator-activated receptor gamma2 and lipoprotein lipase, Glyc yrrhizic acid, a noninhibitory AGRA analog, or alpha -bromopalmitate, a non degradable fatty acid, had no effect. Modulation of skeletal GJC may repres ent a new pharmacological target by which inhibition of marrow adipogenesis can take place with the parallel enhancement of osteoblastogenesis, thus p roviding a novel therapeutic approach to the treatment of human age-related osteopenic diseases and postmenopausal osteoporosis.