Influence of glucoregulation quality on C-reactive protein , interleukin-6 and tumor necrosis factor-α level in patients with diabetes type 1

Background/Aim. Results of studies which have proved an increased inflammatory activity in diabetes type 1, have been published over recent years. One of possible mechanisms that are used to explain chronic inflammation in diabetes is the state of hyperglycemia leading to the enhanced synthesis of glycosylation end products (AGEs) which activate macrophages, increase the oxidative stress and affect the synthesis of interleukins (IL-1, IL-6), tumor necrosis factor-α (TNF-α) and C-reactive protein (CRP). The aim of the study was to determine the inflammatory markers (CRP, IL-6, TNF-α) in patients with diabetes type 1 and to establish their correlation with glucoregulation parameters and other cardiovascular risk factors as well as to compare them with the healthy controls. Methods. The study included 76 patients with diabetes type 1 and 30 healthy controls. We determined values of inflammatory markers (CRP, IL-6, TNF-α) and glucoregulation parameters (fasting glucose HbA1c). Results. The values of CRP (p = 0.014), IL-6 (p = 0.020) and TNF-α (p = 0.037) were statistically significantly higher in the diabetic patients than in the healthy controls. There was a positive correlation between CRP with postprandial glycemia (p = 0.004); the multivariate regression analysis revealed a statistically significant correlation between CRP and age (p = 0.001), smoking (p = 0.055), fasting glucose (p = 0.021) and triglycerides (p = 0.048) as well as between IL-6 and LDLcholesterol (p = 0,009). No statistically significant correlations were found between glycosilated hemoglobin (HbA1c) and the inflammatory markers (CRP, IL-6 and TNF-α). Conclusion. The patients with type 1 diabetes were found to have a low level of inflammatory activity manifested by the increased values of CRP, IL-6 and TNF-α.


Introduction
Numerous epidemiological and clinical studies as well as those performed on autopsy material have proved that atherogenesis develops earlier in patients with diabetes type 1 than in healthy population, thus making the progression of cardiovascular complications far more expressed 1,2 .It is perfectly clear that atherogenesis is a chronic inflammatory process and this fact has been corroborated by results of numerous studies which have found increased values of inflammatory markers such as C-reactive protein (CRP), interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α).Many studies, whose results have been published in recent years, have shown that diabetes type 1 is also associated with the increased inflammation 2 .One of the mechanisms which might explain chronic inflammation in diabetes is the condition of hyperglycemia, which leads to an increased synthesis of advanced glycation endproducts (AGEs) resulting from the interaction of glucose in high concentrations with structural and circulating proteins.Advanced glycation end products are considered to activate macrophages, increase the oxidative stress and affect the synthesis of IL-1, IL-6, TNF-α and CRP.Of all the proteins of the acute phase and plasma inflammatory markers, C-reactive protein (CRP) has been most widely studied and it is believed to have a very important role in the endothelial dysfunction and the process of atherosclerosis.It is also considered to be one of the important and independent predictors of future cardiovascular events.Hepatocytes produce CPR as a response to the increased level of IL-6, IL-1, TNF-α [1][2][3] , which is a sensitive infection marker and it is produced as a systemic inflammatory response to a local or systemic infection.It is widely used in clinical setting to follow not only the disease course but also the effects of the applied antiinflammatory and antibiotic therapy 3,4 .A high sensitivity C-reactive protein is used to detect small changes of CRP levels associated with an increased cardiovascular risk in healthy population 5 .Interleukin 6 is an intercellular mediator and primary indicator of the liver CPR 6 .Although it originates from T cells, other cells such as macrophages, monocytes, smooth-muscle cells, epithelial/endothelial/mesangial cells, fibroblasts, synovial cells, osteoblasts as well as chondrocytes may also be provoked to produce IL-6 7 .An increased IL level is present in many autoimmune diseases, such as rheumatoid arthritis, systemic lupus erythematosus as well as in diabetes of both types 1 and 2 [8][9][10] .The circulating IL-6 stimulates hypothalamic-hypophyseal axis whose activation is responsible for the central obesity, hypertension and insulin resistance 11,12 .TNF-α is a pleiotropic cytokine produced by various cells such as macrophages, endothelial and smooth-muscle cells.It is one of the most important cytokines in the intercellular communication 13 .Recent studies have proved that TNF-α activation may directly or indirectly affect pathogenesis and induce macrovascular complications in diabetes as well as atherosclerotic vascular lesions 14,15 .It has a major role in the amplification of inflammatory cascade.TNF-α also plays a certain role in damaging the pancreas beta cells by being responsible not only for diabetes type 1 pathogenesis but for the development of insulin resistance associated with obesity and diabetes type 2, as well 15,16 .
The aim of the study was to analyse the level of inflammatory markers CRP, IL-6 and TNF-α in a group of patients with diabetes type 1 and a group of healthy controls and to correlate their values with the parameters of glycoregulation such as glycosylated hemoglobin (HbA 1c ), fasting and postprandial glycemia as well as lipid and lipoprotein status.

