In the present study, we did a bioinformatics analysis for primary tumors and metastatic lesions of ovarian cancer samples from GEO database to explore new genes and their mechanisms in ovarian cancer metastasis. While some information was available on the factors and pathways involved in the promoting invasiveness [10, 11], the progression of ovarian cancer was an intricate, multistage process along with numerous molecular changes, which further needed to be explored. Therefore, we systematically searched relevant studies in the GEO data library, and comprehensively analyzed the available datasets to find out common and unique signatures during the process of metastasis in ovarian cancer. Our results showed that the alterations of matrisome components and changes of gated channel activity were involved in this process. Moreover, we firstly identified ZFHX4 as the hub gene contributing to the ovarian cancer metastasis, and EMT regulation and ECM remodeling were considered as its potential mechanisms.
It has been well demonstrated that gene expression profiling of ECM components underwent deregulation during early colonization [12]. In turn, ECM remodeling could be conducive to cancer metastasis [13]. In the primary sites, basement membranes, which lined the basal surface of tumors and presented as dense collagen structures, were broken through proteolytic ECM degradation and non-proteolytic force by neoplastic cells and cancer-associated fibroblasts (CAFs) when tumor rapid growth led to diffusion-limited oxygen and nutrient supply [14–16]. Next, collagens in interstitial matrix were linearized by tumor-derived lysyl oxidase (LOX), which created migratory tracks used for the migration of the tumor cells [17, 18]. In the metastatic sites, however, CAFs deposited more collagens into matrix to support new lesions growth and help them evade immune surveillance [19, 20]. In accordance with the previous studies, we found that genes encoding alpha chains for fibrillar collagens, such as COL5A1, COL5A2, COL1A1 and COL1A2, were the top up-regulated DEGs in the metastatic samples compared to the primary ones. Besides collagens, altered glycoproteins, another kind of component in the ECM, might also contribute to the tumor metastasis. For instance, the expression of POSTN, which encoded a secreted extracellular matrix protein named Periostin, showed significant increase in the GSE30587. Yue.al reported that stromal Periostin could fuel the migration and invasion of ovarian cancer cells by binding to integrin αvβ3 and subsequently activating the PI3K/Akt pathway and inducing the EMT [21]. The regulation of TGFBI was disordered as well according to our results, and its encoding protein (transforming growth factor β induced protein, TGFBIp) was located in the extracellular matrix to bind to type I, II and IV collagens. As mentioned in the recent papers, macrophage-derived TGFBIp promoted ovarian cancer cell migration via inducing an immunosuppressive microenvironment [22, 23].
Ion channel disorder appeared to be responsible for the enhanced migration of cancer cells and ECM dysregulation [24, 25]. Aberrant activation of Ca2+-permeable channels on the endoplasmic reticulum membrane (STIM1) and plasma membrane (ORAI1) led to impulse Ca2+ influx [26]. The oscillatory Ca2+ signaling facilitated invadopodial precursor assembly via Src activation and increased proteolytic activity of invadopodia by recruitment of ECM-degrading membrane type 1 matrix metalloproteinase (MT1-MMP) [27]. Then, Ca2+-activated K+ channels, such as SK3, enhanced the driving force for Ca2+ entry through hyperpolarization of resting membrane potential [28, 29]. Moreover, abnormal opens of sodium ion channels also contributed to invadopodia formation via F-actin polymerization [30]. Na+/H+ exchangers on the membrane caveola resulted in a per-imembrane acidification, which rendered secreted cathepsins, pH-dependent ECM-digesting proteases, more active [31]. In our study, inorganic ion trans-membrane transports for calcium (TRPM3), sodium (SLC family) and potassium (KCNK1) were significantly up-regulated in the metastatic ovarian cancer samples compared to the primary ones, which were in accord with previous researches.
