Evaluation of CAF markers in GC tissues
We first evaluated the expression specificity of FAP, FSP1 and α-SMA in GC. Our results showed that in addition to fibroblasts, the positive immunostaining signals of α-SMA were also observed in smooth muscle cells and pericytes of and GC cells (Figure 1A); FSP1can also stained myofibroblast, immune cells and epithelial cells (Figure 1B). However, FAP is specific expressed in the CAFs (Figure 1C). Therefore, FAP is a truly specific marker for CAFs in GC TME, and thus be used as the CAFs marker in our subsequently analysis.
Expression of EZH2, FOXM1 and FAP protein in gastric tissues
The representative mIF result of protein expression in each group of tissue samples is shown in Figure 2. In comparison with the matched normal gastric samples, both the percent of positive cells and the average fluorescence intensity of E2H2 protein positive cells were significantly increased in cancer tissues (both P < 0.001, with one-way ANOVA, Figure 3A-B). However, there was no significant difference between the percent of positive cells and the average fluorescence intensity of EZH2 positive cells between GIN and normal tissue, yet the percent of EZH2 positive cell between cancer tissue and GIN (all P > 0.05, with one-way ANOVA, respectively, Figure 3A-B). For FOXM1 protein expression, although there was no significant difference in its expression between tumor tissue and normal samples (both P > 0.05, with one-way ANOVA, respectively, Figure 3C-D); both the percent of positive cells and the average fluorescence intensity of FOXM1 was increased in cancer tissues than in GIN tissue samples (P < 0.001 and P = 0.002, with one-way ANOVA, Figure 3C-D). Interestingly, the percent of positive cells and the average fluorescence intensity of FOXM1 in the GIN tissue samples was also reduced compared with the normal samples (both P < 0.001, with one-way ANOVA, Figure 3C-D). Compared with GIN and normal tissue samples, both the percent of positive cells and the average fluorescence intensity of FAP positive cells were significantly increased in cancer tissues (all P < 0.05, with one-way ANOVA, Figure 3E-F). However, there was no significant difference between GIN and normal samples (both P > 0.05, with one-way ANOVA, Figure 3E-F).
Correlation of EZH2, FOXM1 and FAP protein expression in gastric tissues
We then investigated the correlation of the percent of EZH2, FOXM1 and FAP positive cells in each kind of gastric tissue samples (Figure 4). There was no significant correlation between the percent of FAP positive cells with those of EZH2 in GC (Figure 4A), GIN (Figure 4D) or normal (Figure 4G) samples. In GC samples, the percent of FAP positive cells was positive correlated with those of FOXM1 (r = 0.476, P < 0.001, Figure 4B), and the percent of EZH2 positive cells was positive correlated with those of FOXM1 (r = 0.499, P < 0.001, Figure 4C). In GIN samples, the percent of FAP positive cells was positive correlated with those of FOXM1 (r = 0.419, P = 0.021, Figure 4E), and the percent of EZH2 positive cells was positive correlated with those of FOXM1 (r = 0.565, P < 0.001, Figure 4F). In normal samples, the percent of FAP positive cells was positive correlated with those of FOXM1 (r = 0.703, P < 0.001, Figure 4H), whereas there was no significant correlation between the percent of FOXM1 positive cells with those of EZH2 (Figure 4I). In addition, the percent of EZH2 and FOXM1 positive cells was slightly positive correlated with those of FAP in 75 GC cases (r = 0.363, P < 0.001, Figure 5A), and the percent of EZH2 or FOXM1 positive cells was positive correlated with those of FAP in 81 GC cases (r = 0.539, P < 0.001, Figure 5B). Similarly, we also investigated the correlation of the average fluorescence intensity of EZH2, FOXM1 and FAP in each kind of gastric tissue samples (Figure 6). The average fluorescence intensity of FAP was slightly positive correlated with those of EZH2 in GC (r = 0.210, P = 0.042, Figure 6A), GIN (r = 0.359, P = 0.047, Figure 6D) or normal (r = 0.245, P = 0.018, Figure 6G) samples. The average fluorescence intensity of FAP was slightly positive correlated with that of FOXM1 in GC (r = 0.206, P < 0.001, Figure 6B), GIN (r = 0.637, P < 0.001, Figure 6E) and normal (r = 0.419, P = 0.021, Figure 6H) samples. The average fluorescence intensity of EZH2 was significant positive correlated with that of FOXM1 in GC (r = 0.602, P < 0.001, Figure 6C) and GIN (r = 0.801, P < 0.001, Figure 6F) samples, whereas there was no positive correlation between them in normal samples (Figure 6I). Furthermore, we assessed the colocalization of EZH2, FOXM1 and FAP by mIF, which also demonstrated a spatial correlation between EZH2, FOXM1 and FAP positive cells (Figure 7).
Correlation between protein expression and clinicopathological factors in GC tissues
Based on the average fluorescence intensity of these three proteins, we divided 93 GC tissue samples into high protein level group and low protein level group according to their Youden value, and then analyzed the correlation between the expression level of these protein and the clinicopathological parameters of GC patients (Table 1). Chi-squared analysis demonstrated that EZH2 protein expression was significantly correlated with tumor location (P = 0.005). Moreover, FOXM1 expression were significantly correlated with tumor invasion depth (P = 0.030). However, there was no significant correlation between FAP expression and any clinicopathological factors.
Prognostic correlation of EZH2, FOXM1 and FAP in GC tissues
Patients with low FAP protein expression exhibited a significantly better OS (P = 0.016, Figure 8A) and DFS (P = 0.003, Figure 8B) than those with high FAP protein expression. Analogously, these data also demonstrated that high EZH2 expression conferred a the significantly worse OS in patients with GC (P = 0.023, Figure 8C). However, patients with higher EZH2 protein expression showed slightly worse DFS than those with lower EZH2 expression (P = 0.085, Figure 8D); and FOXM1 protein expression showed no prognostication value on OS or DFS (data not shown). In addition, patients with low EZH2 and FAP expression had much better OS (P = 0.008) and DFS (P = 0.009) than patients with other combinations of EZH2 and FAP expression (Figure 8E-F). Our results suggest that the combination of EZH2 and FAP expression may be utilized as powerful factors for prognostication in GC.
Univariate analysis of survival revealed that pTNM stage (P = 0.018), peritoneal metastasis (P = 0.014), lymphatic metastasis (P = 0.004) and vascular invasion (P = 0.008), high expression of FAP (P = 0.018) and EZH2 (P = 0.034) were prognostic indicators of OS (Table 2); vascular invasion (P = 0.009), lymphatic metastasis (P = 0.008) and high expression of FAP protein (P = 0.006) were prognostic indicators for DFS (Table 3). Multivariate Cox regression analysis showed that high expression of EZH2 (P = 0.033) in addition to lymphatic metastasis (P = 0.014) were independent risk factors of OS (Table 2); whereas high FAP protein expression (P = 0.018) in addition to lymphatic metastasis (P = 0.017) were independent prognostic predictors for DFS in GC patients (Table 3). In the present cohort, there was no relationship between FOXM1 expression and DFS or OS in patients with GC (Table 2-3). Overall, these results suggest that EZH2 and FAP are potential independent prognostic factor for GC.