Chronic low dose tumor necrosis factor-α (TNF) suppresses early bone accrual in young mice by inhibiting osteoblasts without affecting osteoclasts

• TgTNF mice expressing low levels of TNFα (< 5 pg/ml) were studied.
• Young, growing TgTNF mice lack an expected osteoblast surge at 4–6 weeks of age.
• TgTNF osteoblasts express reduced Wnt and bone mRNA and delayed mineralized nodules.
• Canonical Wnt signaling is reduced in TgTNF mice at 4, but not 12 weeks of age.
• Osteoclast numbers, markers, and activity do not differ between WT and TgTNF mice.

The inflammatory cytokine tumor necrosis factor-α (TNF-α) is known to cause bone resorption and inhibit bone formation in arthritis and aging but less is known about TNF effects in the young growing skeleton. While investigating the mechanism of bone loss in TNF transgenic mice, we identified an early TNF-sensitive period marked by suppression of osteoblasts and bone accrual as the sole mechanism of TNF action, without an effect on osteoclasts or bone resorption. TgTNF mice express low concentrations of hTNFα (≤ 5 pg/ml). Osteoblasts cultured from TgTNF mice express reduced levels of RUNX2, Osx, alkaline phosphatase, bone sialoprotein, and osteocalcin and have delayed formation of mineralized nodules. Early accrual of bone in TgTNF mice is suppressed until 6 weeks of age, after which the rate of bone accrual normalizes without catch up. Histomorphometry revealed that TgTNF mice fail to generate a transient surge in osteoblast number that is seen in wild type (WT) mice at 4 weeks. Osteoclasts, TRAP staining, erosive surfaces, serum CTx, and OPG/RANKL expression did not differ between young TgTNF and WT mice. Canonical Wnts and signaling through β − catenin were reduced in TgTNF mice at 4 weeks and partially recovered by 12 weeks, associated with reduced cytoplasm to nuclear transfer of β-catenin and Wnt regulated genes. TgTNF mice were crossed with BatGal Wnt reporter mice. Active Wnt signaling in tibial trabecular lining cells was reduced in TgTNF mice at 4 weeks compared to control littermates. Our results demonstrate that a low dose inflammatory stimulus is sufficient to inhibit the early surge in osteoblasts and optimal bone formation of young mice independent of changes in osteoclasts. TNF inhibition of the Wnt pathway contributes to the suppression of osteoblasts.