The morbidity and mortality level of esophageal cancer in china is the highest in the world according to the report of the Chinese Centers for Cancer Registration and the Centers for Disease Control and Prevention [6
]. Nevertheless, the prognosis remains relatively poor in patients with esophageal cancer, with a 5-year survival rate of 10% to 20%. Additional studies have indicated that 70% of patients already present with tumor metastases when clinical symptoms first appeared, and with a cervical lymph node metastatic rate of 73.0% to 74.5% [7
]. In significant number of cases Barrett's esophagus develops to esophageal adenocarcinoma [8
A large-scale retrospective proportional mortality study by Wang and colleagues [9
] estimated that tobacco smoking was responsible for 27.9% of esophageal cancer deaths in middle-aged men and 2.8% in middle-aged women. Alcohol drinking is another important risk factor for esophageal cancer. They report separately estimates the esophageal cancer burden attributable to low intake of fruit and low intake of vegetables. Tobacco smoking, alcohol drinking, low vegetable intake and low fruit intake were responsible for 46% of esophageal cancer mortality and incidence in China. In addition, relapse and tumor metastasis were the main causes responsible for failure of surgical intervention.
Chemotherapy remains an important component of combined therapy against the relapse and metastasis of esophageal cancer [7
]. However, the drug resistance or tolerance to chemotherapy displayed by esophageal carcinoma cells may be related to their comparatively low sensitivity to chemotherapy [10
]. The resistance mechanisms exhibited by esophageal carcinoma cells may be associated with the following. Over-expression or enhanced functional capabilities of the ABC transport protein and second other endogenous factors that have changed during the development of the tumor. These may include altered GST (Glutathione S-transferase), which could in turn inactivate or detoxify chemotherapeuticagents, enhance DNA synthesis, or inhibit topoisomerase α activity, and change RNase activity [10
The cell membrane plays a key role in tumor growth and progression [12
]. The transporter of cell membrane plays a key role in pharmacology, which can functionally obstruct the absorption of chemotherapeutic agents, in which drug efflux mediated by the ATP-binding cassette (ABC) multidrug transporter protein accounts for one of the main causes of tolerance to chemotherapy [3
ABC genes are divided into seven distinct subfamilies (ABCG2, MDR1, MRP, and so on). In recent years, research studies of the functional rolesplayed by MDR1 (Multidrug Resistance 1), MPR and ABCG2 of the ABC family have increased dramatically [13
]. Moreover, ABCG2, which is the second member of group G in the ABC family is expressed highly in placental syncytial trophoblast, intestinal epithelial apical membrane, hepatic tubule membrane and brain microvascular endothelial cells. Additionally, ABCG2 plays an important role in blood–brain, blood-testis, and maternal-fetal barrier function. This transporter protein is also important in the efflux of xenobiotics, which can not only protect cells against the damage caused by extraneous substances or drugs, but it may also inhibit the anti-tumor potency or toxicological effects of chemotherapeutic drugs [14
The membrane-associated ABCG2 consists of two distinct domains capable of undergoing conformational changes. The structure of ABCG2 consists of six reverse half-transporters, with the nuclear-binding domain at the amino-terminus, and transmembrane domain at the carboxyl-terminus capable of forming homodimers or homotrimers that can mediate the transfer or efflux of hydrophobic anions or cations, such as mitoxantrone, topotecan, doxorubicin, epirubicin, etoposide, among others [15
There are few reports detailing ABCG2 expression in esophageal cancer. However, our study has shown that ABCG2 staining patterns in the esophageal squamous cell membrane and cytoplasm were consistent with its nature, and was located in the microsomal membrane and cytoplasm. Table indicated that the total percent expression of squamous cell carcinoma was 66.7%, which was found to be significantly different among the pathological grade groups. The phenomenon that a low grade tumor has a low expression and that the high grade form of the tumor has high expression levels indicates that the expression rates of ABCG2 and its intensity of staining may be closely associated with the differentiation state of the tumor. In our study, expression of ABCG2 was also significantly different and this was dependent on TNM staging.
The epidemiology of esophageal cancer demonstrates a strong gender bias with a sex ratio of 8–9:1 in favor of males [16
]. The expression of ABCG2 and V-ATPase, as we found, depended upon the gender of the patients with a sex ratio of 2.4:1 and 2.3:1 in favor of males. The expression of ABCG2 and V-ATPase was strongly associated with gender.
Our data also supported the notion that ABCG2 expression in esophageal squamous cancer cells associated with TNM staging, particularly in the context of high TNM staging and its association with high expression and enhanced positive staining intensity of ABCG2 expression. Others have shown that ABCG2 expression plays an important role in tumor stem cell proliferation, maintenance of stem cell phenotype and promotion of tumor cell development [17
]. The corollary of these observations is that our data indicate that ABCG2 expression could be associated with the extent of malignancy of esophageal squamous cancer cells, the TNM staging and the metastatic features of this disease. In addition, ABCG2 expression may be associated with drug tolerance seen in esophageal squamous cancer cells. Accordingly, others have found that the drug tolerance of particular tumors is related to the intracellular and extracellular environment [18
V-ATPase is a type of ATPase, which located on the microsomes, and expressed on the cell membrane. Multidrug resistance was related to the extracellular environment and the change of PH in cytoplasm. V-ATPase plays an important role in regulating intracellular pH [3
]. Concordantly, we found it important to identify V-ATPase expression in esophageal squamous cancer cells. The relationship between V-ATPase activities, the relative expression of ABCG2 and the drug tolerance effect collectively exert marked influence on the functional behavior of ABCG2.
Expression of V-ATPase in esophageal squamous cancer cells, the observed pathological grading, and TNM staging informed us that overall functional expression of V-ATPase could be associated with both the pathological grading and TNM staging in esophageal squamous cancer cells. We found that the higher pathological grading and TNM staging, the higher the expression rate and more intense the positive staining for V-ATPase. For example, when comparing the lymphatic metastasis group and non-lymphatic metastasis group, expression of V-ATPase was much higher in the lymphatic metastasis group than was found in the non-lymphatic metastasis group. Since the expression of V-ATPase was closely associated with ABCG2 (Table , P
0.001), in esophageal squamous cancer cells, this data indicated that it was very likely that both proteins promoted their reciprocal expression.
The expression relationship between ABCG2 and V-ATPase in esophageal squamous cell carcinoma and in the pathological grading and TNM staging of esophageal squamous cell carcinoma
It was previously shown that over-expression of V-ATPase plays an important role in maintaining the alkaline environment of the cytoplasm by regulating the cytosolic pH as a means to counter the otherwise acidic extracellular environment (15). V-ATPases were also shown to exacerbate the invasive and migratory ability of metastatic cells. Moreover, others have shown that the capacity of ABCG2 to mediate the efflux of the drug like compound topotecan, enhanced the lowering of the pH environment, and that at pH 5.5, the drug transporting ability of ABCG2 was at least five times greater than that found at physiological pH [19