Enhanced immunoreceptor tyrosine-based activation motif signaling is related to pathological bone resorption during critical illness

Osteoporosis and the Critically Ill Patient

Improved survival after critical illness has led to recognition of impaired recovery following critical illness as a major public health problem. A consistent association between critical illness and accelerated bone loss has been described, including changes in bone turnover markers, bone mineral density, and fragility fracture rate. An association between accelerated bone turnover and increased mortality after critical illness is probable. Assessment of the effect of antifracture agents on fracture rate and mortality in the high-risk population of postmenopausal women with prolonged ventilation is warranted.

Early and Sustained Changes in Bone Metabolism After Severe Burn Injury

CONTEXT

Severe burn injury causes a massive stress response, consecutively heightened serum levels of acute phase proteins, cortisol, and catecholamines with accompanying disturbance in calcium metabolism.

OBJECTIVE

Evaluation of early and prolonged changes of serum bone turnover markers (BTMs) and regulators of bone metabolism.

DESIGN

Longitudinal observational design.

SETTING

University clinic.

PATIENTS

A total of 32 male patients with a median age of 40.5 years and a median burned total body surface area of 40% (83% patients with full thickness burn injury).

INTERVENTIONS

None.

MAIN OUTCOME MEASURES

Comparison of changes of BTM/regulators of bone metabolism in the early (d 2–7) and prolonged (d 7–56) phases after trauma.

RESULTS

All investigated BTM/regulators significantly changed. During the early phase, pronounced increases were observed for serum type 1 collagen cross-linked C-telopeptide, intact N-terminal propeptide of type I procollagen, sclerostin, Dickkopf-1, bone-specific alkaline phosphatase, fibroblast growth factor 23, and intact parathyroid hormone levels, whereas 25-hydroxyvitamin D, albumin, serum, and ionized calcium levels decreased. Changes of osteoprotegerin, osteocalcin, and phosphate were less pronounced but remained significant. In the prolonged phase, changes of intact N-terminal propeptide of type I procollagen were most pronounced, followed by elevated sclerostin, osteocalcin, bone-specific alkaline phosphatase, and lesser changes for albumin levels. Calcium and ionized calcium levels tardily increased and remained within the limit of normal. In contrast, levels of intact parathyroid hormone, fibroblast growth factor 23, C-reactive protein, and to a lesser extent serum type 1 collagen cross-linked C-telopeptide and phosphate levels declined significantly during this phase of investigation.

CONCLUSIONS

Ongoing changes of BTM and regulators of bone metabolism suggest alterations in bone metabolism with a likely adverse influence on bone quality and structure in male patients with severe burn injuries.

Autonomic dysfunction and osteoporosis after electrical burn

INTRODUCTION

Several studies have shown the importance of the sympathetic nervous system in bone metabolism. There is an evidence of sympathetic skin response (SSR) impairment in electrical burn patients up to 2 years after their injuries. The acute phase of burn is accompanied by increased bone resorption. Whether the prolonged dysfunction of sympathetic nervous system may result in bone metabolism derangement even after the acute phase of electrical burn is the inspiring question for this study. And we tried to find correlation between SSR abnormality and areal bone mineral density (BMD) in electrical burn patients 6 months or more after the incidents.

METHODS AND MATERIALS

42 electrical burn patients (≥6 months prior to study) who did not have a known joint or bone disease, history of neuropathy (central or peripheral), diabetes mellitus or consumption of any drug affecting the autonomic nervous system or evidence of neuropathy in nerve conduction study were recruited. We also gathered a control group of 50 healthy subjects (without electrical burn or the exclusion criteria). They went under dual energy X-ray absorptiometry and SSR study. Data were analyzed statistically with SPSS 16.0 making use of independent t-test and Pearson correlation coefficient. P<0.05 was considered significant statistically.

RESULTS

Areal BMD was significantly lower in electrical burn patients than control group (P<0.001). SSR latency was significantly prolonged and its amplitude was significantly reduced in burn patients compared to control group (P<0.001). In burn patients there was an inverse correlation of areal BMD of lumbar vertebrae, left femur neck and total femur with SSR latency and a direct correlation of areal BMD with SSR amplitude. In control group there was just direct correlation of areal BMD of lumbar vertebrae and left femur neck with SSR amplitude.

CONCLUSION

Electrical burn patients are at risk of reduced areal BMD long after their injuries. Sympathetic derangement and impaired SSR are correlated with reduction in areal BMD in these patients.

Critical illness-induced bone loss is related to deficient autophagy and histone hypomethylation

BACKGROUND

Survivors of critical illness are at increased risk of fractures. This may be due to increased osteoclast formation during critical illness, leading to trabecular bone loss. Such bone loss has also been observed in Paget's disease, and has been related to deficient autophagy. Deficient autophagy has also been documented in vital organs and skeletal muscle of critically ill patients. The objective of this study was to investigate whether deficient autophagy can be linked to critical illness-induced bone loss.

METHODS

Osteoclasts grown in vitro and their precursor cells isolated from peripheral blood of critically ill patients and from matched healthy volunteers were analysed for the expression of autophagy genes (SQSTM1, Atg3 and Atg7), and proteins (p62, Atg-5, and microtubule-associated protein light chain 3-II (LC3-II)) and for autophagy and epigenetic signalling factors via PCR arrays and were treated with the autophagy inducer rapamycin. The effect of rapamycin was also investigated at the tissue level in an in vivo rabbit model of critical illness.

RESULTS

Many more osteoclasts formed in vitro from the blood precursor cells isolated from critically ill patients, which accumulated p62, and displayed reduced expression of Atg5, Atg7, and LC3-II compared to healthy controls, suggesting deficient autophagy, whilst addition of rapamycin reduced osteoclast formation. PCR arrays revealed a down-regulation of histone methyltransferases coupled with an up-regulation of negative regulators of autophagy. Critically ill rabbits displayed a reduction in trabecular and cortical bone, which was rescued with rapamycin.

CONCLUSIONS

Deficient autophagy in osteoclasts and their blood precursor cells at least partially explained aberrant osteoclast formation during critical illness and was linked to global histone hypomethylation. Treatment with the autophagy activator Rapamycin reduced patient osteoclast formation in vitro and reduced the amount of bone loss in critically ill rabbits in vivo. These findings may help to develop novel therapeutic targets to prevent critical illness-induced bone loss.