DM is a common endocrine disorder characterized by hyperglycemia and predisposes to chronic complications affecting the eyes, blood vessels, nerves and kidneys. Besides its more commonly recognized complications, such as macro vascular disease, retinopathy, nephropathy and neuropathy, diabetes related bone disease has gained growing attention.

The relationship between diabetes and related bone disease are known for over 50 years and

both diseases share similar socioeconomic characteristics. ( \cite{Hough_1989}(Carnevale 2004)

In order to clarify this relationship, many different parameters have been evaluated in the time.

OPG and RANKL are current parameters which are studing to reveal the mechanisms of this known relationship. Recently, tumor necrosis factor (TNF) and TNF superfamily members RANK, RANKL, OPG have been shown to play a key role in osteoclast activation together with other cytokines and calcitotropic hormones.

Bone tissue is constantly renewed due to mechanical stress and hormonal changes. This regeneration depends on the balance between osteoclastic bone destruction and osteoblastic bone formation.

RANKL is expressed by osteoblasts and other bone marrow stromal cells and RANK is expressed by other members of the preosteoclasts and osteoclast series. \cite{Qiu_2006}

The RANKL - RANK interaction activates transcription factors regulating osteoclastogenesis, resulting in osteoclastic assembly and differentiation. OPG synthesized from osteoblasts inhibits this interaction by acting as a trap receptor for RANKL. Thus, proliferation, differentiation and bone resorption of osteoclasts are prevented. \cite{9568710} \cite{Bell_2003}

OPG is a glycoprotein-structured molecule synthesized by the TNFRSF11B gene located on chromosome 8 and it is a cytokine receptor from the TNF receptor superfamily.\cite{9108485}

RANKL is composed of 314 amino acids and was originally described to have a gene sequence containing 5 exons, mapped in the chromosome 13q14. \cite{19716455}

Several factors such as HbA1c, duration of disease, the presence of certain complications, and the treatment that is used, have been used in determining DM severity. Even some of these variables can be used together to calculate disease severity index.

Since it is a simpler and more questionable indicator, we used whether or not patients use insulin in determining the severity of the disease. We separated the patient group into two separate groups, those receiving oral anti-diabetic treatment and those receiving insulin treatment.

PTH promote bone resorption and inhibit Ca extraction to maintain adequate plasma calcium levels. PTH acts its effects on bone thorough osteoclasts. However, interestingly PTH receptors are located on osteoblasts. Communication between osteoblasts and osteoclasts is thought to be provided by RANKL. \cite{Huang_2003}

BAP is considered to be a highly specific marker of the bone-forming activity of osteoblasts. \cite{24009918} Previous studies suggested that serum BAP, is the useful for predicting BMD. \cite{15978265}\cite{2012}

In our study, we aimed to investigate OPG and RANKL levels in control and patient groups in order to evaluate the negative effects of DM on bone. In addition, we assessed the association of parameters such as HbA1c, parathormone, bone-specific ALP with OPG and RANKL. At the same time, we also wanted to evaluate whether OPG and RANKL levels were associated with DM severity in our study.

Discussion

DM2 is a disease characterized by hyperinsulinemia and hyperglycemia due to insulin resistance.

Hyperglycaemia has an important role in the pathogenesis of diabetic complications by increasing protein glycation and the gradual build-up of advanced glycation endproducts (AGEs) in body Tissues. Recent studies suggest that interaction of AGEs with RAGE alter intracellular signalling, gene expression, release of pro-inflammatory molecules and free radicals that contribute towards the pathology of diabetic complication. \cite{15620429}

DM is a multisystemic disease with effects on bone tissue , impairs the quality of bone structure and consequently increased bone fragility. Energy hemostasis affects bone metabolism and It has been reported that osteocalcin mediates this interaction. \cite{17693256}

İnsuline induces the production of under-carboxylated osteocalcin by post-translational modification of osteocalcin in osteoblasts.

Increased under-carbocylated osteocalcin stimulates pancreatic insulin synthesis and secretion while increasing adiponectin production in the fat tissue, thereby altering energy expenditure by increasing sensitivity to insulin production in tissues. \cite{18362359}

Adiponectin is a major cytokine secreted from adipocyte in glucose metabolism.\cite{Yamamoto_2014} Adiponectin, which is known to increase insulin sensitivity, has been reported to be decreased in patients with DM.

Previous studies indicate that adiponectin negatively affects bone formation. \cite{Kanazawa_2008}

Similar to us, we observed that OPG levels were higher in patients with DM than in the control group. It has been reported that the main molecule regulating OPG levels is adiponectin. Adiponectin and OPG have a negative correlation.

In previous studies on OPG and RANKL, groups were frequently performed in patients with advanced-stage DM in microvascular pathology or in groups of patients with cardiovascular system problems. \cite{Gordin_2013} \cite{Bjerre_2013}

The unique aspect of our study was the investigation of OPG and RANKL levels in patients with DM who are using OAD or insulin and in the control group. Thus, we aimed to examine the effect of disease severity on these parameters.

In our study, we observed that the OPG and RANKL values of patients with DM2 whom blood glucose regulated with insulin were significantly higher than the control group. Yaturi et al. It was stated that OPG level was increased in type 2 DM patients. In the same study, it was stated that there was a significant relationship between OPG level and variables that are indicative of insulin resistance like fasting glucose .\cite{18789716}

At the same time, the observed relationship between OPG and HbA1c or DM2 duration is another finding suggesting that OPG is affected by disease severity.

Gaudio et al. reported that plasma OPG levels in type 2 DM patients were significantly higher in the study group than in the control group, but that RANKL levels were significantly lower. \cite{25056244} We also found that OPG levels were significantly higher in type 2 DM patients, but RANKL levels were significantly higher in our study.

However, the study of Gaudio et al. was performed only in postmenopausal women and there was no treatment-related evaluation among the groups. However, Junk et al. was reported that the RANKL level distribution may vary depending on sex.

Jung K, Lein M, Hösslin K et al. Osteoprotegerin and receptor activator of nuclearfactor-kappa Bligand (RANKL) in the serum of healthy adults Int J BiolMarkers. 2002; 17(3): 177-81.

Shao et al. reported that high serum glucose levels could suppress osteoblast proliferation, but the use of metformin (an oral antidiabetic) ameliorated this suppression.\cite{Shao_2014}

Hyperglycemia has an important role in the pathogenesis of diabetic complications by increasing protein g

lycation and the gradual build-up of advanced glycation end products (AGEs) in body Hyperglycemia affects the skeleton at both cellular and extracellular bone matrix levels. In vitro studies have shown that high glucose levels augment osteoclast differentiation/fusion resulting in a more resorptive environment. \cite{Larsen_2002}