Recent developments in endocrinology, made possible by the combination of mouse genetics, integrative physiology and clinical observations have resulted in rapid and unanticipated advances in the field of skeletal biology. Indeed, the skeleton, classically viewed as a structural scaffold necessary for mobility, and regulator of calcium–phosphorus homoeostasis and maintenance of the haematopoietic niche has now been identified as an important regulator of male fertility and whole-body glucose metabolism, in addition to the classical insulin target tissues. These seminal findings confirm bone to be a true endocrine organ. This review is intended to detail the key events commencing from the elucidation of osteocalcin (OC) in bone metabolism to identification of new and emerging candidates that may regulate energy metabolism independently of OC.
K J Oldknow, V E MacRae and C Farquharson
V E MacRae, C Farquharson and S F Ahmed
Childhood chronic inflammatory disease can be associated with transient and permanent growth retardation. This study examined the potential for spontaneous growth recovery following pro-inflammatory cytokine exposure. Murine ATDC5 chondrogenic cells and postnatal metatarsals were exposed to interleukin (IL)-1β, IL-6 and tumour necrosis factor-α (TNFα), and their growth and proliferative capacity were determined following recovery. TNFα and IL-1β reduced chondrocyte proliferation and aggrecan and collagen types II and X expression at minimum concentrations of 10 ng/ml and 0.1 ng/ml respectively. TNFα but not IL-1β exposure led to increased caspase-3 activity and altered cellular morphology, consistent with reduced viability. Cytokine exposure particularly inhibited proteoglycan synthesis. This effect was dose and duration dependent. Compared with the control, IL-1β and TNFα led to a 71% and 45% reduction in metatarsal growth after 8 days of exposure respectively (P < 0.05). An additive effect of IL-1β combined with TNFα was observed (110% decrease; P < 0.05). Metatarsals exposed to IL-1β or TNFα individually for a 2-day period, and allowed to recover spontaneously in the absence of cytokines for a further 6 days, showed normal growth trajectories. In combination, growth was 59% lower (P < 0.01) compared with control metatarsals at the end of the recovery period. Exposure to the combination for 4 days followed by a 4-day recovery period resulted in 87% decrement compared with controls (P < 0.05). IL-6 did not alter any parameter studied. IL-1β and TNFα exert diverse inhibitory effects on ATDC5 chondrocyte dynamics and metatarsal growth. The extent of recovery following cytokine exposure depends on the duration of exposure, and may be incomplete following longer periods of exposure.
V E MacRae, T Burdon, S F Ahmed and C Farquharson
Proinflammatory cytokines inhibit growth plate development. However, their underlying mechanisms of action are unclear. These effects may be mediated by ceramide, a sphingosine-based lipid second messenger, which is elevated in a number of chronic inflammatory diseases. To test this hypothesis, we determined the effects of C2-ceramide, a cell permeable ceramide analogue, on the growth of the ATDC5 chondrogenic cell line and on cultured fetal mice metatarsals. In ATDC5 cells, C2-ceramide significantly induced apoptosis at both 40 (82%; P < 0.05) and 25 μM (53%; P < 0.05). At 40 μM, C2-ceramide significantly reduced proliferation ([3H]-thymidine uptake/mg protein) (62%; P < 0.05). C2-ceramide did not markedly alter the differentiation state of the cells as judged by the expression of markers of chondrogenesis and differentiation (sox 9, collagen II and collagen X). The IGF-I signalling pathway is the major autocrine/paracrine regulator of bone growth. Both in the presence and absence of IGF-I, C2-ceramide (25 μM) induced an equivalent reduction in proliferation (60%; P < 0.001). Similarly, C2-ceramide (40 μM) induced a 31% reduction in fetal metatarsal growth both in the presence and absence of IGF-I (both P < 0.001). Furthermore, C2-ceramide reduced ADCT5 proliferation in the presence of AG1024, an IGF-I and insulin receptor blocker. Therefore, C2-ceramide-dependent inhibition appears to be independent of IGF-mediated stimulation of bone growth. Indeed, biochemical studies demonstrated that C2-ceramide (25 μM) pretreatment did not alter IGF-I-stimulated phosphorylation of insulin receptor substrate-1, Akt or P44/42 MAP kinase. In conclusion, C2-ceramide inhibits proliferation and induces apoptosis in growth plate chondrocytes through an IGF-I independent mechanism.
R Dobie, V E MacRae, C Huesa, R van't Hof, S F Ahmed and C Farquharson
The suppressor of cytokine signalling (Socs2 −/−)-knockout mouse is characterised by an overgrowth phenotype due to enhanced GH signalling. The objective of this study was to define the Socs2 −/− bone phenotype and determine whether GH promotes bone mass via IGF1-dependent mechanisms. Despite no elevation in systemic IGF1 levels, increased body weight in 4-week-old Socs2 −/− mice following GH treatment was associated with increased cortical bone area (Ct.Ar) (P<0.01). Furthermore, detailed bone analysis of male and female juvenile and adult Socs2 −/− mice revealed an altered cortical and trabecular phenotype consistent with the known anabolic effects of GH. Indeed, male Socs2 −/− mice had increased Ct.Ar (P<0.05) and thickness associated with increased strength. Despite this, there was no elevation in hepatic Igf1 expression, suggesting that the anabolic bone phenotype was the result of increased local GH action. Mechanistic studies showed that in osteoblasts and bone of Socs2 −/− mice, STAT5 phosphorylation was significantly increased in response to GH. Conversely, overexpression of SOCS2 decreased GH-induced STAT5 signalling. Although an increase in Igf1 expression was observed in Socs2 −/− osteoblasts following GH, it was not evident in vivo. Igf1 expression levels were not elevated in response to GH in 4-week-old mice and no alterations in expression was observed in bone samples of 6-week-old Socs2 −/− mice. These studies emphasise the critical role of SOCS2 in controlling the local GH anabolic bone effects. We provide compelling evidence implicating SOCS2 in the regulation of GH osteoblast signalling and ultimately bone accrual, which maybe via mechanisms that are independent of IGF1 production in vivo.