Effects of metabolic syndrome on renal function after radical nephrectomy in patients with renal cell carcinoma

Nephrectomy, partial or radical, remains the standard treatment for renal cell carcinoma (RCC). However, increased risk of chronic kidney disease (CKD) must still be considered. This study aimed to evaluate the effects of concomitant metabolic syndrome (MetS) on renal function in patients with RCC after radical nephrectomy. Medical records of 310 patients who underwent radical nephrectomy for clear-cell RCC at 900th Hospital of the Joint Logistics Support Force, PLA from December 2012 to January 2017 were reviewed retrospectively. Estimated glomerular filtration rate (eGFR) and CKD stages were calculated at one week preoperative as baseline and then at postoperative 1 week, 3 months, 12 months and 24 months. MetS patients were identified and enrolled in the MetS group (n = 31), and a non-MetS group was selected by propensity score matching (n = 31). Non-neoplastic renal parenchyma specimens taken at least 2 cm from edge of tumor were evaluated. Baseline characteristics between the two groups were comparable. At 24 months postoperative, mean eGFR levels of the MetS group were significantly lower than those in the non-MetS group (62.7 vs. 73.3 ml/min/1.73 m2; p = 0.004). CKD stages were still more severe in the MetS group than those in the non-MetS group (p = 0.006). The proportions of global sclerosis, tubular atrophy and interstitial fibrosis were all significantly more prevalent in MetS patients, compared to non-MetS patients (all p < 0.05). In RCC patients with MetS, the possibility of declining eGFR and CKD progression must be considered after radical nephrectomy. Routine monitoring of renal function must be emphasized.


Introduction
Renal cell carcinoma (RCC) is one of the most common urologic tumors and accounts for 5% and 3% of all malignancies in men and women, respectively [1]. Nearly 62,700 new-onset cases and at least 14,000 related deaths yearly have been reported in the United States [1]. In China, the estimated new cases are about 68,300 with around 25,600 deaths per year [2]. Diagnosis and management of RCC have both improved in recent decades, however RCC is still an aggressive and often fatal disease [3]. Clear-cell RCC is a histological subtype of RCC and at least one-third of clearcell RCC patients experience metastasis and recurrence, leading to unsatisfactory outcomes [4].
The standard treatment for RCC is nephrectomy, either partial or radical [5]. Radical nephrectomy could achieve optimal oncological outcome, however because of the loss of functional renal parenchyma and sequential renal function impairment, results in the increased risk of chronic kidney disease (CKD) [6]. Clinically significant CKD after nephrectomy and the increased morbidity and mortality of RCC due to CKD has been intensively discussed and evaluated recently. Older age, smoking, male sex, obesity, hypertension, diabetes mellitus, tumor size, cancer severity, types of nephrectomy and surgical procedures have been reported as risk factors of the development or progression of CKD after Yong Zhang and Tingkun Wu contributed to this manuscript equally.
surgical management of RCC [7][8][9][10][11]. However, a consensus of how to use so many patient and surgical factors to identify the patients with higher risk of CKD is still not achieved since the diversity of the results still high, some parameters, for example, the role of obesity presented by body mass index (BMI) is controversial in different studies [11][12][13].
Metabolic syndrome (MetS) is a cluster of chronic conditions, including abdominal obesity, hyperlipidemia, hypertension, cardiovascular disease and diabetes mellitus that is associated with organ injury. MetS may affect the kidneys, resulting in microvascular renal injury, diabetic nephropathy, CKD or end-stage renal disease (ESRD) [14][15][16][17][18]. In addition, in a large number of studies, MetS has been found to be associated with RCC [12,19,20]. However, the etiology of RCC still remains unclear, although some MetS components have been confirmed as etiological factors of RCC as listed in several guidelines published by well known international organizations such as the European Association of Urology, the American Urological Association, and the Chinese Urological Association [19]. Furthermore, MetS has been used as predictor for RCC outcomes [12]. The progression-free survival (PFS) of RCC was shorter in patients with MetS, compared to those without MetS [12]. The correlation of MetS and RCC and the effects on each other complicate the evaluation of the mechanism of CKD caused by surgical management of RCC.
Nephrectomy is inevitable in certain amounts of patients with RCC, we are seeking risk factors associated with outcomes and quality of life after nephrectomy continuously for the improvement the care quality and optimization individual treatment goal. Base on the abovementioned established knowledge, we tried to evaluate directly the correlation between MetS and post-nephrectomy CKD. To clarify the impact of MetS on patients with RCC and, more specifically, the incidence and progression of CKD following radical nephrectomy for RCC, this retrospective study aimed to clarify the role of concomitant MetS on renal function in patients with RCC after undergoing radical nephrectomy for clear-cell renal carcinoma.

