RANKL/OPG ratio and DKK-1 expression in primary osteoblastic cultures from osteoarthritic and osteoporotic subjects

To evaluate the expression of Dickkopf-1 protein factor (DKK-1), DKK-2, and β-catenin, components of the Wnt pathway, in human osteoarthritic (OA) and osteoporotic (OP) osteoblasts and to correlate it to cell metabolic activity, proliferation, and receptor activator of nuclear factor-κB ligand/osteoprotegerin (RANKL/OPG) expression.

Primary human osteoblast cultures were obtained from healthy, OA, and OP donors. In each cell population we evaluated DKK-1, DKK-2, nonphosphorylated β-catenin and RANKL/OPG expression, osteocalcin and alkaline phosphatase (ALP) synthesis, and cell proliferation, both in basal condition and after vitamin D3 stimulation.

DKK-1 and DKK-2 showed opposite patterns of expression in OA and OP osteoblasts. The RANKL/OPG ratio was significantly higher in the OP group because of a greater expression of RANKL, whereas it was significantly lower in the OA group because of a higher expression of OPG. Treatment with vitamin D3 increased the RANKL/OPG ratio and DKK-2 expression and reduced DKK-1 expression in each cell population, but did not affect β-catenin levels. Both osteocalcin and ALP production and cell proliferation were enhanced in OA cells and reduced in the OP ones.

These data confirm that OA and OP are characterized by opposite bone changes, consisting of reduced bone remodeling processes with increased osteoblast activity in OA, and enhanced bone resorptive activity with reduction of osteoblast metabolism in OP, and suggest that the Wnt pathway is involved in the pathogenesis of both diseases.