Interleukin‐1β and TNF‐α Act in Synergy to Inhibit Longitudinal Growth in Fetal Rat Metatarsal Bones

We hypothesized that pro‐inflammatory cytokines can act locally in the growth plate to impair longitudinal growth. In a model of cultured fetal rat metatarsal bones, we found that IL‐1β and TNF‐α act in synergy to inhibit longitudinal growth, an effect linked to decreased proliferation and increased apoptosis of growth plate chondrocytes. IGF‐I could partially reverse all these effects. Introduction: Children with chronic inflammatory conditions, such as Crohn's disease or rheumatoid arthritis, experience impaired longitudinal growth. The inflammatory process itself, which includes upregulation of the pro‐inflammatory cytokines interleukin (IL)‐1β, IL‐6, and TNF‐α, is believed to be at least partly responsible for the poor growth in these patients. This study aimed to clarify whether these cytokines can act locally in the growth plate to suppress longitudinal growth and whether any negative effects can be reversed by insulin‐like growth factor‐I (IGF‐I). Materials and Methods: The effects of cytokines on longitudinal bone growth were studied in fetal (day E20) rat metatarsal bones kept in culture. After a 7‐day culture, the bones were sectioned, and chondrocyte proliferation was assessed by bromodeoxyuridine (BrdU) incorporation and apoptosis by TUNEL. Results: When added separately, IL‐1β and TNF‐α impaired longitudinal bone growth only at a high concentration (100 ng/ml each; p < 0.05 versus control). In contrast, when added in combination, IL‐1β and TNF‐α potently inhibited growth at far lower concentrations (from 3 ng/ml each; p < 0.001 versus control) and also decreased chondrocyte proliferation and increased apoptosis. Growth failure induced by the combination of IL‐1β and TNF‐α (10 ng/ml each) could be counteracted by anti‐IL‐1β (100 ng/ml; p < 0.001), anti‐TNF‐α (100 ng/ml; p < 0.001), or IGF‐I (100 ng/ml; p < 0.01). IL‐6 did not affect longitudinal growth even when added in combination with IL‐1β or TNF‐α (10 ng/ml each). Conclusions: We show that IL‐1β and TNF‐α act in synergy to locally suppress longitudinal growth, an effect that can be partially reversed by IGF‐I. Although growth hormone (GH)/IGF‐I may improve longitudinal growth in children with chronic inflammatory diseases, our results suggest that the inflammatory process itself must be targeted to achieve normal growth.