Inflammation is associated with a number of chronic conditions, such as cardiovascular disease and cancer. Reducing inflammation may help prevent or treat these conditions. C-reactive protein (CRP) and interleukin-6 (IL-6) are well-studied proinflammatory cytokines [
Smoking is the major risk factor for lung cancer, the most common cancer worldwide. Cigarette smoking has been associated with increases in CRP and previous investigations have demonstrated that increased CRP levels are a secondary effect of cigarette smoking and reflect tissue injury [
There is great interest in studying the relationship between inflammatory markers and smoking in an attempt to provide explanations for smoking-mediated morbidity and mortality. The current study is unique with the inclusion and comparison of verified former smokers with a similar smoking history (pack-years and years of smoking) to the current smokers. The profile of inflammation markers is expected to differ following smoking cessation. Therefore, we are reporting here the associations of CRP and IL-6 with smoking status among community-dwelling men and women.
Current smokers (CS) and former smokers (FS) between 40 and 80 years of age with 25 pack-years or more of smoking history were recruited to a randomized, double-blind, and placebo controlled chemoprevention trial of green tea and black tea. The study protocol received approval by the University of Arizona Institutional Review Board. Once informed consent had been obtained, each participant completed a set of smoking, dietary, and health assessment questionnaires and was evaluated for health and respiratory history. A blood sample was collected for a comprehensive metabolic panel. Two additional fasting morning blood samples were collected from which plasma, serum, buffy coat, and erythrocytes were isolated and stored at −80°C prior to the sample analysis
Aliquots were thawed immediately prior to the analysis. Each immunoassay was completed using a commercially available kit from Quantikine (R & D Systems, Minneapolis, MN, USA). Briefly, specified quantities of the specimen or standard are added to microplates with cells precoated with the CRP antibody or the IL-6 antibody, depending on the assay. The antibodies bind the CRP or IL-6 present in the samples and standards. Any unbound substances are washed away and a monoclonal antibody specific for CRP or a polyclonal antibody specific for IL-6 is added. Again, unbound substances are washed away, substrate solution is added, the plate is incubated, and amplifier solution is added. Color develops in proportion to the amount of CRP or IL-6 present in the well and after stopping the color development in the prescribed time, the wells are measured spectrophotometrically for CRP or IL-6 quantification. Both CRP and IL-6 analyses were completed in duplicate for 154 participants. All data entry and values were checked by additional laboratory personnel for quality control.
Means and standard errors were calculated for descriptive statistics. Because of value skewness, CRP and IL-6 values were log-transformed. Pearson correlations were calculated to describe the unadjusted relationships between the smoking parameters, inflammatory markers, and other relevant variables. Multivariable linear regression models were created to identify independent predictors of log (CRP) and log (IL-6), using variables that had significant correlations (
One hundred and fifty-four current and former smokers (83 women and 71 men) with a mean age of 60.3 ± 0.7 years (mean ± SE) were included in the study. Characteristics of the study population by gender and smoking status are presented in Table
