Tomm34 is commonly expressed in epithelial ovarian cancer and associates with tumour type and high FIGO stage

Epithelial ovarian cancer accounts for approximately 90% of ovarian tumours and is usually diagnosed only at advanced stages of the International Federation of Gynecology and Obstetrics (FIGO) system, accounting for its high mortality rate [1, 2]. Epithelial ovarian cancer is not a single disease and a dualistic model has been proposed to describe morphological subtypes and cell of origin [2, 3]. Type I ovarian cancers are low grade and develop slowly (including endometrioid, clear cell, mucinous and low grade serous adenocarcinomas) whereas type II cancers are the most common form, representing high-grade serous adenocarcinomas. It is also recognized that high and low-grade serous adenocarcinomas originate from precursor lesions in the fallopian tube, whilst the other histological type I tumours arise from endometriosis, germ cells or transitional cells, with an important role for distinct genetic alterations influencing the tumour characteristics [4‐6]. Most notably, mutations in the p53 tumour suppressor are an overwhelming characteristic of Type II high grade serous tumours [7, 8]. Whilst many low grade lesions can be treated with surgery alone, high grade epithelial ovarian cancer is a difficult-to-treat disease that requires surgery plus combination chemotherapy, and even then recurrence is common (70–80% within two years), although this is improving with current targeted therapies for subsets of patients [9].

Increased chaperone activities are a universal feature of cancer and anti-Hsp90 or anti-Hsp70 therapeutics are under investigation for the treatment of various cancer types [10], including ovarian cancer [11, 12]. Chaperone activities are dependent on their interactions with co-chaperones that provide either protein folding or protein degradation functions. The protein folding activity of chaperones is hyperactive in cancers due to enhanced interactions of phosphorylated Hsp90 with Hsp70/Hsp90-organising protein (HOP, also known as STIP1) and reduced interaction with C-terminal Hsp70 interacting protein (CHIP, also known as STUB1) [13].

Translocase of the outer mitochondrial membrane 34 (Tomm34) is an additional component of the cellular chaperone system involved in protein folding. As the name suggests, Tomm34 was initially identified as being involved in mitochondrial protein processing [14, 15]. Subsequent studies have shown that Tomm34 interacts with both Hsp70 and Hsp90 and modifies their protein folding activities [16‐20]. In cancer, high levels of Tomm34 have been reported in bladder, colorectal and breast cancers compared to their normal tissue counterparts [21‐26]. In these cancers, Tomm34 promotes colorectal cancer cell growth [21] and is a biomarker of poor outcome in early invasive breast cancer [22] and bladder cancer [26]. As a tumour-associated protein, Tomm34 peptide vaccination is under investigation as a therapeutic option for colorectal cancer, with significant Tomm34 cytotoxic T-lymphocyte (CTL) response observed [27‐29].

Previous data have indicated that the ovary also expresses TOMM34 mRNA [21] although its expression in ovarian cancer has not been reported. Here, we investigated the levels of Tomm34 in ovarian cancers of mixed subtypes using immunohistochemistry. The data were correlated with tumour type and clinicopathological variables and demonstrate that Tomm34 is expressed at high levels in type II carcinomas and correlates with high FIGO stage.

Cancer cells require a high level of protein synthesis due to their rapid proliferation and exhibit a high degree of stress due to genomic instability and lack of both oxygen and essential nutrients. To overcome such difficulties, cancer cells show up-regulation of many stress-induced proteins, including components of the protein chaperone system, particularly Hsp70 and Hsp90 [30‐32]. Chaperone inhibition is therefore a clinically promising area [10], including for the treatment of ovarian cancer [11, 12]. Metabolic abnormalities are also a common finding in human cancers and the high rate of glucose uptake by tumour cells is utilised clinically for cancer imaging by 18F-2-deoxyglucose uptake and accumulation. Although it was originally thought that aerobic glycolysis in cancer cells (the Warburg effect) represented a lack of mitochondrial function, it is now clear that altered mitochondrial function represents a redeployment of glycolytic nutrients from catabolism to anabolism, required to meet the biosynthetic requirements of rapidly proliferating cancer cells, and NADPH production to help maintain redox balance [33‐36].

These alterations to mitochondrial function relate to altered chaperoning of mitochondrial proteins [37]. Thus, targeting abnormal mitochondrial function and chaperoning of mitochondrial proteins is also a promising strategy for cancer therapeutics [38]. Hsp90 and Hsp70 play critical roles in mitochondrial protein stabilisation and folding [39] and various co-chaperones such as p23, Hsp40 and HOP are also involved [40‐42]. In this respect, Tomm34 was originally identified as a potential mitochondrial import protein [15] and Tomm34 antibodies inhibit transport of preproteins into the mitochondria, whilst expression stimulates mitochondrial preprotein maturation and TOMM34 siRNA inhibits this process [14]. Tomm34 is also reported to be increased as a component of compensatory adaptations to maintain normal rates of protein import in response to mitochondrial abnormalities [43]. On the other hand, and in agreement with our immunostaining data, Tomm34 exists predominantly in the cytoplasm rather than in mitochondria, suggesting it is involved in the transport of mitochondrial preproteins in an unfolded state prior to import [14, 17, 44].

Our data indicate that Tomm34 is commonly expressed at high levels in human ovarian cancers, except for the MOC and CCOC subtype, where high level Tomm34 is rarely seen. Within the different sub-types of ovarian cancer, high levels of Tomm34 associate with higher stage and higher grade cancers and similar findings are seen within the sub-type of serous cancers. Most notably, we have found that Tomm34 associates with poor survival of patients with p53-mutant ovarian cancers. These data are also comparable to the findings that Tomm34 is a marker of poor outcome and a predictor of distant metastasis in breast cancer [22, 23]. Thus, as with breast cancer patients, Tomm34 may serve as part of a panel of markers for prognostic determination in ovarian cancers. In this respect, proteomic analysis of an animal model of ovarian cancer revealed up-regulation of numerous proteins involved in metabolic processes, including endoplasmic stress responses, mitochondrial systems and chaperones such as Hsp70 and Grp78 [45].

The mechanism for high level expression of Tomm34 is unclear from our studies. Data from the COSMIC database (http://​cancer.​sanger.​ac.​uk/​cosmic) indicate that only 1.5% of 729 ovarian cancers tested show copy number variation gain of TOMM34, indicating that gene amplification is a rare event. However, Tomm34 is transcriptionally regulated by NRF-1 and NRF-2 [46, 47] and these latter are implicated in directing metabolic reprogramming during stress [48] and are often hyperactive in cancers. Although further research will be required to elucidate the mechanisms that regulate Tomm34 in ovarian and other cancers, our data imply that Tomm34 has pro-tumourigenic actions and may serve as a useful prognostic indicator and potential therapeutic target in ovarian cancers.

The co-chaperone Tomm34 is frequently expressed in epithelial ovarian cancers. Immunohistochemical identification of Tomm34 may be a useful adjunct to provide prognostic information and may serve as an immunogenic target for therapy.