Downregulation of metastasis suppressor 1(MTSS1) is associated with nodal metastasis and poor outcome in Chinese patients with gastric cancer

Gastric carcinoma (GC), which is the second most common cause of cancer-related death in the world, deprives more than 700,000 lives per annum [1]. Its incidence varies considerably worldwide and has recently been decreasing in developed countries, but remains stably in developing countries [2‐4]. Furthermore, the fact that gastric cancer is insensitive to conventional chemotherapy and is rarely amenable to radiotherapy leaves the survival durations in patients with gastric cancer unchanged in recent years. This highlights the need for the determination of prognostic factors predicting the outcome and the development of novel therapeutic strategies. Previous studies have indicated that disease stage and lymph node metastasis are the most important prognostic factors in gastric cancer. Moreover, some molecular markers have been identified and attempted to use clinically [5‐7]. Nevertheless, other potential prognostic factors related to survival in these patients remain unclear.

Metastasis suppressor 1 (MTSS1), also known as MIM (missing in metastasis), was originally identified by Lee et al [8] as a potential metastasis suppressor gene that was present in non-metastatic bladder cancer cell lines, but was not expressed in a metastatic bladder cancer cell line. This gene, mapped to human chromosome 8q24.1, encodes a 5.3 kb mRNA and a polypeptide predicted to be an actin-binding protein of 356 amino acids with homology to the WASp (Wiscott-Aldrich Syndrome protein) family [8]. Functional analyses of MTSS1 have shown that MTSS1 induced actin-rich protrusions resembling microspikes and lamellipodia at the plasma membrane and promoted disassembly of actin stress fibres [9]. Actin filament assembly is associated with cytoskeletal structure organization and many forms of cell motility [10]. These data have suggested that MTSS1 protein may be important in regulating cytoskeletal dynamics, and as a consequence it would play a potential role in the invasion and metastatic behaviour of cancer cells.

The study surrounding MTSS1 is quite small, yet this protein has been the subject of controversy. Preliminary analysis by Northern blotting demonstrated that MTSS1 is widely expressed but is most abundant in spleen, thymus, testis, and prostate, with low levels also detected in uterus and colon [8]. Since this pioneering article, other reports have indicated that MTSS1 played a role as a metastasis suppressor in prostate cancer [11, 12], bladder cancer [8, 11, 13] and benign lesions, but up-regulated in basal cell carcinomas [14]. However, other evidences showed that MTSS1 is unlikely to be a metastasis suppressor. It acts as a scaffold protein that interacts with actin-associated proteins to modulate lamellipodia formation [15]. Ma et al suggests that MTSS1 is a regulator of carcinogenesis in hepatocellular carcinoma [16]. And it is a member of the sonic hedgehog (SHH) signalling pathway that modulates Gli responses during growth and carcinogenesis [14].

Although these studies cited above suggested MTSS1 as a promising candidate biomarker and playing an important role in tumorigenesis, little is known about the function of MTSS1 in gastric cancer. In our study, we sought to determine the expression of MTSS1 in resected gastric cancers and investigate the correlation of MTSS1 expression and clinicopathologic features and survival, in an attempt to discover the potential influence of MTSS1 on the development of gastric cancer.

Although metastasis suppressor 1 (MTSS1) may be a critical regulator of carcinogenesis in different cancers, study of MTSS1 has been mainly restricted to several cancers and available data seem to be controversial, leaving the involvement of MTSS1 in cancer not clearly defined. Given that gastric cancer is one of the most malignant cancers in the world and that tumor recurrence and metastases are the major causes of death in patients with gastric cancer, resulting in a poor prognosis of the disease, we set out to investigate the role of MTSS1 in gastric cancer. In this study, we found that loss of MTSS1 expression was significantly associated with important clinical determinants of prognosis for gastric cancer including poorly differentiated tumors, diffuse-type in Lauren's classification, large tumor size, deep invasion level, the presence of nodal metastases and advanced disease stage. And patients without MTSS1 expression had shorter median survival durations than that with MTSS1 expression. Moreover, our immunohistochemistry data clearly revealed that MTSS1 expression significantly decreased from normal tissues to primary tumors to nodal metastases. To the best of our knowledge, this is the first clinical evidence that MTSS1 might play an important role in gastric cancer progression and metastasis.

