Ovarian cancer is the seventh most common malignant tumor in the world. In 2012, it was estimated that there were 238,719 incident cases and the age-standardized rate was 6.1/100,000. In 2020, it is expected by the American Cancer Society that there will be approximately 21,750 new ovarian cancer cases in the United States, and 13,9400 women will die from it. Owing to its occult onset and innocuous symptoms, most of the patients with ovarian cancer are diagnosed in advanced stage. Although the development of new anti-tumor drugs and the improvement of surgical treatment, the survival rates decline dramatically from 92% for patients with Stage I to 17–28% for those with advanced disease (Stages III-IV); majority of advanced stages patients eventually relapse and chemotherapeutic resistance. Although serum biomarker CA125 was widely used in clinical practice for diagnosis and differentiation, population-based screening serum cancer antigen (CA125) assessment and use of risk for ovarian cancer algorithm (ROCA) did not identify significant mortality reduction and has been proved to be ineffective. Thus, it is urgent to clarify the underlying mechanism of oncogenesis of EOC and find out tumor biomarkers to facilitate early diagnosis and targeted therapy or prevention.
As a new tumor biomarker, HE4 has aroused full attention in recent years. However, most of the research focuses on its clinical application of early and differential diagnosis, relapse, prognosis, chemotherapeutic resistance, as well as other clinical aspects for EOC[12, 35], and there are few studies on its mechanism in ovarian cancer yet; it may be the reason that HE4 has not to be anchored as a therapeutic target due to a unanimous conclusion on its roles in the tumorigenesis and progression of EOC. As early as in 2011, Gao L. et al.  reported that they found overexpression of HE4 obviously promoted ovarian cancer cell apoptosis and adhesion, they noticed HE4 may inhibite ovarian cancer cell proliferation, migration and invasiveness, as well as xenograft tumor formation in vivo; thus they concluded that HE4 might play a protecting role in the progression of EOC. Further, in 2014, Kong et al. found in vitro that this protective infulence may be attained by regulating the MAPK and PI3K/AKT pathways. On the contrary, other researchers noted that HE4 high expression promotes cell migration, adhesion, proliferation, and spreading, which can be associated with its effects on the EGFR-MAPK signaling pathway[18, 36]. What is more, HE4 contains fucosylated modification (Lewis y antigen), Lewis y overexpression can promote HE4-mediated invasion and metastasis in ovarian cancer cells. Impressively, overexpression of Lewis y antigen enhanced tyrosine phosphorylation of EGFR and HER/neu, which improved cell proliferation by the PI3K/Akt and Raf/MEK/MAPK pathways; hence, Lewis y antigen and HE4 may affect alike signaling pathways that promote tumor growth and malignancy. HE4 overexpression promotes ovarian cancer cell xenograft tumor growth in vivo, antisense target of HE4 can suppress this effect, HE4 interacts with tumor microenvironment constituents (EGFR, IGF1R, Insulin) and transcription factor HIF1α, these results provide some convincing proof that HE4 is tied to growth factor signal and the MAPK/ERK pathway. Annexin A2 (ANXA2) was identified as a robust interacting partner of HE4 by mass spectrometry and co-immunoprecipitation, the HE4-ANXA2 complex can promote ovarian cancer cell invasion and migration in vitro and tumor distant metastasis of lung in vivo, downregulation of HE4 decreases expression of MKNK2 and LAMB2, which were associated with signaling pathways of MAPK and focal adhesion.
In recent years, a growing number of investigations have gradually found that HE4 promotes cell proliferation, adhesion, invasion, migration, and chemoresistance in ovarian cancer[20–26, 42–47]. It was found that HE4 overexpression or rHE4 treatment in EOC cells resulted in upregulation of many transcripts coding for extracellular matrix proteins, including LAMC2, LAMB3, SERPINB2 and GREM1; moreover, in cells overexpressing HE4 or exposed to rHE4 in culture medium, the protein levels of LAMC2 and LAMB3 were continously increased, and in the presence of fibronectin, the focal adhesions were elevated in cells treated with rHE4.
It was known that ovarian cancer participates in evading immunosurveillance and orchestrating a suppressive immune microenvironment, a series of studies by James NE, et al.[23, 44, 45] found that, upon exposure of purified human peripheral blood mononuclear cells(PBMCs) to HE4, osteopontin (OPN) and DUSP6 appeared as the most inhibited and upregulated genes; the proliferation of human ovarian carcinoma cells in conditioned media from HE4-exposed PBMCs was enhanced, while the effect was attenuated by adding recombinant OPN or OPN-inducible cytokines (IL-12 and IFN-γ); HE4 can compromise both OPN-mediated T cell activation and cytotoxic CD8+/CD56+ cells through upregulation of self-produced DUSP6, thus promoting the tumorigenesis of ovarian cancer[23, 44, 45, 48]. Other researchers found that HE4 promotes carcinogenesis of ovarian cancer by combining with histone deacetylase 3 (HDAC3) to activate PI3K/AKT pathway, and that HE4 knockdown suppresses the invasive cell growth and malignant progress of ovarian cancer by inhibiting JAK/STAT3 pathway. Until now, a few studies have begun to delineate HE4’s role in chemoresistance of ovarian cancer. It was noted that overexpression of HE4 promotes the collateral resistance of ovarian cancer cells to cisplatin and paclitaxel, and down-regulation of HE4 partially reverses the resistance of multiple chemotherapeutic agents; the HE4-mediated chemoresistance might be related to a variety of factors, including deregulation of MAPK signaling (EGR1 and p38 inhibition), and alterations of tubulin levels or stability; recombinant HE4 could upregulate the levels of α-tubulin, β-tubulin and microtubule associated protein tau (MAPT) [41, 43]. Similarly, in vitro, HE4 represses apoptosis induced by carboplatin, and recombinant HE4 results in increased BCL-2 expression and decreased Bax (Bcl-2 associated X protein) expression in carboplatin treated ovarian cancer cells, which reduces the ratio of Bax/Bcl-2; in addition, HE4 also suppresses EGR1 expression, which may contribute to the overall reduction of pro-apoptotic factors that lead to EOC chemoresistance. HE4 can enhance the regulation of DUSP6 and they are positively correlated, DUSP6 deactivates extracellular-signal-regulated kinase (ERK), the inhibition of DUSP6 can alter gene expression of ERK pathway response genes (EGR1 and c-JUN) and sensitize ovarian cancer cells to chemotherapeutic agents(paclitaxel or carboplatin)[23, 48]. The resensitization of ovarian cancer cells to cisplatin and paclitaxel caused by HE4 knockdown is due to the corresponding decreases of ERK and AKT during gene knockouts, and the activation of these pathways inhibits the apoptotic signal of tumor cells.