ZFHX4 is one of the five members of zinc finger homobox family with molecular weights of 397 kDa [32]. The protein comprises 22 typical C2H2 structures of zinc finger and 4 homeodomains, and its coding gene extends over 180 kb and is located on 8q13.3-q21.11[32]. ZFHX4 is predicted to be active in nucleus, and enable DNA-binding transcription factor activity. Fèvre-Montange et.al firstly reported that ZFHX4 was highly expressed in parapineal tumors, a rare but aggressive type of pineal region tumor, using oligonucleotide arrays in 2006 [33].Similarly, Wang et.al found that ZFHX4 amplifications were more common in the metastatic colorectal cancer patients than those without metastases [34]. In ovarian cancer, elevated expression of ZFHX4 was also demonstrated to be correlated with poor outcomes, which was mainly influenced by metastasis, in bioinformatics databases and real-world cohort samples [35, 36]. Our results were in accord with these studies, which indicated that ZFHX4 reduced the survival times of ovarian cancer patients through promoting metastasis.
Currently, researches for the functions of ZFHX4 largely focused on the therapy resistance and consequent recurrence. Chudnovsky et al. suggested that ZFHX4 interacted with the CHD4, an essential member of the NuRD (nucleosome remodeling and deacetylase) complex, to regulate the therapy-resistant tumor initiating cells in glioblastomas [37]. However, the detailed mechanism by which ZFHX4 regulated metastasis of ovarian cancer remained unknown. According to our bioinformatics analysis, we, for the first time, linked ZFHX4 with EMT-associated genes. In 2020, “the EMT International Association (TEMTIA)” issued a consensus statement on the famous academic journal “NATURE REVIEWS MOLECULAR CELL BIOLOGY” to provide guidelines and definitions for EMT [38]. In the statement, experts emphasized central roles of transcription factors belonging to the ZEB families (Zeb1 and Zeb2) in the execution of EMT in cancer metastasis [38]. Coincidentally, Zeb1 and Zeb2 were also two members of zinc finger homobox family, and they shared similar molecular structures with ZFHX4 [39], which rendered the function of ZFHX4 as an EMT inducer reasonable. Besides, ZFHX4 could be modulated by canonical pathways for EMT regulation, such as TGF-β signaling pathway [40, 41], which made further efforts to support the contributions of ZFHX4 to EMT. Simultaneously, we found the positive correlation between ZFHX4 and ECM-related genes. ZFHX4, on the one hand, was conducive to the collagen formation, but on the other hand, induced degradation of ECM. The results might be explained by the fact that individual cancer cell migrated faster in a mesenchymal phenotypes, while collective cell migration depended on the cell-matrix junctions [42]. Next, significant correlations were found between ZFHX4 and lots of ion channels. Eskandari et al. reported that Kv11.1 channel reprogrammed EMT in colon cancer via regulating zinc finger homobox proteins [43], suggesting that ZFHX4 might also act as a mediator converting the signals from ion transporter activation to EMT occurrence and ECM reprogramming in ovarian cancer. Although ZFHX4 took an active part in promoting cancer metastasis, it functioned as a tumor suppressor in tumor proliferation. This rather interesting finding might be explained by the opinion that expression changes of certain genes necessary for the initial tumorigenesis were not essential for subsequent metastasis, which was proposed by Mitra et al [12]. In their study, they observed that most of shared deregulated genes in the primary tumors versus normal tissues and metastasis versus primary tumors were altered in the different directions [12].
There were surely some limitations in the study. Firstly, DEGs were screened from primary tumors and metastatic lesions in GEO database, while the candidate genes were generated from ovarian cancer patients with various stages in TCGA database. Secondly, laboratory-based experiments have not been conducted to validate the mechanisms of ZFHX4 in metastasis in ovarian cancer. In the future studies, clinical samples from ovarian cancer patients would be collected to detect the expressions of ZFHX4. Then, the relationships between ZFHX4 and EMT- and ECM-related proteins would be further confirmed by in vitro or (and) in vivo experiments.