Accumulating evidences suggested that poorly differentiated tumors have higher growth and recurrence rates than well-differentiated tumors do [17]. Previous study reported that down-regulation of MTSS1 expression might correlate with the transition of tumor cells from distinct epithelium-like morphology to less differentiated carcinomas [18]. Consistently, in our study, MTSS1 protein expression was lost more frequently in poorly differentiated tumors than in well-differentiated tumors, suggesting that MTSS1 is a differentiation marker for gastric cancer. However, another report reveals that an incremental increase in MTSS1 expression was detected from healthy normal liver donors to non tumor tissue specimens and then to their matched hepatocellular carcinoma tumor tissue specimens. Specifically, a higher level of MTSS1 expression was observed at the early stages of the disease, suggesting that MTSS1 may play an important role in promoting the early development of hepatocellular carcinoma [16]. Inasmuch as the role of MTSS1 has not been clearly defined to date because of contradicting published data, we speculate that the role of MTSS1 could be cancer or tissue type specific.

Risk factors for gastric cancers have been explored in a number of studies, including status of lymph nodes, depth of tumor invasion, age at diagnosis, TNM stage and some molecular markers [5, 7, 19‐23]. In the present study, we confirmed that age, histology, nodal metastasis and TNM stage were independent predictors for gastric cancer. Furthermore, MTSS1 expression was found to be significantly correlated with prognosis in univariate survival analysis and it still kept its prognostic value in multivariate survival analysis. Positive associations between MTSS1 expression and other clinicopathologic features, such as tumor size, depth of tumor invasion, status of lymph nodes, TNM stage, differentiation and Lauren's classification were indicated. All these findings suggested that MTSS1 expression alone was a potential molecular marker for predicting outcome in patients who undergo gastrectomy for gastric cancer.

Metastasis is a fatal step in the progression of gastric cancer and has become one of the most challenging problems in tumor therapy [24]. The spread of tumor cells is a complicated and multistage process and requires altered expression of many different genes [25]. Nixdorf et al [13] indicated that down-regulation of MTSS1 occurred in BL17/2 bladder tumor cell lines with invasive abilities. Expression of MTSS1 has also been shown to be reduced in prostate cancer and can contribute to tumor growth and development, as well as metastasis [12]. In this study, we observed significantly lower expression of MTSS1 in the nodal metastases of gastric cancers than that in primary tumors. The high frequency of down regulation of MTSS1 expression in primary gastric tumors and lost MTSS1 expression in metastases and their direct association with poor outcome of gastric cancer indicated that MTSS1 could suppress tumor invasion and metastasis, and might be a candidate prognostic factor for lymph node metastasis and tumor progression. This notion is clearly supported by a recent study showing that biological overexpression of MTSS1 significantly suppressed the invasive, migratory, growth and adherence properties of a human breast cancer cell line [26]. However, Bompard et al [15] found that MTSS1 expression is not dependent on the metastatic state of the cells. We also observed MTSS1 expression in six nodal metastases but not in matched primary tumors. This discrepancy makes us presume that there is a possibility that the components which regulate MTSS1 may create specific interactions with different microenvironments between primary regions and target organs. Expression of MTSS1 is regulated by an epigenetic event that is differently represented in different sites. The mechanism for the down-regulation of MTSS1 to tumor progression, especially metastasis, is not clear. It was reported that its expression is regulated by DNA methylation, a potential hallmark of a gene likely to be involved in tumor initiation and progression [11]. Additionally, several studies indicated that MTSS1 may be due to actin binding, reorganisation or changes in cell adhesion or tyrosine phosphorylation upon loss of MTSS1 [27]. As reorganization of the actin cytoskeleton has been extensively studied in cancer and is the primary mechanism of cell motility and migration, which are critical steps, involved in tumor carcinogenesis [28]. Further functional investigations are worthwhile to explore the precise mechanism of the carcinogenic effect of MTSS1 with the goal of developing potential therapies targeting MTSS1 as an indicator for gastric cancer.

To the best of our knowledge, this is the first report demonstrating the clinicopathological significance of MTSS1 expression in gastric cancer. Our results showed that loss of MTSS1 expression was associated with large tumor size, low differentiation, deep invasion level, advanced tumor stage, the presence of nodal metastasis, and poor outcome in patients who underwent gastrectomy. MTSS1 expression may serve as a useful biomarker for the prediction of outcome of gastric cancer.