Patients
Patients with clear-cell RCC who underwent radical nephrectomy at the 900th Hospital of the Joint Logistics Support Force, PLA, from December 2012 to January 2017 were included, and their medical records were reviewed and screened retrospectively. First, patients with eGFR less than 60 ml/min/1.73 m 2 , with advanced CKD or with severe infection, heart disease, liver disease, hematopoietic disease and other malignancies before the surgery were all excluded. Next, patients with MetS were identified and enrolled in the MetS group. Propensity scoring matching was conducted, matching patients without MetS by age, gender, tumor diameter, and the surgical approach of patients in the MetS group and were included in the non-MetS group.

Ethical considerations
The protocol for this study was reviewed and approved by the Institutional Review Board of the 900th Hospital of the Joint Logistics Support Force, PLA. Informed consent of patients was waived because of the retrospective design of this study in which patients remained anonymous.

Main measures
Patients' eGFR levels were calculated at baseline (1 week preoperative) and 1 week, 3 months, 12 months and 24 months postoperative using the Chronic Kidney Disease Epidemiology Collaboration equation (CKD-EPI) [21] as follows: where SCr is serum creatinine (in mg/dl), κ is 0.7 for females and 0.9 for males, α is -0.329 for females and -0.411 for males, min indicates the minimum SCr/κ or 1, and max indicates the maximum SCr/κ or 1. Stages of chronic kidney disease were defined according to the KDOQI Chronic Kidney Disease Guidelines 2012 [22].
Clinical and biochemical values were collected from patients' medical records, all of which were performed in our hospital using standard procedures. Age, gender, body weight, height, blood pressure (BP), FBG, lipid profile, serum creatine (SCr), serum uric acid, hemoglobin (HB) within 30 days before radical nephrectomy were recorded for further statistical analysis.

Pathological evaluation
Non-neoplastic renal parenchyma specimens collected at least 2 cm from the edge of the clear-cell renal carcinoma were evaluated by two experienced pathologists after hematoxylin and periodic acid-silver methenamine staining using light microscope. The pathologists were blinded to the clinical information of the patients. Glomerular, tubular, interstitial, and vascular features and the percentage of globally or segmentally sclerosed glomeruli were determined. Grade of tubular atrophy, interstitial fibrosis, and vascular sclerosis were scored according to the Banff pathological classification [24]. Within the selected MetS and non-MetS patients, specimens of 21 patients were missing and 7 were less than 2 cm from the edge of tumor, therefore these were not included in the pathological examination.

