Proteoglycan 4, a Novel Immunomodulatory Factor, Regulates Parathyroid Hormone Actions on Hematopoietic Cells

Proteoglycan 4 (PRG4), a critical protective factor in articular joints, is implicated in hematopoietic progenitor cell expansion and megakaryopoiesis. PRG4 loss-of-function mutations result in camptodactyly-arthropathy-coxa vara-pericarditis (CACP) syndrome, which is characterized primarily by precocious joint failure. PRG4 was identified as a novel parathyroid hormone (PTH) responsiveness gene in osteoblastic cells in bone, and was investigated as a potential mediator of PTH actions on hematopoiesis. Sixteen-week-old Prg4−/− mutant and Prg4+/+ wild-type mice were treated daily with intermittent PTH (residues 1-34) or vehicle for 6 weeks. At 22 weeks of age, Prg4 mutant mice had increased peripheral blood neutrophils and decreased marrow B220+ (B-lymphocytic) cells, which were normalized by PTH. The PTH-induced increase in marrow Lin−Sca-1+c-Kit+ (hematopoietic progenitor) cells was blunted in mutant mice. Basal and PTH-stimulated stromal cell-derived factor-1 (SDF-1) was decreased in mutant mice, suggesting SDF-1 as a candidate regulator of proteoglycan 4 actions on hematopoiesis in vivo. PTH stimulation of IL-6 mRNA was greater in mutant than in wild-type calvaria and bone marrow, suggesting a compensatory mechanism in the PTH-induced increase in marrow hematopoietic progenitor cells. In summary, proteoglycan 4 is a novel PTH-responsive factor regulating immune cells and PTH actions on marrow hematopoietic progenitor cells.