Our study demonstrated that middle third primary gastric cancer had a worse prognosis, and was an independent risk factor for gastric cancer prognosis. Tumor location has been shown to be an important factor that influences the classification of gastric cancer, risk of lymph node metastasis and surgical approaches. The general consensus is that proximal gastric cancer tends to exhibit higher aggressiveness and poorer prognosis than distal gastric cancer[5, 12, 13]. This observation may be attributed to several factors associated with distal gastric cancer, including younger patient age, reduced intraoperative blood loss, relatively lower rate of lymph node metastasis, and a lower proportion of stage IV[12, 13]. Previously, Li et al. investigated the association between tumor location and patient prognosis in 2145 gastric cancer cases, and found that tumors located in the gastric body had a lower 5-year survival rate than tumors located in the gastric fundus, gastric cardia and antrum[14, 15]. In addition, Liu et al. showed that in 198 young gastric cancer patients, those with middle third tumors had a 50.6% overall survival rate, which was significantly lower than those with upper-third or lower-third tumors[15]. Our findings are consistent with these studies and highlight the importance of tumor location in determining patient prognosis.
Next, we found that middle third gastric cancer was associated with increased tumor size, which was consistent with a retrospective study by Li et al. involving 2477 gastric cancer patients[14]. Their study showed that tumors in the gastric body were larger (p = 0.009) and associated with lower grade of differentiation and deeper infiltration (T stage) than tumors in the cardia, fundus, and antrum[14]. In another independent cohort study that included gastric cancer patients from South Korea and the United States, the largest tumor size and deepest invasion depth were observed in upper third tumors, while middle third tumors were the second largest in size, and lower third tumors were the smallest[16]. Tumor size has previously been recognized as an independent prognostic factor for gastric cancer[17, 18]. Our study revealed that patients with larger tumors and tumors located in the middle third of the stomach had an elevated risk of poor prognosis.
Using non-targeted metabolomics, we aimed to delineate the characteristic metabolic profiles of gastric cancer at different stomach locations to determine the association between tumor location and tumor size, as well as prognosis. Sixteen differentially expressed metabolites were identified between gastric cancer tissue and normal tissue. These metabolites were enriched in three major pathways including retrograde endocannabinoid signaling, arginine biosynthesis, and alanine, aspartate and glutamate metabolism. An increasing number of studies have indicated a strong association between abnormal amino acid metabolism and the carcinogenesis or progression of gastric cancer[19, 20]. Cancerous cells that lack arginine have been shown to exhibit distinctive traits, including mitochondrial dysfunction, transcriptional reprogramming, and ultimately cell death, and may therefore be the basis for the development of therapeutic approaches that specifically target the synthesis of arginine[21, 22]. Five amino acids including arginine identified by LC-MS analysis were found to be significantly differentially expressed between the plasma of patients with gastric ulcer and gastric cancer, and may therefore act as potential biomarkers for the early detection of gastric cancer[23]. Metabolomics studies based on GC-TOF-MS and UHPLC-QE-M also found dysregulation of arginine metabolism in the tongue coating of patients with gastric precancerous lesions[24]. In addition, a characteristic metabolic panel that included arginine was shown to have high diagnostic efficiency for distinguishing between gastric cancer and superficial gastritis and atrophic gastritis[25]. Together, all these studies suggest that abnormal arginine metabolism may be involved in the carcinogenesis and progression of gastric cancer.
Here, we examined the metabolic profiles of gastric cancer located in the middle third region, as well as other locations. Our findings revealed that the distinctive metabolic phenotype of middle third gastric cancer involved abnormal metabolism of alanine, aspartate, and glutamate, which are closely associated with arginine biosynthesis. Furthermore, our study found that patients with middle third gastric cancer experienced a more unfavorable prognosis. Thus, abnormal arginine metabolism in middle third gastric cancer could potentially influence tumor size in this specific region and subsequently impact patient prognosis.
To further understand the involvement of abnormal arginine metabolism in gastric cancer, we examined the role of ASS1, a key rate-limiting enzyme in this pathway, in gastric cancer cells. ASS1 catalyzes the synthesis of argininosuccinic acid from citrulline and aspartic acid, ultimately leading to the production of arginine via the action of argininosuccinate lyase. Interestingly, we found that ASS1 levels were significantly increased in gastric cancer tissues with a more pronounced elevation observed in middle third gastric cancer. Our findings were consistent with those of Tsai et al., and provided further support for the association between aberrant arginine metabolism and the development of gastric cancer[26]. Finally, we found that knockdown of ASS1 significantly inhibited the proliferation, colony formation, and migration of gastric cancer cells, and promoted apoptosis, further supporting a cancer-promoting role for ASS1 in gastric cancer. The abnormal upregulation of ASS1 in gastric cancer is not only a direct reflection of the association between dysregulation of the arginine synthesis pathway and the carcinogenesis of gastric cancer, but also indirectly corroborates the relationship between abnormal elevation of argininosuccinic acid, and the unfavorable prognosis observed in middle third gastric cancer. Together, these results suggest the potential value of targeting the arginine anabolic pathway in the targeted treatment of gastric cancer.