Statistical analyses
Categorical data are presented as count and percentage, and their associations with MetS were identified using Fisher's exact test. Normally distributed continuous data are displayed as mean with standard deviation (mean ± SD), and comparisons between the two patient groups were performed using the independent two samples t-test. Changes in eGFR levels at difference time points within groups were performed using the paired t-test. Bonfferroni correction was applied for multiple comparisons, including the eGFR levels between groups and the eGFR levels within groups at various time points. Other continuous or ordinal data are displayed as median with interquartile range (IQR), and comparisons between the two patient groups were performed using the non-parametric Mann-Whitney test. Linear mixed models were performed to show the time effect (from preoperative 1 week to postoperative 24 months) and group effect (MetS vs. non-MetS) of the repeat measures. Associations between patients' clinical characteristics, baseline eGFR, and MetS components versus CKD stages at 24 months after radical nephrectomy were performed as odds ratios (ORs) with 95% confidence intervals (CIs) using binary logistic regression models. To find the independent risk factors for CKD, variables with p-value < 0.1 found in the univariable binary logistic model were chosen and stepwise entered into the multivariable model. All the results of univariable models and final multivariable model would be presented in the corresponding tables. All statistical analyses were performed using IBM SPSS version 22.0 (IBM Corp. Armonk, NY, USA). A two-tailed p-value less than 0.05 indicates statistical significance.

Clinical characteristics
During the study period of this matched case-control study, 310 patients who underwent radical nephrectomy for clear-cell renal carcinoma were screened for demographic and clinical information. Thirty-one patients were diagnosed with MetS and included in the MetS group. Based on propensity score matching, another 31 patients without MetS were matched by age, gender, tumor diameter, surgical approach and included in the non-MetS group for further analysis. The clinical characteristics of the two groups are presented in Table 1. Significant differences were found between the two groups, including BMI, triglyceride (TG), FBG, systolic blood pressure (BP), diastolic BP, and mean arterial pressure (MAP), which were significantly higher in patients of the MetS group compared to those in the non-MetS group; while high density lipoprotein cholesterol (HDL-C) of the MetS group was significantly lower than that of the non-MetS group. The proportions of obesity, hyperlipidemia, hypertension in the MetS group were significantly larger than those in the non-MetS group.
Histories of diabetes mellitus and cardiovascular disease, smoking and drinking did not differ between the two groups. For the cardiovascular disease, no patient in the non-MetS group had cardiovascular disease, and 5 (16.1%) patients in the MetS group had, including cerebral ischemic stroke (n = 2), coronary artery disease (n = 1), ST elevation myocardial infarction (n = 1) and one patient had both coronary artery disease and vertebrobasilar ischemia.

Medication use for hyperlipidemia, hypertension, and diabetes mellitus
No patient in the non-MS group used medications for hyperlipidemia; 5 patients in the MS group were prescribed medications for hyperlipidemia, including atorvastatin (n = 3) pitavastatin (n = 1) and rosuvastatin (n = 1). Two patients in the non-MS group were prescribed medications for hypertension, both used calcium channel blockers (CCB), 16 patients in the MS group used medications for hypertension, including CCB (n = 13), β-blockers (n = 4), and ACEI/ ARB (n = 7). Only one patient in the non-MS group used insulin for diabetes mellitus, 5 patients in the MS-group were prescribed medication for diabetes mellitus, including metformin (n = 5), oral hypoglycemics other than metformin (n = 4) and insulin (n = 1). Figure 1 shows the changes in trends of eGFR levels from preoperative 1 week to postoperative 24 months between the two groups. Significant time effects and group effects on eGFR levels from preoperative 1 week to postoperative 24 months were observed in the linear mixed model (p < 0.001, data not shown). No significant differences were found between the two groups in preoperative eGFR levels. At 1 week after radical nephrectomy, the mean eGFR level of both groups decreased significantly from 107.2 to 69.3 ml/min/1.73m 2 in the non-MetS group and from 96.7 to 62.7 ml/min/1.73m 2 in the MetS group.

Effects of MetS on CKD stages after radical nephrectomy
Preoperative and postoperative CKD stages are shown in Fig. 2. Pre-operative CKD stages of the two groups ranged from normal to mild (Stages 1-2, eGFR > 60 ml/ min/1.73m 2 ). Postoperative 1-week CKD stage of the two groups were both mainly mild to moderate (Stages 2-3a, eGFR 45-89 ml/min/1.73m 2 ). At 3 months after radical nephrectomy, the CKD stages of patients in the MetS group were significantly more severe than those in the non-MetS group (p = 0.005); 5 (16.1%) patients were Stage 1, 19

Renal pathological findings
Renal pathological findings in patients with and without MetS are listed in Table 2, n = 18 in each group. The proportion of global sclerosis was significantly larger in patients with MetS (median 3.60% vs. 1.73%, p = 0.028) than in those without MetS. However, the difference in segmental glomerulosclerosis between the two groups did not reach statistical significance. Notably, tubular atrophy (p = 0.023) and interstitial fibrosis (p = 0.010) were more prevalent in patients with MetS.

