SGEF is a potential prognostic and therapeutic target for lung adenocarcinoma

Background SH3-containing guanine nucleotide exchange factor (SGEF), a RhoG-specific guanine nucleotide exchange factor (GEF), was consider as a key signal that determines cancer cell invasion. Although SGEF has been considered to highly express in glioma and prostate cancer. However, it is not well illustrated in LAC. Methods In this experiment, expression of SGEF was detected in 92 LAC and corresponding normal tissue samples by immunohistochemistry. In addition, we evaluated the invasion and migration of lung adenocarcinoma cells by the gain and loss of SGEF expression. Furthermore, RhoG activity was measured by GST pull-down assay. Results SGEF is highly expressed in LAC tissues than in normal lung tissues and was associated with the TNM stage. Lung adenocarcinoma patients with low SGEF subgroup had longer overall survival compared to those with high expression. Furthermore, univariate analysis showed that SGEF expression was an independent prognostic factor for overall survival in lung adenocarcinoma. Silencing of SGEF effectively suppressed the invasion and migration of human lung adenocarcinoma cells in vitro by inhibiting RhoG activity, and over-expression of SGEF could reverse this phenomena. Conclusion SGEF is a novel prognostic target in human lung adenocarcinoma. Electronic supplementary material The online version of this article (10.1186/s12957-018-1331-8) contains supplementary material, which is available to authorized users.


Background
LAC is the familiar common histological subtype of nonsmall-cell lung cancer (NSCLC), which is the first cause of cancer-related deaths worldwide [1]. Although lung cancer mortality rate has been decreasing owing to advances made in the treatment of lung adenocarcinoma (LAC), the prognosis of advanced lung adenocarcinoma is very poor because of its invasion and metastasis [2]. Therefore, it is worthwhile for us to have a better understanding of the molecular mechanism of invasion and migration of LAC and find a more sensitive and novel target for predicting the prognosis.
Recently, about 22 Rho family members were identified to be crucial for forming filopodium [3], which are demonstrated embedded in or protruding from the lamellipodial actin network to promote lung cancer cell migration and invasion [4]. The activity of Rho GTPases was regulated by Rho guanine nucleotide exchange factors that switch bound GDP to bound GTP [5], indicating that some GEF factors may act as oncogene by activating Rho activation.
SGEF is a RhoG-specific guanine nucleotide exchange factor (GEF) [6]. As shown in Additional file 1: Figure S1, its full length contains an amino-terminal proline-rich region (Pro), a Dbl homology (DH) domain, and pleckstrin homology (PH) domain, as well as Src homology 3 (SH3) domain [7]. DH-PH domain is thought to be correlated with protein-protein interactions and a center of exchange SGEF activation [7]. For instance, the domains of DH/PH in SGEF strongly exchange RhoG [8] and interact with HPV E6 oncoprotein which contains PDZ domain, accelerating invasive phenotype of the HPV-induced cervical cancer. Furthermore, EGFR stability was meditated by SGEF through inhibited EGFR trafficking promoting prostate cancer cell progression. Overall, SGEF is complex and multivariate in the cancer progression. However, there is no information about the functional and clinical value of SGEF in LAC. Therefore, it deserves to explore the SGEF expression and function in LAC.
In summary, we showed that SGEF is highly expressed in cancer than in adjacent cancer and its expression was associated with invasion depth and lymph node metastasis. Furthermore, we found that silencing SGEF in lung cancer cell leads to less invasive phenotype by regulating RhoG activity. Clinically, our data indicated that SGEF may be a novel prognostic marker in LAC.

Clinical samples
The tissue microarray including 92 tumor tissues and 87 corresponding non-cancerous tissue specimens was obtained from lung adenocarcinoma patients diagnosed and operated at the Xinqiao Hospital, Third Military Medical University, from 2008 to 2013 and followed up until 2016. A total of 13 patients were lost to follow up among these patients. None of the patients had received radiotherapy or chemotherapy before surgery.

Detection of EGFR status
Increased EGFR gene copy number was assessed by standard FISH analysis. Patients were considered to be FISH positive if they displayed high polysomy (greater than or equal to four copies of the EGFR gene in greater than or equal to 40% of cells) or gene amplification, defined by the presence of tight EGFR gene clusters and a gene/chromosome per cell ratio of at least 2, or an average of greater than or equal to 15 copies of EGFR per cell in greater than or equal to10% of analyzed cells.

Biostatistics mining of NCBI database
The LAC patients' mRNA expression microarray data were downloaded from GDS2771 in NCBI GEO. There are 90 normal smokers not diagnosed with lung cancer and 97 smokers with lung cancer. For the survival analysis of SGEF, we use online (http://kmplot.com/analysis/ index.php?p=service&cancer=lung) to generate Kaplan-Meier curves, which include 866 gastric cancer patients with available clinical data. The median expression was used as the final cutoff value.

Rho GTPase activation assays
The activation of RhoG proteins was performed as described previously [7]. Briefly, pull-down assays were done using purified GST-ELMO with cell extracts from LAC cells transfected with SiSGEF and SiCtrl. Cellular extracts were prepared from by lysing the cells in 250 ml of 50 mM Tris, pH 7.4, 10 mM MgCl2, 500 mM NaCl, 1% Triton X-100, 0.1% SDS, 0.5% deoxycholate, and protease inhibitors and then equalized for protein concentration. Supernatants were incubated with purified GST-ELMO conjugated to glutathione sepharose beads after clearing it at 12,000 g. RhoG-GTP was investigated by WB and compared with the total RhoG present within the same cell lysate.

