ÁREAS FUNCIONALES BAJO EL ENFOQUE HUMANISTA Pensar la administración humanista en las áreas funcionales es ubicarla 

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Several currently available treatments for type 2 diabetes mellitus (T2DM) have been reported to negatively affect bone. The incretin-based therapies including, GLP-1 mimetics and DPPIV inhibitors, are new classes of drugs introduced for the treatment of T2DM. Although these agents are largely known to improve glycaemic control, their long-term effects on bone quality have not yet been fully established. This study examined the effects of daily treatment with the well-established GLP-1 mimetic, Exendin-4 (25 mmol/kg bw, i.p) and DPP-IV inhibitor, Sitagliptin (50 mmol/kg bw, orally) on metabolic control, bone mineral density, microarchitecture and strength inhigh-fat fed mice. Exendin-4 and Sitagliptin had no effect on body weight of high-fat fed mice. Reductions of non-fasting glucose (Exendin-4 : 58% decrease, P < 0.01; Sitagliptin : 41% decrease, P < 0.001) were accompanied by significant increases of non-fasting insulin (Exendin-4 : 83% increase, P < 0.01; Sitagliptin : 55% increase, P < 0.01) in treated compared to nontreated mice. Glucose tolerance (22-32%, decrease, P < 0.05) and insulin sensitivity (50-57% increase, P < 0.001) were improved with Exendin-4 and Sitagliptin treatment. As compared to lean mice, diabetic high-fat mice exhibited significant increases in total fat mass (27% increase, P < 0.01). However, no significant differences in fat mass of Exendin-4 or Sitagliptin treated mice when compared to high-fat controls. Administration of Exendin-4 or Sitagliptin did not affect on total, lumbar, femoral and tibial BMD and BMC in high-fat fed mice. In parallel, bone mineral content, as determined by GLmeanand absorbing material density at both cortical and trabecular bones, did not change between Exendin-4- or Sitagliptin-treated and their saline- treated diabetic controls. Administration of Exendin-4 marginally increased trabecular bone volume which contributed to the increase in trabecular numbers and reduction in trabecular separation. These changes however, did not reach statistical significance when compared to saline controls. Similarly, Sitagliptin treatment did not improve parameters measured at trabecular bone in high-fat fed mice. From assessment by microCT, high-fat diet altered cortical bone geometry resulting in marked increases in cortical bone diameter (10% increase, P < 0.05), bone marrow (18-25% increase, P < 0.001) and CSMI (49% increase, P < 0.05). These parameters were not improved by Exendin-4 or Sitagliptin therapy. In three-point bending tests, augmented resistance to fracture was observed (32% increase, P < 0.01) in diabetic

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mice treated with Exendin-4 but not Sitagliptin, while ultimate force, ultimate displacement and stiffness remained unchanged in both treated groups. From this study, it is concluded that Exendin-4 improved whole-bone mechanical strength, whilst Sitagliptin has neutral effects on bone in high-fat fed mice.

6.2 INTRODUCTION

Bone remodeling is a continuous process throughout life and is coordinated by bone- forming osteoblasts and bone-resorbing osteoclasts (Matsuo & Irie 2008). In this dynamic organ, maintenance of normal osteoblast-osteoclast communication is vital as imbalance in bone formation and resorption will reduce bone quality, leading to increased risk of fracture (Seeman 2002). Diabetes is closely associated with bone impairments and type 2 diabetes mellitus patients are highly susceptible to bone fragility fractures, despite having normal or elevated bone mineral density (Vestergaard, 2007). Emerging evidence on further bone loss with existing antidiabetic therapies such as thiazolidinediones (TZDs) (Schwartz et al. 2006) has lead to major concern on the safety of currently available diabetes treatment on bone. As newly-introduced class of antidiabetic drugs, the impact of incretin-based therapies on bone strength and quality remain largely unknown.

Glucagon-like peptide-1 (GLP-1) is released in response to nutrient ingestion and stimulates glucose-dependent insulin secretion (Baggio & Drucker 2007). GLP-1 also plays a role in delaying gastric emptying, inhibiting glucagon release and promoting satiety (Drucker & Nauck 2006). Furthermore, administration of GLP-1 stimulates pancreatic beta-cells proliferation and reduces islet apoptosis (Li et al. 2003, Xu et al. 1999). Therefore, the collective actions of GLP-1 make this molecule a promising therapeutic approach for type 2 diabetes mellitus (T2DM). As such, GLP-1-based therapies have now been developed for T2DM and the first approved drug in this class was Exendin-4 (Exenatide) (Lovshin & Drucker 2009). Exendin-4 shares 53% sequence homology to mammalian GLP-1, and substitution of alanine with glycine at position 2 of the peptide protects the molecule from rapid degradation by the ubiquitous serine protease, dipeptidyl peptidase-4 (DPP-4).

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DPP-4 inhibitor-based drugs were later developed to directly inhibit the actions of DPP-4 to degrade incretin hormones (Barnett 2006). As a result, this prolongs the glucoregulatory and insulinotropic actions of both incretin peptides, namely GLP-1 and its sister incretin glucose-dependent insulinotropic polypeptide (GIP). Treatment with DPP-4 inhibitors enhances beta-cells regeneration and survival in STZ-induced diabetes model of mice (Pospisilik et al. 2003) and a more recent study found preservation of islet beta-cell mass and increase in pancreatic insulin content in isolated islets from high-fat fed mice treated with Sitagliptin (Mu et al. 2006). Sitagliptin (Januvia), was released onto the market in 2006 as orally administered drug for once-daily administration (Lovshin & Drucker 2009). Sitagliptin was proven to improve glycaemic control either in monotherapy or in combination with metformin or thiazolidinedione treatment (Aschner et al. 2006, Charbonnel et al. 2006, Rosenstock et al. 2006). However, the effect of drugs such as Sitagliptin on bone turnover, strength and quality in diabetes remain largely unknown.

In the present study we assessed the capacity of Exendin-4 and Sitagliptin to improve aspects of metabolic control, bone micromorphology and whole-bone mechanical strength in animal dietary-induced high-fat fed mice that present with overt obesity and insulin resistance.