Factors associated with moderate to severe CKD at 24 months after radical nephrectomy
As shown in Table 3, age, baseline eGFR level, hypertension, diabetes mellitus, and MetS were associated with moderate to severe CKD at 24 months after radical nephrectomy (p < 0.05). The risk of moderate to severe CKD was increased by each year of increase in age, with a crude OR of 1.08 (p = 0.03). The risk of moderate to severe CKD was decreased by each 1 unit of increase in baseline eGFR level, with a crude OR of 0.95 (p = 0.003). Patients with hypertension and diabetes mellitus were more likely to have moderate to severe CKD at 24 months after radical nephrectomy, with crude ORs of 9.69 (p = 0.001) and 4.50 (p = 0.037), respectively. In addition, patients in the MetS group were more likely to have moderate to severe CKD at 24 months after radical nephrectomy, with crude OR of 4.28 (p = 0.017).
In the multivariable analyses, the 7 factors with p-value less than 0.1, including age, baseline eGFR, obesity, hypertension, diabetes mellitus, MetS and smoking were stepwise entered into or removed from the multivariable model. The final model included that baseline eGFR and hypertension as displayed in the right half of Table 3, which meant baseline eGFR and hypertension significantly and independently associated with the occurrence of CKD. After adjusting for baseline eGFR, only hypertension still showed a significant association with moderate to severe CKD at 24 months after radical nephrectomy (adjusted OR = 6.11, p = 0.009).

