Co-evolution of cancer microenvironment reveals distinctive patterns of gastric cancer invasion: laboratory evidence and clinical significance

Tumor progression represents the greatest threat to patients with gastric cancer (GC). The 5-year survival is significantly decreased from over 80% in early GC to below 28% in advanced GC [1]. Over the past 25 years, the majority of cancer studies have focused on functional consequences of activating and/or inactivating mutations in critical genes and signal pathways that regulate cell proliferation and/or cell death as cancer is often defined as a disease of cell proliferation [2]. However, such studies have largely ignored the fact that interactions between cancer cells and stroma are critical for growth and invasion of epithelial tumors [3]. It has been recognized that invasion is regulated not only by intrinsic genetic changes in cancer cells as 'initiators' of carcinogenesis, but also regulated by stroma cell as 'promoter' [4, 5]. A seminal event in cancer progression is the ability of cancer cells to mobilize the necessary machinery to break surrounding extracellular matrix (ECM) barriers while orchestrating a host stroma response that ultimately supports tissue-invasive and metastatic processes [6]. Proteolytic ECM remodeling is considered both prerequisite and consequence of invasive cell migration [7]. The cancer cell and stroma both modulate the process of invasion by remodeling the ECM with tumor-associated proteases such as matrix metalloproteinase (MMPs), which subsequently breakdown proteins of the ECM such as collagens and release the cryptic information [8, 9]. Many studies have focused on the role of extracellular proteases. It was supposed that cancer cells break through the ECM barriers and invade surrounding tissues in two fashions: a protease-independent and Rho kinase (ROCK)-dependent amoeboid migration mode and a protease-dependent and ROCK-independent mesenchymal migration mode [10]. Further more, the process of pericellular proteolysis leads to ECM degradation and realignment during cell movement and integrate it into established steps of cell migration [11].

It has long been recognized that the behavior of tumor systems is complex, which means that understanding the individual component like pericellular proteolysis in more detail does not necessarily explain the collective behavior of many individuals, and thus usually evokes Aristotle's quote in that 'The whole is more than the sum of its parts' [12]. Therefore, instead of investigating a single component of cancer matrix, this study focused on the whole tumor microenvironment related to GC invasion, by evaluating tissue destructive proteolytic enzymes MMP9 and MMP2, tissue barriers against invasion like type IV collagen, tumor infiltrating macrophages, and tumor angiogenesis, all of which are essential components of tumor stroma and involved in the process of invasion (Figure 1.). Furthermore, the interactions between cancer cells and tumor stroma termed as 'invasion pattern' corresponding to the dynamic stroma remodeling were also delineated so as to formulate new concepts on cancer invasion at the histological level.

Invasion is the first observable step of cancer progression. Cancer invasion occurs in a particular context of tissue microenvironment which is under constant evolution largely due to the interactions of cancer cells and the surrounding stromal cells [13, 14]. However, such co-evolution of cancer-microenvironment has long been under appreciated. Most studies focused on molecular level gene mutations and signal pathways in cancer cells during tumor progression, while other studies focused on TNM staging at the clinical level [15, 16]. The molecular level studies focused on the "temporal evolution" of cancer molecules, while the clinical studies focused on the "spatial evolution" of cancer tissues. The underlying theory behind these studies is to focus on cancer itself. A major drawback of such study, however, is the lack of appreciation of the "temporal and spatial co-evolution of cancer and its environment", which is the real context of tumor progression [17].

It is based on such understanding that this study focused on major ingredients of tumor microenvironment, particularly the cancer invasion front, as well as cancer cells. These components included in this study were MMPs and type IV collagen, two major factors for and against cancer invasion, and TAMs which are double-edge swords facilitating or deterring cancer invasion. Moreover, tumor angiogenesis was also evaluated because provides potential routes for tumor dissemination as a result of the co-evolution of cancer microencironment and cancer cells and promoted by those components.

MMPs are major proteolytic enzymes to breakdown ECM during cancer invasion. Traditionally, extracellular proteolysis and BM breaching are two absolute requirements for cancer invasion, while type IV collagen forms physical barrier against cancer invasion [18]. High levels of proteases facilitate ECM degrading, thereby creating a path for the migration of cancer cells. As a result of this path through the ECM, the invading cancer cells could gain access to vasculature and lymphatic systems [19]. This progress would rely on invadopodia which are membrane protrusions that localize enzymes required for ECM degradation, and MMP9 would be required in the initial steps of invadopodia formation [20]. In support to this theory, this study revealed high expression of MMP9 in advanced GC tumor tissue, especially nearby the BM. Although the difference of MMP2 expression is significant in terms of the recurrence and metastatic status, the MMP9 expression was not associated with tumor stage, lymph node status, metastasis status, recurrence or not. Similar unexpected result was showed in terms of the relationship of type IV collagen and tumor progression.

