Molecular Mechanisms of Esophageal Cancer

Figure 31-1 The central Asian esophageal cancer belt extending from Iran to China. Modified from Kamangar F, Malekzadeh R, Dawsey SM, et al. Esophageal cancer in northeastern Iran: a review. Arch Iranian Med. 2007;10:70-82.

Chronic Inflammation and Esophageal Cancer

The inflammatory response is a highly complex and coordinated system to promote cellular regeneration and proliferation. This response provides an environment rich in inflammatory cells, growth factors, adhesion molecules, and angiogenic mediators, all of which can potentiate and initiate tumorigenesis. ¹¹ Rudolf Virchow first postulated a possible link between tumorigenesis and the inflammatory system in the 19th century, when he observed the presence of leukocytes within tumors. ¹² Only recently, however, have many of the underlying molecular mechanisms between inflammation and cancer initiation been elucidated^11,13,14(Figure 31-2 ).

Nuclear Factor-Kappa B

Nuclear factor-kappa B (NFκB) is a pro-inflammatory transcription factor initially described as a B-cell–specific factor for the immunoglobulin-κ light chain gene. ¹⁵ As part of the Rel protein family of transcription factors, NFκB exists in the cytoplasm as an inactive dimer until activated by a diverse set of extracellular stimuli and signals, such as inflammatory cytokines and growth factors.^16,17These stimuli trigger a common pathway of phosphorylation, ubiquitination, and proteasome-dependent degradation of NFκB’s regulatory protein, inhibitor of NFκB (IκB), to activate NFκB. ¹⁷ Once activated, NFκB is quickly translocated into the nucleus and binds to promoter regions responsible for the transcription of genes that encode for multiple cytokines (IL6, IL8, TNFα), cellular adhesion molecules (ICAM1, E-selectin), cell cycle regulators (p21, cyclin D1), apoptosis regulators (surviving, Bcl-2), and other transcription factors (p53, c-myc).^13,16,17

Aberrant activation of NFκB has been implicated in the initiation and progression of many cancers, including esophageal cancer, because of its broad role in inflammation, apoptosis, and cell survival ¹³ (Figure 31-3 ). Barrett’s esophagus (BE), as the main risk factor, is due to chronic epithelial damage from gastroesophageal bile and acid reflux. ²² NFκB activation has been described as a central event in the development of BE, and thus in EAC’s initiation and progression.^18,19A 2004 study was the first to show that NFκB expression was increased in the epithelial cells of BE compared to normal esophageal epithelial cells. ¹⁸ In addition, this study revealed that 61% of resected EAC tumors displayed NFκB immunoreactivity, and NFκB-positive tumors were more likely to be of advanced stage. ²¹ Others have also validated these data ¹⁹ and have even identified NFκB as an emerging molecular prognostic marker: NFκB tumor positivity is correlated with chemoradiation resistance and poor outcomes.^18,20,21

Although BE and gastroesophageal reflux are well-documented risk factors for EAC, the molecular pathogenic correlation between risk factors and the development of esophageal squamous-cell carcinoma is less clear.^5,9,22The mechanism for epithelial injury is quite different from that in adenocarcinoma and is usually due to thermal injury from hot liquids and foods, corrosive irritation from retained esophageal contents due to achalasia, or caustic ingestion of corrosive agents.^23–25Nonetheless, abnormal activation of NFκB has also been linked to ESCC. In vitro models have shown that NFκB pathways are highly expressed in esophageal SCC cell lines. ²⁶ Hatata and colleagues found that NFκB was overexpressed in 61% of ESCC resected tumors. ²⁷ They also reported that patients with NFκB positivity in their tumors correlated with significantly poorer survival times when compared to patients with NFκB negative-staining neoplasms (15 versus 42 months, P = .007).

Figure 31-2 Role of inflammation in tumor initiation and promotion (A) Tumor initiation. Reactive oxygen species (ROS) and reactive nitrogen intermediates (RNI) produced by inflammatory cells may cause mutations in neighboring epithelial cells. Also, cytokines produced by inflammatory cells can elevate intracellular ROS and RNI in premalignant cells. In addition, inflammation can result in epigenetic changes that favor tumor initiation. Tumor-associated inflammation contributes to further ROS, RNI, and cytokine production. (B) Tumor promotion. Cytokines produced by tumor infiltrating immune cells activate key transcription factors, such as NFκB or STAT3, in premalignant cells to control numerous pro-tumorigenic processes, including survival, proliferation, growth, angiogenesis, and invasion. As parts of positive feed-forward loops, NFκB and STAT3 induce production of chemokines that attract additional immune/inflammatory cells to sustain tumor-associated inflammation. Modified from Grivennikov S, Greten FR, Karin M. Immunity, inflammation, and cancer. Cell. 2010;140:883-899.

