Aging-related hyperphosphatemia triggers the release of TNF-α from macrophages, promoting indicators of sarcopenia through the reduction of IL-15 expression in skeletal muscle

AIMS: The association between aging-related hyperphosphatemia and sarcopenia has been documented, and evidence suggests that inflammaging(definition) is involved in the manifestation of sarcopenia. The present study investigates whether hyperphosphatemia triggers inflammation, thereby inducing the appearance of sarcopenia along with the cytokines involved in these processes. MATERIALS AND METHODS: RAW 264.7 macrophages were incubated with β-glycerophosphate (BGP), as a phosphate donor, at different time intervals, to assess the production of proinflammatory markers. Conditioned medium from macrophages was collected and added to cultured C2C12 myoblasts to analyse whether proinflammatory molecules, released by macrophages, modified myogenic differentiation, cell senescence(definition) or myokine IL-15 expression. A neutralising antibody anti-TNF-α and recombinant IL-15 were added to evaluate the role of these cytokines in the observed effects. Additionally, TNF-α, IL-15, serum phosphate, and sarcopenia signs were evaluated in 5-month-old mice, 24-month-old mice and 24-month-old mice fed with a hypophosphatemic diet. KEY FINDINGS: BGP increased TNF-α expression in macrophages through NFkB activation. Conditioned medium from BGP-treated macrophages impaired myogenic differentiation in differentiating myoblasts and promoted cellular senescence and reduced IL-15 expression in undifferentiated myoblasts. These effects were mediated by TNF-α. Old mice displayed reduced expression of muscle IL-15 and elevated circulating TNF-α, along with increased serum phosphate levels, which correlated with the appearance of sarcopenia indicators. The hypophosphatemic diet prevented these changes in old mice. SIGNIFICANCE: Hyperphosphatemia induces TNF-α production in macrophages, which contributes to the reduced expression of muscular IL-15. This mechanism may play a role in inducing sarcopenia in elderly mice.