Tumor microenvironment plays dynamic and different roles in different stages of cancer progression, which could partly explain these unexpected results. It has been evident that although cancer cells and some traditionally proteins account for invasion and metastasis are no different, the microenvironments at the primary tumor site, the invasive front and the metastatic site are different [21]. Although no statistically significant result was showed regarding of the relationship of type IV collagen and tumor progression, OS was significantly improved in type IV collagen positive group compared to negative group (the median OS was 25.5 months and 10.0 months, respectively, P = 0.044). Further more, GC patients with negative MMP9 expression displayed improved overall survival compared to patients with positive MMP9 expression (Median OS was 44.0 and 13.5 months, respectively. P = 0.036). Nevertheless, the roles of proteases in cancer are now known to be much broader than simply degradation of ECM during tumor invasion and metastasis. The proteolysis of ECM by MMPs may reveal cryptic matrix binding sites, MMPs can act as tumor suppressor by revealing cryptic matrix binding sites, releasing matrix-bound growth factors and activating a variety of cell surface molecules [22]. For instance, angiostatin and tumstatin are angiogenesis inhibitors generated from the NC1 domain of the 3 chain of type IV collagen [23]. Thus, we supposed that MMPs-mediated degradation of BM and ECM can act as both positive and negative regulators of tumor progression which resulted in the unexpected results predicted in the traditional view because of the change of the tumor stroma during the cancer progression.

Macrophages are versatile, plastic inflammatory cells that respond to environmental signals with polarized genetic and functional programs. The presence and significance of macrophages infiltration in developing neoplasms is now well recognized, and infiltrating macrophages play an important role in tumor cell invasion into surrounding normal tissues [24, 25], including expression of growth factors, matrix proteases, promotion of angiogenesis and suppression of adaptive immunity, all of which influence the ECM and hypoxia, two non-cellular components that potently influence stromal-epithelial interactions [21, 26] (Figure 1). A protumoral role of tumor-associated macrophages (TAMs) is consistent with studies from humans, wherein a high density/number of TAMs is associated with poor prognosis in different cancers (cervix, prostate, breast, bladder) [27, 28]. In agreement with these results, our study also found that macrophages infiltration was correlated with serosa invasion, distant metastasis and TNM stage. The OS was longer in low macrophages density group than in high macrophages density group, although the level of significance was only marginal (P = 0.056). Additionally, the cumulative disease-free survival (DFS) rate was significantly higher in low macrophages density group than in high macrophages density group. We found that the interface of tumor nest and stroma is the main location of infiltrating macrophages in gastric cancer, and phagocytosis of cancer cells by macrophage, indicating the coexistence of M1 and M2 phenotypes in GC tissues.

In cancer, tumor cells require new blood vessels for sustenance, local growth and escape to distant sits through hematogenous spreading and metastasis [29]. No matter the mechanism of the invasion, angiogenesis maybe the common last step of invasion in primary tumor environment. In our study, tumor angiogenesis was studied by calculating the MVD, and the MVD was higher in patients with GC lymph node metastasis and advanced GC (P = 0.019 and 0.010, respectively). Interestingly, our results indicate that type IV collagen and macrophages were the negative and positive factors for tumor angiogenesis, respectively, in keeping with what we have mentioned above. In the early stage, MMPs destroy the ECM and established a potential pathway for cancer cell migration but the revealed molecule from type IV collagen inhibits the tumor angiogenesis [30]. Whereas in the advanced stage, type IV collagen was almost destroyed and no molecules that inhibit tumor angiogenesis were released, that's why MVD was higher in type IV collagen negative group than in positive group (P = 0.026). It has been well established that M2 type macrophages can promote the tumor angiogenesis [31], and we found that MVD was higher in high density macrophages group than in low density group (P = 0.040). Histomorphology analysis also indicates that the locations of infiltrating macrophages and MVD are accordant (Figure 2E and Figure 2F). One limitation of this study, however, is that it did not differentiate between M1 and M2 cells. Further work in this direction would be more informative.

The current study suggests that GC invasion is influenced by co-evolution of cancer cells and their microenvironment, and histological study on tumor tissue can directly show such interactions. Based one our observations, we analyzed invasion patterns in an attempt to characterize the invasive behaviours of GC beyond the simplistic gene mutation or overall TNM stage, whose values were limited for ignoring the interaction of cancer cells and stroma. Rather, this study focused on the micro-ecology system of cancer invasion front (Figure 1), and identified four invasive patterns, including Washing pattern, Ameba-like pattern, Spindle pattern, and Liner pattern, each representing distinctive interactions between cancer cells and their microenvironment. In the Washing pattern, successive waves of cancer cells may induce progressive conditioning of the microenvironment to facilitate cancer cells spreading along a plane rather than deep penetration. In the Ameba-like pattern, extensive tissue destruction may have occurred in the adjacent tissue even though the local tumor border appears intact. Therefore, invasive tunnels may have already developed beneath the seemly intact tumor margin. In the Spindle pattern, simultaneous coordinated polarization of cancer cells at the leading edge of tumor front may cooperate in invasion by constantly changing the local microenvironment. In linear pattern, a few coordinated "pioneering cancer cells" form deep penetrating invasion tunnels along a line, paving the way for follower cancer cells. Among these four patterns, washing pattern may correlate with best prognosis as crossing ECM barriers occurs relatively late. In contrast, Linear pattern may relate to the worst prognosis because cancer cells may have already deeply penetrated the ECM in spite of the density of the surrounding type IV collagen, and such cancer may have already become a potentially systemic disease even it is diagnosed as early stage by conventional pathology. However, the significance of invasion patterns was not fully evaluated in this study because of the limited sample size, which is the major limitation of our study. Large scale studies are needed to further develop this concept.

In summary, proteolytic enzymes MMP9, MMP2 and macrophages in stroma contribute to GC progression by facilitating tumor angiogenesis. The co-evolution of tumor cells and their microenvironment results in four patterns of tumor invasion, which could be useful for new prognostic models and novel treatment strategies.