Activation of Additional Inflammatory Mediators and Esophageal Cancer

Abnormal activation of downstream NFκB targets, such as COX-2 and IL6, has also been implicated independently in the pathogenesis of esophageal cancer. Cyclooxygenase-2 (COX-2), is one of two isozymes that catalyze the rate-limiting conversion of arachidonic acid to prostaglandins, prostacyclins, and thromboxanes. ²⁸ COX-2 protein expression has been found to be significantly increased in patient samples with BE and EAC when compared to normal esophageal tissue. ²⁹ In addition, bile acid exposure to esophageal cells has been shown significantly to induce COX-2 expression. ²⁹ COX-2 overexpression has also been found in ESCC tumors, specifically well-differentiated tumors, and its progressive expression correlates well with advancing clinical stage. ³⁰ Although in vitro investigations into selective COX-2 inhibition for the treatment of EAC and ESCC have shown some promise,^31,32clinical trials have yet to demonstrate efficacy.^33,34

Interleukin-6 (IL6) is a multifunctional cytokine that has a range of functions from acute-phase protein induction to cellular growth and differentiation. ³⁵ Once bound to its extracellular receptor (IL6R), IL6 induces gene transcription through two signaling pathways, the JAK-STAT (Janus family tyrosine kinase-signal transducer and activator of transcription) pathway and the Ras-MAPK (mitogen-activated protein kinase) pathway^36,37(Figure 31-4 ). Like NFκB and COX-2, IL6 has been demonstrated to be a key mediator in the metaplastic conversion of normal esophageal squamous epithelium to BE as well as the further dysplastic conversion to EAC.^38,39Because of its multifunctionality and production by multiple cells throughout the body, IL6 is readily detected in the serum and has shown promise as a noninvasive diagnostic tumor marker. In 2011, Lukaszewicz-Zajac and colleagues demonstrated that when compared to traditional esophageal cancer tumor markers of carcinoembryonic antigen (CEA) and squamous-cell cancer antigen (SCC-Ag), elevated IL6 serum concentrations correlated better with early esophageal cancer diagnosis. ⁴⁰ Overall, the elevated serum IL6 concentrations could be detected in either EAC or ESCC with 87% sensitivity and 92% specificity with a statistically significant area under the receiver operating characteristic curve of 0.92 when compared to either CEA or SCC-Ag. ⁴⁰

Figure 31-3 Diagram showing activation of NFκB and target genes in esophageal cancer. Modified from Abdel-Latif MM, Dermot Kelleher K, Reynolds JV. Potential role of NF-κB in esophageal adenocarcinoma: as an emerging molecular target. J Surg Res. 2009;153:172-180.

Environmental Carcinogenic Exposures and Esophageal Cancer

The link between certain environmental carcinogens, such as tobacco smoking, and tumorigenesis has been well established for many cancers. The ability of these carcinogens to promote tumorigenesis stems from their abilities to cause genomic aberrations leading to unregulated and abnormal cellular growth. ⁴¹ Both EAC and ESCC have many unique genomic and epigenomic abnormalities that have been identified and studied. ¹⁰ However, EAC and ESCC have been found to have similar aberrant genomic changes in cell-cycle regulator genes, such as p53, APC, RB, and cyclin-D1, early in their tumorigenesis^42–44(Figure 31-5 ). Specific environmental risk factors for esophageal cancer and their molecular pathogenesis are now being revealed.