Discussion
In the present study, the trend of eGFR levels increased from preoperative to over 2 years after radical nephrectomy, representing that CKD is a chronic condition. Both MetS and non-MetS groups showed recovery of eGFR postoperatively, however MetS patients demonstrated a significantly lower recovery rate. Also, at 24 months after radical nephrectomy, the severity of CKD in patients with MetS was higher than the severity in patients without MetS. Baseline eGFR level, hypertension and diabetes mellitus appear to be risk factors for moderate to severe CKD. Differences in renal pathological features between the two groups, including global sclerosis, tubular atrophy and interstitial fibrosis, which were all more prevalent in patients with MetS at the time of radical nephrectomy, indicating that MetS may cause more severe kidney injury. Overall, results of the present study confirm that patients with MetS need a longer time for renal function recovery after radical nephrectomy than those without MetS. It is also clear that MetS patients who underwent radical nephrectomy had greater risk of progression of renal insufficiency.
CKD following nephrectomy due to RCC will lead to ESRD or cardiovascular events, which is associated with potential cardiovascular and overall survival, also has a heavy impact on patients' quality of life [25]. Thus, Table 2 Renal pathological changes in patient with and without MetS † Data are displayed as median with interquartile range (IQR). *p < 0.05 by the non-parametric Mann-Whitney U test, indicates a significant difference exists between the two groups  predicting the risk of CKD before surgical management of RCC may be invaluable in helping patients and surgeons to determine the necessity of surgical treatment, and improve both the surgery and follow-up plans to balance the risk of oncological and renal function progression and achieve maximum benefit for the patients. For example, early referral to a nephrologist and initial regular eGFR monitoring and appropriate, timely management is essential [26]. The patient population in the present study had all undergone radical nephrectomy, which is typical for those who have advanced RCC and are more likely to have preoperative CKD, and preservation of renal function is both more necessary and more difficult [27]. The data of patients' baseline characteristics also confirmed these factors. CKD, regardless of the causes, and RCC share many risk factors, including age, gender, smoking, hypertension and diabetes mellitus, and these are also related to MetS components [28]. In a large cohort study with nearly 1200, 000 adults with eGFR < 30 ml/min/1.73 m 2 and a two-fold risk of RCC compared to those with 60-89 ml/ min/1.73 m 2 CKD itself was reported to be an independent risk factor for RCC. In order to clarify the effects of MetS and simplify the study design, we controlled for age and gender. Also, because associations between the tumor size and surgical approach have been studied as well as the development of CKD after nephrectomy [27,29], we also adjusted for these factors in our analysis. However, this strategy limited the generalization of our results and is a limitation of the present study.
MetS is a combination of metabolic disorders. Even the MetS-related abnormalities such as hypertension and diabetes mellitus are shown to increase the risk of CKD after nephrectomy [28]. Therefore, MetS may reflect a more comprehensive presentation of individuals with CKD risk than each individual component of MetS. When Kriegmair et al. [12] evaluated the impact of MetS and each of the single components of MetS on outcomes of patients with RCC, the results of Kaplan-Meier and log-rank analysis revealed that MetS was significantly associated with a shorter PFS (p = 0.018), whereas no significant differences were found in the effects of diabetes mellitus, BMI, hypertension and hypertriglyceridemia on the outcomes. Another study of the outcomes of patients with RCC had controversial results. Diabetes mellitus and hypertriglyceridemia were shown to be associated with worse recurrence-free survival (RFS) in RCC patients, [30,31] but obesity had a positive impact on RCC outcomes [32]. No consensus has yet been reached in previously published studies. The pathogenetic mechanisms of MetS for inducing diseases or influencing disease outcomes, including insulin resistance, inflammation, endothelial dysfunction, oxidative stress, or all of above, may lead to nephron injury and further renal function impairment [28]. Thus, using MetS as a relatively comprehensive marker for CKD risk can probably identify CKD risk before it actually occurs.
In the present study, global sclerosis, tubular atrophy and interstitial fibrosis were seem more frequently in patients with MetS. Histopathological abnormalities identified in the non-neoplastic parenchyma of the kidney are associated with renal function decline after radical nephrectomy [33]. These findings are consistent with the results reported by Alexander et al. [34] In addition, obesity has been shown to induce glomerulosclerosis in an animal model, resulting in ESRD [35]. Interstitial fibrosis and tubular atrophy are both related to hyperlipidemiainduced oxidative stress, the increased reactive oxygen species (ROS), vascular endothelial growth factor (VEGF) and transforming growth factor-β (TGF-β), which together increase the level of matrix synthesis and reduce matrix degradation, resulting in glomerular, tubular and vascular damage [36,37]. This study has several limitations, including its retrospective nature, which limits generalizations to other populations and precludes inferences of causation, for example, we cannot rule out the possible effects of underlying diseases and medications used for these diseases, since we cannot know whether the status of the diseases and whether they were control well by the medicines in each individual. Furthermore, MetS was not defined by NCEP/ ATP-III criteria, which may also introduce bias for generalization. The descriptive nature of our study indicated we only can conclude base on the correlations, in addition, the prevalence of various lipid and hematological parameters was based on single assessment, which may lead to a misclassification bias. It also had a small sample size from a single hospital. Further prospective study with a large multicenter cohort or population-based study would help to confirm results of the present study.
In conclusion, clinicians treating RCC patients with concomitant MetS should be aware of the possibility of decreasing eGFR and CKD progression more than in RCC patients without MetS after radical nephrectomy, and routine monitoring of renal function should be emphasized in the post-nephrectomy follow-up plan. A large prospective cohort study should be conducted to further support the findings of the present study. Availability of data and materials The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Compliance with ethical standards
Conflict of interest The authors declare that they have no competing interests.

Ethics approval
The protocol for this study was reviewed and approved by the Institutional Review Board of the 900th Hospital of the Joint Logistics Support Force, PLA. Informed consent of patients was waived because of the retrospective design of this study in which patients remained anonymous.