Retinoic acid suppresses interleukin 6 production in normal human osteoblasts

Systemic long-term retinoid therapy for chronic skin diseases significantly reduced bone turnover markers within days and led to bone abnormalities. Retinoic acid (RA) plays a key role in the regulation of mouse bone cell proliferation, differentiation and functions. Meanwhile, there is little information of RA effect on human osteoblast and osteoclast cell development and function. Interleukin 6 (IL-6) is a pleiotropic cytokine with profound effects on bone metabolism. Thus, the present study examined the RA effect on cell differentiation, alkaline phosphatase and osteocalcin production as well as IL-6 production in normal human osteoblasts. The number of large differentiated osteoblast cells decreased in RA-treated cultures P<0.05. The production of bone specific markers, alkaline phosphatase and osteocalcin, was also reduced in RA-treated cultures. Normal human osteoblasts produced 31.0+/-4.8 pg IL-6 per ml in control cultures. Within 24 h, RA at all four concentrations reduced Il-6 production from normal human osteoblasts. The pharmacological concentration of 10(-5) M RA suppressed 90% of IL-6 production. The present study shows for the first time that RA profoundly inhibits IL-6 production in normal human osteoblasts within 24 h and in a dose-dependent manner. RA was shown previously to inhibit IL-6 production in several other normal and malignant human cell types. The associated decrease in osteoblast cell differentiation, alkaline phosphatase and osteocalcin production could result from the rapid RA-inhibition of IL-6 production. Thus, RA inhibition of IL-6 production in normal human osteoblasts may contribute to the bone abnormalities seen after systemic long-term retinoid therapy in some patients.