Methods
This cross-sectional study was performed on a group of 106 subjects, of whom 76 were the patients with diabetes type 1 and 30 were the healthy controls.The study group consisted of the patients with diabetes type 1, which had been diagnosed before they were 36 years of age and who were on insulin therapy in the first year after the diagnosis had been made.They either visited day hospital for diabetes or were hospitalized in the Department of Endocrinology, Clinical Center of Vojvodina.The group of healthy controls consisted of 30 subjects of both sexes, of approximately the same age, normally nourished and without other risk factors for atherosclerosis.
The following data were taken for both groups of subjects: sex, age, length of the disease, age when the disease was diagnosed, smoking habit and family medical history.To assess the state of metabolic regulation in diabetes the values of fasting glycemia were taken 2 hours after breakfast as well as the values of HbA1c; when the latter were less than 7.5% they pointed to the satisfactory glucoregulation in diabetes, whereas when they were over 7.5%, glucoregulation was considered unsatisfactory.Values of total cholesterol, LDL-cholesterol, HDL-cholesterol and triglycerides were measured to analyze the lipid and lipoprotein status.The nourishment status was assessed on the basis of anthropometric measurements, body mass, body height and body mass index (BMI).The waist was measured in centimeters and the recommended values according to the IDF criteria 17 were considered to be desirable: less than 80 cm for women and less than 94 cm for men.The following inflammatory markers were determined: serum fibrinogen concentrations, CRP, IL-6 and TNF-α.The serum samples were kept at -80ºC before being analyzed.C-reactive protein was determined by electrochemiluminescence, and the referral values were from 0 to 5mg/L.TNF-α and IL-6 were determined in the Laboratory for Immunology, Department of Nephrology and Immunology, Clinical Center Vojvodina by commercial ELISA plates according to the standard procedure recommended by the manufacturer.
The collected data were processed by the methods of descriptive and inferential statistics.The following was presented as the numerical characteristics: the arithmetic mean, median, standard deviation and either the value range or the interquartile range, depending on the data nature.Mean values of the numerical characteristics of the two groups were compared by the t-test.Differences in the distribution of numerical non-homogenous characteristics between the two groups were compared by the nonparametric Mann-Whitney test and among three groups by the Kruskal-Wallis test.The correlation of the two characteristics were examined by the Spearman's coefficient of correlation.Multivariate regression analysis was applied to determine the predictions and correlations between dependent variables with independent ones: the linear regression was used when the dependent variable was continuous and the logistic regression model was applied when the dependent variable was dichotomous (binary).

Results
Table 1 shows the characteristics of the study and the control group which did not differ in sex and age structure, nourishment status, values of total cholesterol, HDLcholesterol, LDL-cholesterol and triglycerides.Not surprisingly, the study group had statistically significantly higher values of fasting glycemia (p = 0.000), postprandial glycemia (p = 0.003) as well as the values of HbA1c (p = 0.000) compared to the healthy controls.The values of CPR (p = 0.014), IL-6 (p = 0.0200), TNF (p = 0.037) were statistically significantly higher in the diabetic patients than in the healthy con-trols.The average duration of diabetes in the group of diabetic patients was 20.01 years.
As shown in Table 2, the correlation between CRP and postprandial glycemia was significant (p = 0.021) and positive.No significant correlation was found between fasting glycemia and CRP, TNF-α, IL-6 nor between postprandial glycemia and TNF-α and IL-6.
According to the HbA 1c value, which was over 7.5% in almost 87% of the patients (Table 3) it was concluded that the study group of the diabetic patients had a poor metabolic regulation.No statistically significant correlation was found between HbA 1c and CRP (p = 0.878), IL-6 (p = 0.249) and TNF-α (p = 0.817).
When multiple linear regression was applied, the CRP value was regarded as a dependent variable, and the rest of the mentioned values were regarded as independent variables.The following variables were found to be significant CRP predictors: age, smoking habit, fasting glycemia, triglycerides; whereas sex, age, LDL-cholesterol and triglycerides were significant for predicting IL-6 levels.All independent variables shown in Table 4 (the reduced model) were significantly correlated with the CRP value (F = 6.568, p = 0.000) and IL-6 value (F = 3.121, p = 0.020).According to the obtained coefficient value it can be stated that 31.9% of the changes were in the CRP level; 15% of the changes in the values of IL-6 were explained by the changes in values of independent variables from Table 4 (the reduced model).

