INTERLEUKIN-6 ACTS LOCALLY ON THE GROWTH PLATE TO IMPAIR BONE GROWTH
A recent clinical study in mothers with rheumatoid arthritis (RA) published in this journal, reported that high maternal serum levels of interleukin-6 (IL-6) is associated with low birth weight deliveries1. Low birth weight in turn is associated with impaired growth during childhood2. Interestingly, overexpression of IL-6 leads to reduced bone growth in mice3. Although the systemic effects of IL-6 are well understood, any local effects on the growth plate have not yet been clarified. Aiming to study the local actions of IL-6, we isolated fetal rat metatarsal bones and cultured them ex vivo for 12 days while monitoring bone growth. When exposed to IL-6 and its soluble receptor IL-6R alpha (10+100 ng/ml), growth was decreased by 21.1% compared to control (p<0.001) (Figure 1A). IL-6 mainly targeted the hypertrophic growth plate zone, reducing its height by 53.7% and area by 72.6% (p<0.05 vs. control; Figure 1B). These data further strengthen the above cited report of an association between high maternal IL-6 levels and impaired fetal growth1.
Beside IL-6, the proinflammatory cytokines IL-1 beta and TNF-alpha are often up-regulated in RA patients. To investigate any interaction between these cytokines, fetal rat metatarsal bones were cultured with IL- 6+IL-6R alpha (10+100 ng/ml) in combination with IL-1beta (10 ng/ml) or TNF-alpha (10 ng/ml). Growth was then found to be reduced by 40.6% and 54.0%, respectively (p<0.001 vs. control) (Figure 1A). In supernatants of bones cultured with IL-1 beta and TNF-alpha in combination, a 27-fold increase in IL-6 production was measured while individually these cytokines only marginally affected IL-6 production (Figure. 2). Thus, IL-1 beta and TNF-alpha act in synergy to induce local IL-6 production in growth plate cartilage; an effect which may explain the synergistic growth suppressive effect of IL-1? and TNF-alpha previously reported in cultured fetal rat metatarsal bones4 5.
Tocilizumab, a humanized anti-IL-6R monoclonal antibody preventing IL -6 binding to the IL-6R6, provides sustained clinical improvement and a favourable risk-benefit profile in children with RA7. However, to date it is unclear if anti-IL-6 treatment can rescue bone growth in these patients. To experimentally address this gap of knowledge, we co-cultured fetal rat metatarsal bones with IL-6 antibody (2.5 micrograms/ml) and the detrimental IL-1 beta+TNF-alpha combination for 12 days, but growth could not be reconstituted. This failure, which could be due to difficulties for the IL-6 antibody to penetrate into the growth plate, emphasizes the importance to develop small molecules/peptides directed towards the blocking of IL-6 production or signalling allowing bone growth to be rescued.
In conclusion, we here report the novel finding that IL-6 acts directly at the growth plate level to suppress bone growth and that IL- 1beta and TNF-alpha synergistically trigger this effect by markedly stimulating the local production of IL-6. Our experimental data strengthen the recent clinical report of an association between high maternal IL-6 levels and impaired fetal growth.
1. de Steenwinkel FD, Hokken-Koelega AC, de Man YA, et al. Circulating maternal cytokines influence fetal growth in pregnant women with rheumatoid arthritis. Ann Rheum Dis 2012.
2. Cutfield WS, Regan FA, Jackson WE, et al. The endocrine consequences for very low birth weight premature infants. Growth Horm IGF Res 2004;14 Suppl A:S130-5.
3. De Benedetti F, Rucci N, Del Fattore A, et al. Impaired skeletal development in interleukin-6-transgenic mice: a model for the impact of chronic inflammation on the growing skeletal system. Arthritis Rheum 2006;54(11):3551-63.
4. Martensson K, Chrysis D, Savendahl L. Interleukin-1beta and TNF- alpha act in synergy to inhibit longitudinal growth in fetal rat metatarsal bones. J Bone Miner Res 2004;19(11):1805-12.
5. MacRae VE, Farquharson C, Ahmed SF. The restricted potential for recovery of growth plate chondrogenesis and longitudinal bone growth following exposure to pro-inflammatory cytokines. J Endocrinol 2006;189(2):319-28.
6. Mihara M, Kasutani K, Okazaki M, et al. Tocilizumab inhibits signal transduction mediated by both mIL-6R and sIL-6R, but not by the receptors of other members of IL-6 cytokine family. Int Immunopharmacol 2005;5(12):1731-40.
7. Yokota S, Imagawa T, Mori M, Efficacy and safety of tocilizumab in patients with systemic-onset juvenile idiopathic arthritis: a randomised, double-blind, placebo- controlled, withdrawal phase III trial. Lancet 2008;371(9617):998-1006.
Figures for this eLetter can be found at the ePage with the ARD issues.
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