SUMMARYNuclear factor kappa B (NF-k B) is a transcription factor pivotal for the development of inflammation. A dysregulation of NF-k B has been shown to play an important role in many chronic inflammatory diseases including rheumatoid arthritis, inflammatory bowel disease and psoriasis. Although classical NFk B, a heterodimer composed of the p50 and p65 subunits, has been well studied, little is known about gene regulation by other hetero-and homodimeric forms of NF-k B. While p65 possesses a transactivation domain, p50 does not. Indeed, p50/p50 homodimers have been shown to inhibit transcriptional activity. We have recently shown that Interleukin-10 exerts its anti-inflammatory activity in part through the inhibition of NF-k B by blocking I k B kinase activity and by inhibiting NF-k B already found in the nucleus. Since the inhibition of nuclear NF-k B could not be explained by an increase of nuclear I k B, we sought to further investigate the mechanisms involved in the inhibition of NF-k B by IL-10. We show here that IL-10 selectively induced nuclear translocation and DNA-binding of p50/p50 homodimers in human monocytic cells. TNF-a treatment led to a strong translocation of p65 and p50, whereas pretreatment with IL-10 followed by TNF-a blocked p65 translocation but did not alter the strong translocation of p50. Furthermore, macrophages of p105/p50-deficient mice exhibited a significantly decreased constitutive production of MIP-2 a and IL-6 in comparison to wild type controls. Surprisingly, IL-10 inhibited high constitutive levels of these cytokines in wt macrophages but not in p105/p50 deficient cells. Our findings suggest that the selective induction of nuclear translocation and DNA-binding of the repressive p50/p50 homodimer is an important anti-inflammatory mechanism utilized by IL-10 to repress inflammatory gene transcription.

Changes in whole body energy levels are closely linked to alterations in body weight and bone mass. Here, we show that hypothalamic signals contribute to the regulation of bone mass in a manner consistent with the central perception of energy status. Mice lacking neuropeptide Y (NPY), a well-known orexigenic factor whose hypothalamic expression is increased in fasting, have significantly increased bone mass in association with enhanced osteoblast activity and elevated expression of bone osteogenic transcription factors, Runx2 and Osterix. In contrast, wild type and NPY knockout (NPY −/−) mice in which NPY is specifically over expressed in the hypothalamus (AAV-NPY+) show a significant reduction in bone mass despite developing an obese phenotype. The AAV-NPY+ induced loss of bone mass is consistent with models known to mimic the central effects of fasting, which also show increased hypothalamic NPY levels. Thus these data indicate that, in addition to well characterized responses to body mass, skeletal tissue also responds to the perception of nutritional status by the hypothalamus independently of body weight. In addition, the reduction in bone mass by AAV NPY+ administration does not completely correct the high bone mass phenotype of NPY −/− mice, indicating the possibility that peripheral NPY may also be an important regulator of bone mass. Indeed, we demonstrate the expression of NPY specifically in osteoblasts. In conclusion, these data identifies NPY as a critical integrator of bone homeostatic signals; increasing bone mass during times of obesity when hypothalamic NPY expression levels are low and reducing bone formation to conserve energy under ‘starving’ conditions, when hypothalamic NPY expression levels are high.

Background & AimsGastrointestinal peptides are increasingly being linked to processes controlling the maintenance of bone mass. Peptide YY (PYY), a gut-derived satiety peptide of the neuropeptide Y family, is upregulated in some states that also display low bone mass. Importantly, PYY has high affinity for Y-receptors, particularly Y1R and Y2R, which are known to regulate bone mass. Anorexic conditions and bariatric surgery for obesity influence circulating levels of PYY and have a negative impact on bone mass, but the precise mechanism behind this is unclear. We thus examined whether alterations in PYY expression affect bone mass.MethodsBone microstructure and cellular activity were analyzed in germline PYY knockout and conditional adult-onset PYY over-expressing mice at lumbar and femoral sites using histomorphometry and micro-computed tomography.ResultsPYY displayed a negative relationship with osteoblast activity. Male and female PYY knockout mice showed enhanced osteoblast activity, with greater cancellous bone mass. Conversely, PYY over-expression lowered osteoblast activity in vivo, via a direct Y1 receptor mediated mechanism involving MAPK stimulation evident in vitro. In contrast to PYY knockout mice, PYY over expression also altered bone resorption, as indicated by greater osteoclast surface, despite the lack of Y-receptor expression in osteoclastic cells. While evident in both sexes, cellular changes were generally more pronounced in females.ConclusionsThese data demonstrate that the gut peptide PYY is critical for the control of bone remodeling. This regulatory axis from the intestine to bone has the potential to contribute to the marked bone loss observed in situations of extreme weight loss and higher circulating PYY levels, such as anorexia and bariatric obesity surgery, and may be important in the maintenance of bone mass in the general population.

On initial inspection, bone remodeling, the process whereby the skeleton adapts through time, appears to be relatively simple. Two cell types, the bone-forming osteoblasts and the bone-resorbing osteoclasts, interact to keep bone mass relatively stable throughout adult life. However, the complexity of the regulatory influences on these cells is continuing to expand our understanding of the intricacy of skeletal physiology and also the interactions between other organ systems and bone. One such example of the broadening of understanding in this field has occurred in the last decade with study of the central, neural regulation of bone mass. Initial studies of an adipose-derived hormone, leptin, helped define a direct, sympathetic pathway involving efferent neural signals from the hypothalamus to receptors on the osteoblast. Since the leptin-mediated pathway has been continuously modified to reveal a complex system involving neuromedin U, cocaineand amphetamine-related transcript and serotonin interacting within the hypothalamus and brainstem to regulate both bone formation and resorption in cancellous bone, a number of other systems have also been identified. Neuropeptide Y, acting through hypothalamic Y2 receptors, is capable of skeleton-wide modulation of osteoblast activity, with important coordination between body weight and bone mass. Cannabinoids, acting through central cannabinoid receptor 1 and bone cell cannabinoid receptor 2 receptors, modulate osteoclast activity, thereby identifying pathways active on both aspects of the bone remodeling process. This review explores the key central pathways to bone and explores the complexity of the interactions being revealed by this emergent field of research. Journal of Molecular Endocrinology (2010) 45, 175-181 Function of hypothalamusThe brain has long been appreciated as a pivotal regulator of homeostasis in peripheral tissues, including the skeleton. There is now clear evidence for crosstalk between the brain and bone through two distinct routes. The first pathway comprises welldefined hormonal signals arising from neuroendocrine neurons of the hypothalamus and subsequently processed within the pituitary. The second pathway consists of efferent neuronal discharges originating from the hypothalamus and processed through the brainstem. The hypothalamus, with its semipermeable blood-brain barrier, is thus one of the most powerful regulatory regions within the body, integrating signals not only from peripheral tissues but also from within the brain itself. These direct, neural pathways represent an emergent area of study that is identifying novel regulatory axes between the brain and the cells of bone. Moreover, this work is also providing insights into regulatory connections involving skeletal tissue, which are proving to be unexpected, thereby outlining a level of interconnectedness that has been previously unappreciated. This review examines the expanded understanding of the central, neural outputs to bone metabolism and remodeling.