Interrelationship between parathyroid hormone and insulin: effects on DNA synthesis in UMR-106-01 cells

Weak bones in diabetes mellitus – an update on pharmaceutical treatment options

Objectives

Diabetes mellitus is often associated with a number of complications such as nephropathy, neuropathy, retinopathy and foot ulcers. However, weak bone is a diabetic complication that is often overlooked. Although the exact mechanism for weak bones within diabetes mellitus is unclear, studies have shown that the mechanism does differ in both type I (T1DM) and type II diabetes (T2DM). This review, however, investigates the application of mesenchymal stem cells, recombinant human bone morphogenetic protein-2, teriparatide, insulin administration and the effectiveness of a peroxisome proliferator-activated receptor-ϒ modulator, netoglitazone in the context of diabetic weak bones.

Key findings

In T1DM, weak bones may be the result of defective osteoblast activity, the absence of insulin's anabolic effects on bone, the deregulation of the bone–pancreas negative feedback loop and advanced glycation end product (AGE) aggregation within the bone matrix as a result of hyperglycaemia. Interestingly, T2DM patients placed on insulin administration, thiazolidinediones, SGLT2 inhibitors and sulfonylureas have an associated increased fracture risk. T2DM patients are also observed to have high sclerostin levels that impair osteoblast gene transcription, AGE aggregation within bone, which compromises bone strength and a decrease in esRAGE concentration resulting in a negative association with vertebral fractures.

Summary

Effective treatment options for weak bones in the context of diabetes are currently lacking. There is certainly scope for discovery and development of novel agents that could alleviate this complication in diabetes patients.

Bone Cell Bioenergetics and Skeletal Energy Homeostasis

The rising incidence of metabolic diseases worldwide has prompted renewed interest in the study of intermediary metabolism and cellular bioenergetics. The application of modern biochemical methods for quantitating fuel substrate metabolism with advanced mouse genetic approaches has greatly increased understanding of the mechanisms that integrate energy metabolism in the whole organism. Examination of the intermediary metabolism of skeletal cells has been sparked by a series of unanticipated observations in genetically modified mice that suggest the existence of novel endocrine pathways through which bone cells communicate their energy status to other centers of metabolic control. The recognition of this expanded role of the skeleton has in turn led to new lines of inquiry directed at defining the fuel requirements and bioenergetic properties of bone cells. This article provides a comprehensive review of historical and contemporary studies on the metabolic properties of bone cells and the mechanisms that control energy substrate utilization and bioenergetics. Special attention is devoted to identifying gaps in our current understanding of this new area of skeletal biology that will require additional research to better define the physiological significance of skeletal cell bioenergetics in human health and disease.

O-GlcNAcylation: a bridge between glucose and cell differentiation

Glucose is the major energy supply and a critical metabolite for most cells and is especially important when cell is differentiating. High or low concentrations of glucose enhances or inhibits the osteogenic, chondrogenic and adipogenic differentiation of cell via the insulin, transforming growth factor‐β and peroxisome proliferator‐activated receptor γ pathways, among others. New evidence implicates the hexosamine biosynthetic pathway as a mediator of crosstalk between glucose flux, cellular signalling and epigenetic regulation of cell differentiation. Extracellular glucose flux alters intracellular O‐GlcNAcylation levels through the hexosamine biosynthetic pathway. Signalling molecules that are important for cell differentiation, including protein kinase C, extracellular signal‐regulated kinase, Runx2, CCAAT/enhancer‐binding proteins, are modified by O‐GlcNAcylation. Thus, O‐GlcNAcylation markedly alters cell fate during differentiation via the post‐transcriptional modification of proteins. Furthermore, O‐GlcNAcylation and phosphorylation show complex interactions during cell differentiation: they can either non‐competitively occupy different sites on a substrate or competitively occupy a single site or proximal sites. Therefore, the influence of glucose on cell differentiation via O‐GlcNAcylation offers a potential target for controlling tissue homoeostasis and regeneration in ageing and disease. Here, we review recent progress establishing an emerging relationship among glucose concentration, O‐GlcNAcylation levels and cell differentiation.

Review: The diabetic bone: a cellular and molecular perspective

With the increasing worldwide prevalence of diabetes the resulting complications, their consequences and treatment will lead to a greater social and financial burden on society. One of the many organs to be affected is bone. Loss of bone is observed in type 1 diabetes, in extreme cases mirroring osteoporosis, thus a greater risk of fracture. In the case of type 2 diabetes, both a loss and an increase of bone has been observed, although in both cases the quality of the bone overall was poorer, again leading to a greater risk of fracture. Once a fracture has occurred, healing is delayed in diabetes, including nonunion. The reasons leading to such changes in the state of the bone and fracture healing in diabetes is under investigation, including at the cellular and the molecular levels. In comparison with our knowledge of events in normal bone homeostasis and fracture healing, that for diabetes is much more limited, particularly in patients. However, progress is being made, especially with the use of animal models for both diabetes types. Identifying the molecular and cellular changes in the bone in diabetes and understanding how they arise will allow for targeted intervention to improve diabetic bone, thus helping to counter conditions such as Charcot foot as well as preventing fracture and accelerating healing when a fracture does occur.