Discussion
Inflammation and oxidative stress play an important role in the process of atherosclerosis; therefore, patients having diabetes type 1 are at higher risk for cardiovascular morbidity and mortality.Although the majority of studies on these problems dealt with diabetes type 2, recently published results have indicated that there is an increased inflammatory activity in patients having diabetes type 1, as well 1,2,7,11,13,15 .The afore mentioned has been corroborated by our results as well, which have shown that patients with diabetes type 1 have a low degree of an inflammatory activity, which is reflected through the increased values of CRP, TNF-α, IL-6.All of the studied inflammatory markers, CRP, IL-6, TNF-α were significantly higher in the diabetic patients than in the healthy controls.These results are in accordance with the results of studies which have found increased CRP in adults having diabetes type 1 11,12 .Although the mechanism of CRP rise is not completely clear, it seems to be associated with the activation of macrophages, increased oxidative stress and induction of cytokines.In their study, Okano et al. 18 observed statistically significantly higher CRP values in patients with diabetes type 1 than in the healthy controls.Their study did not show statistically significant differences in age, the nourishment status index, values of LDL-cholesterol, HDLcholesterol and triglycerides between the diabetic patients and healthy controls.This study has clearly shown that the increased level of hs-CRP correlated with the early stage of carotid atherosclerosis in young patients having diabetes type 1, measured through the level of carotid intima-media thickness 18 .Alexandraki et al. 19 found higher values of CRP, IL-6 and TNF-α in the patients with diabetes type 1 than in the healthy controls.However, the values of the above mentioned inflammatory markers were lower than in the group of patients having diabetes type 2.
Chronic hyperglycemia results in advanced glycation end products (AGE), which activate macrophage, increase the oxidative stress and affect the synthesis of IL-6, IL-1, TNF-α and CRP.A great number of studies has clearly shown a significant correlation between HbA1c, fasting and postprandial glycemia and the inflammatory markers CRP, IL-6 and TNF-α [19][20][21][22][23][24][25] .A positive correlation was found between the values of CRP and postprandial glycemia and the one with fasting glycemia was determined by the multivariate regression analysis.These results are in accordance with the most recent opinions about the importance of postprandial hyperglycemia in the development of inflammation and chronic complications.Postprandial hyperglycemia is a very frequent phenomenon in patients with both type 1 and type 2 diabetes, and it can be also found in patients with wellregulated diabetes, assessed on the basis of HbA1c values 18- 20 .Postprandial hyperglycemia increases the oxidative stress and together with hypertriglyceridemia increases the production of intercellular adhesion molecules (ICAM) and vascular cell adhesion molecules (VCAM), E-selectin as a marker of the endothelial dysfunction, thus affecting the increased production of inflammatory cytokines [22][23][24][25] .Our results did not show a significant correlation between the inflammatory markers (CRP, IL-6, TNF-α) and the values of HbA 1c .Such findings may be explained by the fact that almost 87% of the patients were metabolically unregulated, two fifth of them having HbA 1c values over 9.5%.These results can be compared with the results of some studies which have also failed to show the existence of a significant correlation between inflammatory markers and HbA 1c 10, 20 , thus making it clear that factors other than hyperglycemia affect inflammation and endothelial dysfunction in diabetes.
Dyslipidemia in diabetes type 1 is mostly the result of a poor metabolic regulation of the diseases, with the consequent increase in triglycerides and decrease in HDLcholesterol 26 .Besides, the development and progression of nephropathy in diabetes type 1 contribute to the development of dyslipidemia together with the increase in the total cholesterol, LDL-cholesterol, total triglycerides and the decrease in the protective HDL2-cholesterol 26,27 .At the same time, hyperlipoproteinemia, and particularly hypercholesterolemia, can be an important risk factor for the progression of diabetic nephropathy, as it has been proved in many studies on patients with diabetes type 1 [28][29][30] .No statistically significant differences in the values of total cholesterol, HDL cholesterol and triglycerides were found between the study group of diabetic patients and the healthy controls.Such results are in accordance with the studies which failed to prove the existence of statistically significant differences in the values of lipid parameters between the healthy subjects and the patients with diabetes type 1 [31][32][33] .
A statistically significant correlation between CRP and triglycerides as well as between IL-6 with triglycerides and LDL-cholesterol was found by the multivariate regression analysis.The same analysis failed to find any correlations between TNF-α and inflammatory markers.Eurodiab study 34 has clearly shown a correlation between the values of triglycerides and HDL cholesterol with inflammatory markers, and it has been confirmed in the healthy population, as well.They have not confirmed a statistically significant correlation between LDL and inflammatory markers 34,35 .In their study, Ladeia et al. 36 found a significant correlation between CRP and the values of triglycerides and the ratio of triglycerides/HDL.The study did not show a correlation of CRP and other lipid parameters.Increased values of TNF-α in patients with diabetes type 1 and its role as an inflammatory cytokine in the pathogenesis of diabetic nephropathy and other micro-and macrovascular complications have been confirmed by numerous studies [36][37][38] .Our study also confirmed increased values of TNF-α in the diabetic patients.However, no correlation was found between TNF-α and lipid and lipoprotein parameters, that being in accordance with the results of some other authors 39,40 .

Conclusion
In the patients with diabetes type 1 there is a low degree of an inflammatory activity which is manifestated by higher values of CRP, TNF-α and IL-6 compared to the healthy controls.Future prospective studies should prove the importance of inflammation in the pathogenesis of chronic microand macrovascular complications in the population of diabetic patients, as well.
Correlations of certain inflammatory markers with glucoregulation parameters, lipid parameters and hypertension offer the possibility of therapeutic modification of inflammation in diabetes indirectly by improving glucoregulation, treating dyslipidemias and hypertension.