Tobacco Smoking

The association between tobacco smoking and lung cancer has long been recognized as due to the abundance of carcinogens in tobacco smoke. The International Agency for Research on Cancer has identified more than 60 substances in cigarette smoke for which sufficient evidence exists for carcinogenicity in either laboratory animals or humans. ⁴⁵ Tobacco smoking is one of the few risk factors shared by both EAC and ESCC. In a study of 474,606 participants, current and former smokers had a significantly increased risk of developing EAC when compared to never-smokers (hazard ratio [HR] 3.7; 95% confidence interval [CI] 2.2-6.2 and 2.8; 95% CI 1.8-4.3, respectively). ⁴⁶ The same study also revealed that current smokers were at an even higher risk for developing ESCC (HR 9.2; 95% CI 4.0-21.3), and individuals who had ever smoked accounted for 77% of ESCC cases. ⁴⁶

Two of the most potent and well-studied tobacco smoke carcinogens are polyaromatic hydrocarbons (PAHs) and the tobacco-specific N-nitrosamine, nicotine-derived nitrosamine ketone (NNK). Their tumorigenic effects arise from their ability to form DNA adducts as well as intra- and interstrand DNA crosslinks. ⁴⁷ The proteins of the nucleotide excision repair pathway and specialized DNA polymerases repair and bypass various types of DNA damage acquired by PAHs or NNK.^48,49However, because of the constant exposure to PAHs and NNK with chronic smoking, the amount of DNA damage done can overwhelm the DNA repair system, resulting in chromosomal aberrations, halted DNA replication, and mutations. ⁴⁷ The resulting alterations may lead to abnormal cellular growth and transformation if proto-oncogenes or tumor suppressor genes are affected.

Alcohol

Chronic alcohol abuse is an important risk factor in the development of ESCC, but has no correlation with increased EAC risk. ⁴⁶ Heavy drinkers (more than 84 drinks per week) have nearly a 25-fold increase in ESCC risk compared to light drinkers (1 to 20 drinks per week). ⁵⁰ In addition, exposure to both tobacco and alcohol has been found to have a synergistic effect on the risk of developing ESCC. ⁵¹

Figure 31-4 IL6 signal transduction IL6-mediated stimulation induces homodimerization of gp130, activating (phosphorylating) JAKs, and Stat1 and Stat3. Activated Stat1 and Stat3 form homodimers or heterodimers, which induce activation of various genes. Modified from Kishimoto T. IL-6: from its discovery to clinical applications. Int Immunol. 2010;22:347-352.

Figure 31-5 Scheme linking cigarette-smoke carcinogens with multiple genetic changes in lung cancer A key aspect is the chronic exposure of DNA to multiple metabolically activated carcinogens, leading to multiple adducts and their consequent mutations. The time period and sequence of genetic events are uncertain. Modified from Pfeifer GP, Denissenko MF, Olivier M, et al. Tobacco smoke carcinogens, DNA damage and p53 mutations in smoking-associated cancers. Oncogene. 2002;21:7435-7451.

The pathophysiology underlying alcohol’s role in tumorigenesis of ESCC involves esophageal irritation, through pathways previously described, and its principal metabolite, acetaldehyde, a known carcinogen causing DNA adducts. ⁵² The primary enzymes responsible for metabolizing acetaldehyde into an inert metabolite are aldehyde dehydrogenase-2 (ALDH2) and alcohol dehydrogenase-1B (ADH1B). ALDH2 and ADH1B are both present in the mouth, and mutations in either of these genes create increased salivary acetaldehyde concentrations, providing direct carcinogenic exposure to the esophageal mucosa.^53,54For example, homozygotes for the ALDH2 gene have been shown to have acetaldehyde levels up to 13 times greater than normal individuals. ⁵³ ALDH2 and ADH1B mutations are rare in Western populations; however, they are very prevalent in eastern Asian populations and are thought to contribute to the high incidence of ESCC in the esophageal cancer belt.^53,55,56

Diet

Diets high in fruit and vegetable intake have been shown to be protective against many cancers because of the high concentrations of anticarcinogenic compounds. ⁵⁷ Many dietary studies have demonstrated that diets low in fruit and vegetables have an increased risk of ESCC.^58,59Dietary intake of nitrogen- and nitrosamine-rich foods have also been associated with ESCC development, especially in the high-risk areas of the esophageal cancer belt where nitrogen-rich foods and water are consumed in high concentrations.^59–62In the Hebei province in China, the highest incidence areas of ESCC correlated with the highest concentrations of nitrite and ammonia nitrogen in drinking water. ⁵⁹ In addition, in many of these areas cultural customs lead to widespread consumption of hot beverages, which was vital in establishing the connection between thermal irritation and esophageal cancer.^63,64

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