Is insulin an anabolic agent in bone? Dissecting the diabetic bone for clues

Diabetic osteoporosis is increasingly recognized as a significant comorbidity of type 1 diabetes mellitus. In contrast, type 2 diabetes mellitus is more commonly associated with modest increases in bone mineral density for age. Despite this dichotomy, clinical, in vivo, and in vitro data uniformly support the concept that new bone formation as well as bone microarchitectural integrity are altered in the diabetic state, leading to an increased risk for fragility fracture and inadequate bone regeneration following injury. In this review, we examine the contribution that insulin, as a potential anabolic agent in bone, may make to the pathophysiology of diabetic bone disease. Specifically, we have assimilated human and animal data examining the effects of endogenous insulin production, exogenous insulin administration, insulin sensitivity, and insulin signaling on bone. In so doing, we present evidence that insulin, acting as an anabolic agent in bone, can preserve and increase bone density and bone strength, presumably through direct and/or indirect effects on bone formation.

Bone formation is impaired in a model of type 1 diabetes

The effects of type 1 diabetes on de novo bone formation during tibial distraction osteogenesis (DO) and on intact trabecular and cortical bone were studied using nonobese diabetic (NOD) mice and comparably aged nondiabetic NOD mice. Diabetic mice received treatment with insulin, vehicle, or no treatment during a 14-day DO procedure. Distracted tibiae were analyzed radiographically, histologically, and by microcomputed tomography (microCT). Contralateral tibiae were analyzed using microCT. Serum levels of insulin, osteocalcin, and cross-linked C-telopeptide of type I collagen were measured. Total new bone in the DO gap was reduced histologically (P < or = 0.001) and radiographically (P < or = 0.05) in diabetic mice compared with nondiabetic mice but preserved by insulin treatment. Serum osteocalcin concentrations were also reduced in diabetic mice (P < or = 0.001) and normalized with insulin treatment. Evaluation of the contralateral tibiae by microCT and mechanical testing demonstrated reductions in trabecular bone volume and thickness, cortical thickness, cortical strength, and an increase in endosteal perimeter in diabetic animals, which were prevented by insulin treatment. These studies demonstrate that bone formation during DO is impaired in a model of type 1 diabetes and preserved by systemic insulin administration.

Growth-inhibitory effect of a high glucose concentration on osteoblast-like cells

Impaired bone formation resulting from a decline of osteoblast activity has been implicated in the pathogenesis of diabetic osteopenia. We examined the effects of high glucose concentration alone, independent of insulin deficiency, on the growth of a human osteoblast-like cell line (MG-63). Sustained exposure to high glucose for 7 days inhibited cell growth in a concentration-dependent manner up to 49.5 mmol/L, as compared with cells cultured with a normal glucose concentration (5.5 mmol/L) or a high mannitol concentration (an iso-osmolar control). Glucose (49.5 mmol/L) attenuated the increment either in DNA content or in [3H]thymidine incorporation induced by insulin-like growth factor I (IGF-I). The IGF-I-induced increase of ornithine decarboxylase (ODC) activity, which plays an important role in cell growth, was also attenuated. The half-life of ODC protein was not shortened by the high glucose culture, but the intracellular content of putrescine (an end product of ODC) was significantly decreased. These changes did not occur in the high mannitol culture, strongly suggesting a specific effect of glucose. In summary, our observations suggest that a high glucose concentration significantly impairs the proliferative response of osteoblastic cells to IGF-I and that the defective cell function caused by sustained exposure to high glucose levels might contribute to impaired bone formation in patients with diabetic osteopenia.

Development of secondary hyperparathyroidism and bone disease in diabetic rats with renal failure

Renal osteodystrophy in diabetic patients on maintenance hemodialysis is characterized by a higher prevalence of low bone turnover and is associated with a relative deficiency of parathyroid hormone (PTH) as compared with non-diabetic hemodialysis patients. The goal of the study was to evaluate how diabetes affected the development of secondary hyperparathyroidism (2 degrees HPT) and bone disease in azotemic rats. Three groups of 5/6 nephrectomized, pair-fed male Wistar rats maintained on a high phosphorus (1.2%) diet were studied: (1) the control group, non-diabetic azotemic rats (NDR); and two streptozotocin-induced diabetic azotemic groups, (2) poorly-controlled diabetic rats (PCDR) which received only enough NPH insulin to maintain the blood glucose between 300 and 400 mg/dl, and (3) well-controlled insulin-treated diabetic rats (IDR) which received a continuous insulin infusion for 14 days via a subcutaneously implanted miniosmotic pump. Serum calcium, phosphorus and creatinine levels were similar among the three groups. Blood glucose levels were greater in the PCDR group than the IDR and NDR groups (358 +/- 11 vs. 83 +/- 9 and 87 +/- 8 mg/dl, respectively; P < 0.001). Rats in the PCDR group weighed less at sacrifice as compared with the IDR and NDR groups (P < 0.05). Serum PTH levels (normal 47 +/- 2 pg/ml) were elevated, but not different among the three groups (136 +/- 34, 147 +/- 21 and 98 +/- 8 pg/ml in the PDCR, IDR and NDR groups, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)