Clinical Review

The Pathobiology of Diabetes Mellitus in Bone Metabolism, Fracture Healing, and Complications

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Also as already discussed, AGEs affect intracellular processes by protein binding and gene regulation and by disrupting the communication between cells and the surrounding matrix. From an extracellular standpoint, AGEs bind to circulating proteins, promoting inflammation and upregulation/downregulation of growth factors, including endothelial nitric oxide synthase, a critical vasodilator. Endothelin 1, on the other hand, is a potent vasoconstrictor. It is upregulated while transforming growth factor b and plasminogen activator inhibitor 1 are upregulated, resulting in further vascular damage.26 The common mechanism for this vasculopathy appears to be superoxide production in the mitochondria, caused by excess glucose oxidation forcing coenzyme Q to donate electrons to oxygen, producing the superoxides. Superoxides in turn inhibit glyceraldehyde 3-phosphate dehydrogenase, which activates the polyol pathway, AGE formation, PKC, and the hexosamine pathway.26 In addition to coenzyme Q, several other enzymes generate reactive oxygen species, including nicotinamide adenine dinucleotide phosphate oxidase, aldehyde oxidase, xanthine oxidase, and glucose oxidase.27 These reactive oxygen species exacerbate oxidative stress, leading to further endothelial cell damage, and cause vascular smooth muscle injury.28

Further influencing the wound-healing environment are the effects of diabetes on blood vessel maintenance and repair as well as angiogenesis in response to local-tissue hypoxia. Vessel-repair mechanisms require endothelial progenitor cells (EPCs), which are released in response to cytokines and neural impulses.29 Bone marrow–derived EPCs have inadequate proliferative and migratory ability in patients with diabetes.28,30 In a diabetic mouse model, EPCs appear in the bone marrow at normal levels, but levels in circulation are lower than anticipated, because of poor proliferation and mobilization, it is thought. In terms of local-tissue hypoxia, hypoxia-inducible factor 1 (HIF-1) is an important transcription factor that promotes the expression of genes that in turn induce angiogenesis. The mechanism of this response is complex, and hyperglycemia has the potential to interfere in various steps of the cycle. In response to local-tissue hypoxia, the HIF-1a subunit must localize to the target site, where it combines with HIF-1b to create the active dimer, HIF-1.31 This active dimer is regulated through degradation of the a subunit in the presence of normal oxygen levels. However, in a state of hypoxia, the molecule is stabilized, promoting angiogenesis and fibroblast migration.32 Recent evidence suggests that hyperglycemia interferes with the dimerization process and that there is a failure of HIF-1a to locate into the nucleus, which is crucial for gene upregulation.31-33

Infection in Diabetes

Throughout the literature, the risk for infection after fracture is consistently higher in patients with diabetes than without diabetes. There likely are many contributing factors, including diminished blood flow and vasculopathy as well as a dampened immune response as a result of defective granulocytic, phagocytic, and chemotactic functions and defective macrophagic activity. Typically, polymorphonuclear leukocytes (PMNs) migrate to bacteria and initiate bacteriocidal activity, and then macrophages phagocytize PMNs and other damaged cells. PMNs demonstrate impaired function in patients with diabetes—reduced phagocytic response and respiratory burst as well as chemotaxis impairment. The diminished phagocytic potential is substantial, with experiments showing an almost 50% reduction in ingestion of Staphylococcus aureus in a patient with diabetes than in one without diabetes.34 Expression of surface integrins, which mediate PMN adhesion to the basement membrane of the tissue, appears to be negatively altered in both T1DM and T2DM, furthering diminishing the chemotactic response of PMNs.35 Impaired leukocyte function may also be a downstream effect of vasculopathy and associated hypoxia/hypoxemia as PMNs use superoxide radicals and other oxidizing agents to create a bacteriocidal environment that is negatively impacted in a low oxygen state.3 In addition, macrophages are disabled in patients with diabetes. (In rats with streptozotocin-induced diabetes, there is inadequate activation of macrophages in the early stages of healing.36) Furthermore, AGEs similar to those mentioned earlier have a significant negative impact on macrophagic function.37 Thus, both the activation and the activity of macrophages appear to be impeded in the setting of diabetes.

Potential New Treatment Modalities

There is tremendous potential for clinical intervention to prevent pathologic outcomes in patients with diabetes, given the complex tissue, cellular, and molecular interactions, particularly those caused by hyperglycemia. At the bone tissue level, increased osteoclastic activity in patients with diabetes has been associated with many complications, including Charcot arthropathy. RANKL modulates differentiation and activation of osteoclasts; thus, RANKL inhibition is a possible therapeutic target.38 Elevated AGE levels have also been observed in patients with Charcot arthropathy, and RAGE, the receptor for AGE, has been seen at lower than expected levels in patients with diabetes. RAGE appears to provide a protective effect against excessive bone resorption; therefore, treatment that increases RAGE levels—such as angiotensin-converting-enzyme inhibitors, statins, and glitazones—may be capable of mitigating the osteoclastic effects in Charcot arthropathy.39

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