Introduction
Obesity is an emerging and increasingly prevalent condition in the Western world. It is associated with well-known metabolic disorders, leading to the development of different diseases, including cancer (1). Chronic inflammation, a typical feature of obesity, increases the imbalance of the tissue microenvironment, promoting the appearance of the preneoplastic status. In fact, the increase in white adipose tissue (WAT) affects the development of the disease due to the release of several adipokines, interleukins and other cytokines by this type of tissue (2). These factors cause greater hormonal signaling, increased proliferation and survival of adjacent and distal cells which could induce tumorigenesis (3). There is convincing evidence that excess body weight is associated with an increased risk for cancer of at least 13 anatomic sites, including colorectal (4). Thence, the relationship between body mass index (BMI) and risk of colorectal cancer (CRC) has been the focus of several investigations (4,5), and these studies have shown that an increase in BMI is related to a higher incidence of CRC. However, only few of the publications have shown poorer results for patients with obesity (6,7) and, therefore, the relationship between obesity and the prognosis of CRC has not been widely studied.
As is known, one of the hallmarks in the development of tumorigenesis is the gain of replicative immortality through the maintenance of telomere length (8). Telomeres are a complex of nucleoproteins, which have a fundamental role in the protection of genomic DNA, and they are shortened with each cell cycle of replication (9). Individuals with short telomeres should be at increased risk for cancer, since short telomeres lead to genomic instability-a hallmark of cancer. However, individuals with long telomeres also display an increased risk for major cancers, thus creating a cancer-telomere length paradox (10). A recent study investigating relative telomere length in white blood cells provides evidence in support of longer telomeres being associated with a higher risk of colorectal cancer, particularly rectal cancer (11). In most cases, shortened telomeres induce a cell-cycle arrest or trigger apoptosis, although for those cells that bypass such signals during tumour progression, a critical length threshold is reached at which telomere dysfunction may ensue (12). Therefore, telomere attrition resulting in replicative senescence, simultaneously by passing cell cycle checkpoints, is a hallmark of malignant transformation of the cell (13).
Telomerase enzyme is the main responsible for telomere maintenance (14). The majority of tumors (80-85%) sustain their capacity to grow indefinitely through the ectopic expression of telomerase, as shown in previous studies (15,16). Telomerase is almost ubiquitous in advanced solid cancers, including CRC, and its expression is essential for cell immortalization (13).
Many studies have shown that telomere dysfunction, understood as a critical telomere shortening, has a dual role in the development of the tumor, since it can act as a tumor suppressor or an oncogenic factor, depending on the cellular context (17). Attrition of telomeres induces the loss of telomere function, which increases genomic instability and activates regulatory molecules such as p53 that lead to senescence and cell death (18,19). On the other hand, this genomic instability causes mutations which could lead to tumorigenesis (20). In addition, it has been described that shorter telomeres are found in patients with higher BMI values (21), due to the impact of oxidative stress and inflammatory processes, suggesting that obesity is related to shortening of telomeres and potentially promotes colorectal carcinogenesis (22-24).
Previous reports, in order to help clinicians optimize their practice, considered crucial to introduce more effective tools that will improve not only early diagnosis, but also prediction of the most likely progression of the disease and response to chemotherapy. In this context, telomere length and telomerase activity have been included among the promising emerging biomarkers in CRC monitoring (25).
The exposed data highlight the importance of studying how obesity influences telomere function and its potential role as a predictor of prognosis in CRC. We report a multivariate predictor model for CRC prognosis. The main novelty of the present work consists of jointly analyzing obesity and telomere status regarding prognosis of CRC patients submitted to curative intention surgery treatment.
Discussion
There is growing evidence that inflammation is a central and reversible mechanism through which obesity promotes cancer risk and progression. The tumor promoting effects of obesity occur locally through the inflammation of adipose tissue and the associated alterations in the microenvironment (2). Therefore, the characterization of biomarkers to identify patients with obesity with high-risk CRC seems paramount for an early diagnosis and improvement in the election of the most appropriate therapeutic protocols.
In this context, considering that telomere shortening has been associated with obesity in several studies, and that telomere maintenance is critical for the progression of cancer, our study was carried out considering a large population of CRC patients, with and without obesity, submitted to surgery with curative intention. Results from this work allowed us to demonstrate that telomere status is related to obesity and clinical prognosis.