Characteristics of the study population by gender and smoking status (
Males |
Females |
|
|
---|---|---|---|
Age (years) | 60.8 ± 1.0 | 59.9 ± 1.0 | 0.54 |
Former smokers | 62.4 ± 1.5 | 60.9 ± 1.4 | 0.45 |
Current smokers | 59.5 ± 1.3 | 59.2 ± 1.4 | 0.89 |
|
0.14 | 0.40 | |
Pack-year | 45.6 ± 2.6 | 40.7 ± 1.9 | 0.12 |
Former smokers | 41.1 ± 3.4 | 44.6 ± 3.5 | 0.49 |
Current smokers | 49.2 ± 3.7 | 37.7 ± 1.9 | 0.004 |
|
0.12 | 0.07 | |
Years of smoking | 34.4 ± 1.1 | 32.6 ± 1.2 | 0.28 |
Former smokers | 29.5 ± 1.5 | 27.6 ± 1.6 | 0.39 |
Current smokers | 38.4 ± 1.3 | 36.4 ± 1.5 | 0.33 |
|
<0.001 | 0.001 |
The differences in the levels of the biomarkers of inflammation and blood lipids between smokers and former smokers are presented in Table
Biomarkers of inflammation and lipid profile by gender and smoking status (
Males |
Females |
| |
---|---|---|---|
Serum C-reactive protein |
7.4 ± 0.13 | 7.4 ± 0.11 | 0.66 |
Former smokers | 7.2 ± 0.20 | 7.3 ± 0.16 | 0.58 |
Current smokers | 7.6 ± 0.17 | 7.4 ± 0.16 | 0.27 |
|
0.10 | 0.87 | |
Serum interleukin-6 |
0.67 ± 0.10 | 0.43 ± 0.10 | 0.09 |
Former smokers | 0.44 ± 0.14 | 0.33 ± 0.13 | 0.57 |
Current smokers | 0.86 ± 0.13 | 0.50 ± 0.14 | 0.08 |
|
0.04 | 0.40 | |
Total cholesterol (mg/dL) | 193.2 ± 5.1 | 206.2 ± 4.0 | 0.04 |
Former smokers | 189.5 ± 6.7 | 204.6 ± 5.5 | 0.08 |
Current smokers | 196.1 ± 7.6 | 207.3 ± 5.7 | 0.23 |
|
0.52 | 0.74 | |
High density lipoproteins (mg/dL) | 47.0 ± 1.5 | 61.5 ± 1.6 | <0.001 |
Former smokers | 47.1 ± 2.3 | 59.0 ± 2.7 | 0.002 |
Current smokers | 47.0 ± 1.9 | 63.4 ± 1.9 | <0.001 |
|
0.99 | 0.17 | |
Low density lipoproteins (mg/dL) | 114.0 ± 4.2 | 119.7 ± 3.6 | 0.30 |
Former smokers | 111.8 ± 5.3 | 118.9 ± 4.6 | 0.32 |
Current smokers | 116.8 ± 1.9 | 120.4 ± 5.3 | 0.67 |
|
0.99 | 0.83 | |
Total triglycerides (mg/dL) | 175.0 ± 18.5 | 139.5 ± 7.1 | 0.06 |
Former smokers | 165.4 ± 18.6 | 142.5 ± 11.2 | 0.28 |
Current smokers | 182.9 ± 30.3 | 137.3 ± 9.2 | 0.12 |
|
0.64 | 0.71 |
Given the fact that there were no significant differences by gender, the correlations among CRP, IL-6, smoking intensity parameters, and other variables are presented by smoking status in Table
Pearson’s correlation of CRP
Variables | CRP | IL-6 | ||
---|---|---|---|---|
|
|
|
| |
Interleukin-6 (pg/mL) | ||||
Former smoker | 0.387 | 0.000 | 1.000 | |
Current smoker | 0.589 | 0.000 | 1.000 | |
Age (years) | ||||
Former smoker | 0.161 | 0.140 | 0.198 | 0.067 |
Current smoker | 0.270 | 0.026 |
0.446 | 0.000 |
Years of smoking | ||||
Former smoker | 0.081 | 0.461 | 0.202 | 0.062 |
Current smoker | 0.017 | 0.889 | 0.242 | 0.047 |
Pack/year | ||||
Former smoker | 0.136 | 0.212 | 0.313 | 0.003 |
Current smoker | 0.028 | 0.823 | 0.256 | 0.033 |
Caloric intake/day | ||||
Former smoker | 0.206 | 0.065 | 0.014 | 0.904 |
Current smoker | 0.092 | 0.474 | 0.054 | 0.677 |
Total fat intake (g/day) | ||||
Former smoker | 0.173 | 0.126 | 0.007 | 0.951 |
Current smoker | 0.090 | 0.488 | 0.124 | 0.338 |
Total cholesterol (mg/dL) | ||||
Former smoker | 0.024 | 0.825 | 0.024 | 0.829 |
Current smoker | 0.134 | 0.275 | 0.086 | 0.484 |
HDL-C (mg/dL) | ||||
Former smoker | −0.273 | 0.011 |
−0.207 | 0.056 |
Current smoker | −0.131 | 0.287 | −0.290 | 0.017 |
LDL-C (mg/dL) | ||||
Former smoker | 0.028 | 0.799 | 0.072 | 0.517 |
Current smoker | 0.182 | 0.141 | 0.061 | 0.623 |
Total triglycerides (mg/dL) | ||||
Former smoker | 0.168 | 0.123 | 0.134 | 0.220 |
Current smoker | 0.196 | 0.110 | 0.268 | 0.028 |
BMI (kg/m2) | ||||
Former smoker | 0.287 | 0.007 |
0.129 | 0.236 |
Current smoker | 0.203 | 0.100 | 0.130 | 0.299 |
CRP: C-reactive protein; HDL-C = high density lipoproteins; LDL-C: low density lipoproteins; BMI: body mass index.
