TNFα inhibits the development of osteoclasts through osteoblast-derived GM-CSF.

Inflammatory cytokines such as tumor necrosis factor-alpha (TNFα) are potent stimulators of osteoclast formation and bone resorption and are frequently associated with pathologic bone metabolism. The cytokine exerts specific effects on its target cells and constitutes a part of the cellular microenvironment. Previously, TNFα was demonstrated to inhibit the development of osteoclasts in vitro via an osteoblast-mediated pathway. In the present study, the molecular mechanisms of the inhibition of osteoclastogenesis were investigated in co-cultures of osteoblasts and bone marrow cells (BMC) and in cultures of macrophage-colony stimulating factor (M-CSF) dependent, non-adherent osteoclast progenitor cells (OPC) grown with M-CSF and receptor activator of NF-κB ligand (RANKL). Granulocyte-macrophage colony stimulating factor (GM-CSF), a known inhibitor of osteoclastogenesis was found to be induced in osteoblasts treated with TNFα and the secreted protein accumulated in the supernatant. Dexamethasone (Dex), an anti-inflammatory steroid, caused a decrease in GM-CSF expression, leading to partial recovery of osteoclast formation. Flow cytometry analysis revealed that in cultures of OPC, supplemented with 10% conditioned medium (CM) from osteoblasts treated with TNFα/1,25(OH)(2)D(3), expression of RANK and CD11c was suppressed. The decrease in RANK expression may be explained by the finding, that GM-CSF and the CM from wt osteoblasts were found to suppress the expression of c-Fos, Fra-1, and Nfatc-1. The failure of OPC to develop into CD11c(+) dendritic cells suggests that cell development is not deviated to an alternative differentiation pathway, but rather, that the monocytes are maintained in an undifferentiated, F4/80(+), state. The data further implies possible interactions among inflammatory cytokines. GM-CSF induced by TNFα acts on early hematopoietic precursors, inhibiting osteoclastogenesis while acting as the growth factor for M-CSF independent inflammatory macrophages. These in turn may condition a microenvironment enhancing osteoclast differentiation and bone resorption upon migration of the OPC from circulation to the bone/bone marrow compartment.

Fibroblast growth factor receptors 1 and 2 in keratinocytes control the epidermal barrier and cutaneous homeostasis.

Fibroblast growth factors (FGFs) are master regulators of organogenesis and tissue homeostasis. In this study, we used different combinations of FGF receptor (FGFR)-deficient mice to unravel their functions in the skin. Loss of the IIIb splice variants of FGFR1 and FGFR2 in keratinocytes caused progressive loss of skin appendages, cutaneous inflammation, keratinocyte hyperproliferation, and acanthosis. We identified loss of FGF-induced expression of tight junction components with subsequent deficits in epidermal barrier function as the mechanism underlying the progressive inflammatory skin disease. The defective barrier causes activation of keratinocytes and epidermal gammadelta T cells, which produce interleukin-1 family member 8 and S100A8/A9 proteins. These cytokines initiate an inflammatory response and induce a double paracrine loop through production of keratinocyte mitogens by dermal cells. Our results identify essential roles for FGFs in the regulation of the epidermal barrier and in the prevention of cutaneous inflammation, and highlight the importance of stromal-epithelial interactions in skin homeostasis and disease.