Invasion and migration assays
For the migration assay, H1975 cell was plated at the density of 2 × 10 4 cells/well in serum-free DMEM in the upper chamber of 24-well transwell plates containing 8.0-μm-pore Millicell inserts, while the lower chambers were filled with DMEM supplemented with 10% FBS as a chemotaxis agent. For the invasion assay, Millicell inserts were coated with 1 mg/mL of Matrigel (BD Biosciences). Cells invaded/migrated to the lower membrane surface were counted under a light microscope at × 200 magnification in at least four randomly selected fields, and the average number of cells per field was calculated. All the experiments were performed in triplicate.

SGEF is over-expressed in lung cancer
In order to explore the expression of SGEF in LAC, we first investigate eight pairs of lung carcinoma specimens and corresponding neighboring non-cancerous tissues  by western blotting and PCR. We found that SGEF is highly expressed in lung carcinoma than in adjacent tissues, regardless of protein (Fig. 1a) or mRNA (Fig. 1b). Consistent with our previous result, we found that the same result was testified in GEO database (Fig. 1c).Furthermore, SGEF expression were investigated in 87 pairs of lung carcinoma specimens and corresponding neighboring non-cancerous tissues. We found that SGEF was mostly localized in the cell cytoplasm, and the proportion of cells with high SGEF expression was significantly higher in the lung adenocarcinoma tissues (58.7%, 54/92) than in the adjacent normal tissues (17.24%, 15/87) ( Fig. 1d and Table 1). Overall, we found that SGEF is highly expressed in lung carcinoma than in matched adjacent tissues.

High SGEF is closely related with advanced TNM stage and poor prognosis in LAC
We next investigated the potential relationship of SGEF expression and clinical pathological parameters. The pathological features of LAC were summarized in Table 2. SGEF expression is positively related with disease stage (Fig. 2a, b, Table 3), tumor depth (P = 0.026), and lymphatic metastasis (P = 0.021). Furthermore, overall survivals for patients with low SGEF were significantly lower than those with high-SGEF tumors (Fig. 2c, P < 0.001). Additionally, similar results were observed in GEO database (Fig. 2d, P < 0.001). Univariate analysis showed that SGEF was an independent prognostic factor in LAC (HR = 1.846; 95% CI 1.029-3.310; P = 0.040, Table 4). Therefore, SGEF seems to serve as a predictor for the prognosis of patients with LAC.

SGEF accelerates the mobility of LAC cell by activating RhoG
As SGEF is correlated with invasion depth and lymphatic metastasis, we want to examine the oncogenic function of SGEF in LAC invasion by using LAC cell H1975 with siRNA to generate SGEF knockdown cells (Fig. 3a).
A transwell assay showed that treatment with SiSGEF in H1975 cells significantly reduced its motile ability (Fig. 3b), while over-expressed SGEF in H1299 cells increased their mobility (Fig. 3c, d). Furthermore, the activating RhoG was also decreased in H1975-siSGEF cell The expression of SGEF in TNM III stage was significantly higher than that in early TNMΙ and TNMΠ stages. c Kaplan-Meier analyses of 92 CRC patients showing that high-SGEF patients (n = 54) have shorter overall survival as compared with low-SGEF patients (n = 38). d Patients with highly expressed SGEF has a relatively higher risk of mortality (P < 0.001) from GEO databases (Fig. 3e). Taken together, SGEF promotes invasion and migration by inhibiting RhoG activity.

Discussion
LAC is currently the major histological subtype of lung cancer, and the average of 5-year survival rate was only 15% [10], owing to the invasion and metastasis. Thus, searching for specific invasive therapeutic target was crucial for treating LAC. SGEF was highly expressed in several malignant cancers, such as glioma, HPV-induced cervical cancer, and prostate tumors [11][12][13]. Herein, we showed that high SGEF expression in LAC is correlated with TNM stage and poor prognosis. Furthermore, we found that SGEF regulated the invasion of LAC cell line by activating RhoG via siRNA. Our result revealed that SGEF may be a novel prognostic and therapeutic target in LAC. Rho GTPases were first identified to promote cell migration and invasion about 20 years ago [14]. RhoG, a subfamily of Rho GTPases, is induced to form actinrich lamellipodia protrusions and derived cell movement by activating Rac1 pathway. Owing to the RhoG role in regulating various cellular processes [15], downregulation of the RhoG pathway is assumed to be crucial for molecular targeting therapy. These facts suggest that some guanine nucleotide exchange factors (GEF), upregulator proteins of Rho GTPases, may be involved in the cancer progression. Furthermore, several GEFs including PREX1 and Vav2 as well as Vav3 are reported to be over-expressed and associated with carcinoma progression [16,17]. Consistent with previous study, we showed that SGEF is upregulated in LAC tissue as compared with that in adjacent normal tissue and was positively associated with TNM stage. Most importantly, SGEF is an independent prognostic factor in LAC and predicts poor prognosis. These findings support the notion that some GEF protein was involved in the progression of cancer.
Recent studies have shown that SGEF was involved in the response of glioma cells to TMZ treatment modulates the DNA repair. Furthermore, SGEF promotes prostate cancer cell progression by interacting with Grb2 for activating ERK pathway and enhancing EGFR stability [18,19]. Herein, we showed that SGEF promotes the invasion and migration of LAC by regulating the activity of RhoG via silencing SGEF expression. These facts indicate that SGEF was involved in the progression of tumor via various pathways, highlighting the importance and complexity of SGEF.
In summary, we investigated the function of SGEF in LAC initiation and progression. Our study indicates that SGEF was a novel prognostic target in LAC. Our data provide a new insight into the mechanism responsible for the development of human LAC.