The multivariate predictors of inflammatory markers by smoking status are presented in Table
Stepwise regression analysis for CRP and IL-6 by smoking status.
Subjects | Explanatory variable |
|
|
Adjusted |
---|---|---|---|---|
CRP (mg/L) | ||||
Former smokers | Total triglycerides | 0.496 | 0.009 | 0.455 |
CRP |
<0.001 | 0.000 | ||
Smokers | HDL-C (mg/dL) | −0.017 | 0.010 | 0.369 |
BMI (kg/m2) | 0.028 | 0.052 | ||
CRP |
<0.001 | 0.000 | ||
IL-6 (pg/mL) | ||||
Former smokers | Total triglycerides | 0.442 | 0.002 | 0.547 |
Age | 0.027 | 0.007 | ||
CRP |
<0.001 | 0.000 | ||
Smokers | Pack/year | 0.012 | 0.005 | 0.436 |
CRP |
<0.001 | 0.000 |
CRP: C-reactive protein; IL-6: interleukin 6; HDL-C: high density lipoprotein; BMI: body mass index, and CRP
Variables included in the model are gender, age, BMI, total triglycerides, HDL-C, years of smoking, pack/year, caloric intake/day, and interaction between CRP and IL-6.
In the current study among participants with more than 30 pack-years of smoking exposure, we did not find any significant difference in CRP levels between smokers and former smokers. However, the serum levels of IL-6 were significantly higher among male smokers compared to former smokers (
The MONICA study from Germany examined gender specific differences for smoking and CRP levels, finding that serum CRP concentrations were significantly higher in male regular smokers than male never-smokers but no significant differences were observed in women [
There is strong evidence that IL-6 serum concentration increases with age [
Smoking cessation lowers risk for lung and other cancer types along with reducing the risk of coronary heart disease, stroke, and peripheral vascular disease [
Numerous studies have now confirmed that CRP levels are elevated in patients with the metabolic syndrome. CRP levels were shown to be strongly associated with BMI, low HDL, triglycerides, and levels of the proinflammatory cytokine, IL-6 [
In conclusion, our data supports previous studies that showed that serum CRP is associated with biomarkers of the metabolic syndrome but not with smoking. However, serum IL-6 levels are mainly affected not only by the metabolic syndrome but also by age and smoking status.
We conducted a cross-sectional study on a sample of 154 current and former smokers. We investigated the relationship between inflammatory biomarkers and smoking status. CRP levels were significantly associated with IL-6 levels regardless of smoking status. The significant predictors of CRP levels were biomarkers of the metabolic syndrome. IL-6 levels were significantly associated with smoking especially among current smokers.
The authors declare no conflict of interests.
This work was partially supported by a grant (DOD PR 023104) from the Department of Defense.