Given the technical requirements of the methodologies used in this study, it is necessary to highlight some limitations. On the one hand, not all the usual weight data from patients were available; therefore, there are some cases where the usual BMI could not be calculated. Furthermore, the technique used to evaluate telomere length requires a significant amount of DNA. There were samples in which it was not possible to obtain enough DNA from the available tissues. The number of cases submitted to analysis in each of the determinations included in this work, as well as the reasons for case exclusions, are detailed in the flow diagram in Fig. 1.
Although the activity of telomerase was positive in most of the tumors considered in our study, we detected a significant shortening of telomeres in the tumor samples compared to the mean values of the telomere length observed in the non-tumor paired tissues, as previously reported by others (16,30). Furthermore, according to the results obtained in the present investigation, the lower mean values of telomere length, both in tumors and in their non-tumor paired tissues, were associated with earlier stages of Dukes, in agreement with other groups (31). Thus, these data prove that shorter telomeres are associated with cancers that would, a priori, confer a better prognosis to patients with CRC. In addition, our results indicate that cancers displaying lower values of shortening of telomere length occur in patients with obesity. We consider these data of interest since, although several studies have established a link between obesity and the risk of colon cancer, little is known about the effect of obesity on the outcomes after diagnosis (7).
It has been reported that a BMI greater than 35.0 kg/m2 at the time of diagnosis in patients with colon cancer is associated with an increased risk of recurrence (7). However, other authors have not confirmed these data, nor a significant correlation between BMI and an increased risk of death in patients with CRC (32). It has also been suggested that BMI prior to diagnosis is an important predictor of survival among patients with non-metastatic CRC (6). More recently, Bhaskaran et al (5) have reported that heterogeneity in the effects of BMI suggests different mechanisms or combinations of mechanisms associated with different tumor locations and in different subgroups of patients.
In our study, the group of patients with BMI values greater than 31.85 kg/m2, showed a significantly worse clinical prognosis. However, Cox multivariate regression analyses did not demonstrate that these results were independent of the Dukes tumor stage.
Interestingly, our results indicate significant differences according to the gender: When the effect of obesity on the prognosis of patients with CRC was analyzed, these differences were only evident in the male population. Although this fact could be explained by considering the relationship between gender and fat distribution (33), other aspects might be further investigated, including additional genetic, hormonal or molecular mechanisms, in order to explain the effect of obesity on the prognosis of CRC in relation to the gender of patients (34).
As for the telomere status, our data support that the prognosis of patients with CRC, whether with or without obesity, is strongly related to the length of the tumor telomere, being these results independent of the stage of the tumor.
In the present study we have identified a specific length of telomere in non-tumor tissues that seems critical to predict the prognosis of cancer. Therefore, patients with a telomere length less than 7.1 kb in non-tumor tissues present a better clinical evolution, considering both subjects with and without obesity. At this point, our results would support the idea that carcinogenic cells have a common biological history with normal tissue (35).
These data allow us to hypothesize the possibility that tumor cells with shorter telomeres activate cellular senescence, thereby conferring a more favorable clinical prognosis to patients affected by CRC. In fact, the progressive shortening of telomeres results in the formation of dysfunctional telomeres that compromise tissue proliferation (19).
The shortening of human telomeres has two opposite effects during the development of cancer. On the one hand, the shortening of telomeres can exert a tumor suppressive effect through the arrest of proliferation. On the other hand, the loss of telomere protection can lead to a telomere crisis, which is a state of extensive genomic instability that can promote the progression of cancer (17). Our research team has previously reported data in CRC that support a worse clinical evolution in patients with tumor telomere maintenance (16).
In conclusion, in the present study we have jointly evaluated the prognostic relevance of obesity and telomere status in patients affected by CRC who had undergone surgery of curative intention. Our results demonstrate that the length of telomeres is a useful biomarker to predict the clinical outcome in these subjects. Patients with shorter telomeres, both in the tumor and their non-tumor paired tissues, had the best clinical evolution, independently of the Dukes' stage. Our data allow us to conclude that patients with obesity had a poorer prognosis, however, these results were not independent from the tumor Dukes' stage.
Further investigations are needed to analyze the effect of obesity on the clinical course of CRC in the context of other factors, such as the gender of the patients.