Endometrial cancer (EC) is the leading gynecological malignancy in the Western world, and one of the top ten most common cancers among women (1). Obesity, polycystic ovarian syndrome and nulliparity are risk factors for EC (1-3). The risk of developing EC increases with age and more than 90% of cases present in peri- and postmenopausal women, with a peak incidence in the sixth decade (4). Based on clinical and molecular characteristics, EC is classified into two subgroups. Type 1, also known as endometrioid type, is the more common one, representing ~80% of uterine cancer cases, and is typically hormone sensitive, linked to an excess of estrogen, and has a better prognosis. Type 2, or non-endometrioid type, accounts for 20%, and is typically estrogen independent and comprises sarcomas, clear cell carcinomas and others. Type 2 is more common in senior women and clinically presents as a more aggressive disease with a higher recurrence rate than type 1 (1,5,6).

Since symptoms such as abnormal vaginal bleedings present early, EC is often diagnosed at an early stage, which likely contributes to the favorable prognosis, with overall 5-year survival higher than 80% (6). However, metastasis, chemoresistance and recurrence remain a challenge.

Tumour progression, notably metastasis and chemoresistance, involves changes in cellular metabolism that benefit tumour cell growth and survival; of these changes, the Warburg effect, or high rates of aerobic glycolysis, is perhaps the most well-known (7,8). Alterations in the levels of PKM2, GAPDH and ATP5B, i.e., enzymes related to cellular metabolism, were shown to be associated with the shorter survival in patients with ovarian carcinomas (9). Alterations in mitochondrial functions are instrumental to the metabolic plasticity of tumour cells (10,11), but also to metastasis, as upregulation of mitochondrial oxidative phosphorylation by metastatic cells was linked to superoxide production and subsequent regulation of cell adhesion processes (12).

The transcriptional coactivator peroxisome proliferator-activated receptor gamma (PPARγ) coactivator 1 (PGC1α) is important in regulating mitochondrial biogenesis and function, and lower levels of PGC1α expression have been noted in various cancers, such as breast, colon (13), and ovarian cancers (14). Of note here, PGC1α has been shown to be influenced by the presence of estrogen. Other mechanisms are possible; thus, type 1 EC is reported to harbor mutations in mitochondrial DNA (mtDNA) which affect Complex I, and which might thereby lead to the observed upregulation of mitochondrial biogenesis and PGC1α (15,16). In line with differential regulatory pathways, both increased and decreased levels of PGC1α have been associated with more aggressive cancer and poor prognosis (17).

The voltage-dependent anion channel type 1 (VDAC1) protein located in the outer mitochondrial membrane regulates mitochondrial import and export of ions and metabolites, including Ca²⁺, ATP and NADH, and thereby regulates oxidative phosphorylation (18). VDAC1 is also involved in regulating cell death through interactions with various proteins including hexokinase (HK), Bcl-2 and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (19). VDAC1 is often used as a marker of mitochondrial content of cells (20).

In the present study, we assessed protein expression of PGC1α and VDAC1 in type I and II ECs and paired non-cancerous tissue, in order to examine a putative correlation between them and clinical data such as subtype, stage and grade, clinical resistance and overall survival.

Tumour pathogenesis and progression go hand in hand with major metabolic alterations, notably altered mitochondrial function(s) (22). The transcriptional coactivator PGC1α is well-studied, particularly in normal tissue, as a major regulator of mitochondrial biogenesis and function, and is generally perceived to promote an oxidative metabolism (13). Regarding its role in cancer and as a prognostic marker, reports vary greatly, as both high and low levels have been found to correlate with worse outcome (17). Here, we studied the expression of PGC1α in EC. The main findings were that its expression was lower in cancer tissue than in benign tissue from the same patient and that there was no correlation between the expression of PGC1α and aggressive course of the disease. We also examined the expression of VDAC located in the mitochondrial outer membrane and which regulates mitochondrial import and export of ions and metabolites. VDAC expression was lower in the tumour tissue than in benign tissue. In agreement with the current understanding of Ki67 as a cellular marker for proliferation, we observed significantly higher Ki67 expression in tumour tissue than in adjacent benign tissue, and an association with shorter time to relapse. Others have shown that survival in EC patients is independently influenced by Ki67 expression (23).

That the decreased expression of PGC1α in EC tissue was similar in the two subtypes of EC was perhaps unexpected considering that type I is estrogen sensitive and that a model for how hyperestrogenism promotes EC progression involving PGC1α has been proposed (24). Moreover, the results contradict those of Ren et al (25) who reported increased PGC1α expression in a small (n=15) cohort of EC type I tumours; however, these were compared to benign tissue from healthy controls, and data were on mRNA rather than the actual protein. In a larger study comparing EC tissue to benign, Cormio et al (16) reported doubled levels of PGC1α and of the mitochondrial transcription factor TFAM which is known to be in part regulated by PGC1α (13). An important difference between their study and the present one is that they assessed protein expression levels by western blotting, i.e., in heterogeneous extracts, whereas we evaluated it specifically in cancer cells.

Similar to our results, others have reported decreased expression of PGC1α in, for instance, colon (26), breast (27) and clear-cell ovarian cancer (14). Furthermore, other studies have associated a decrease in the expression of PGC1α with poor prognosis in human breast cancer and hepatocellular carcinoma (13). By contrast, we could not find any overall correlation in EC between PGC1α expression and prognosis. However, there was a tendency that among grade 1 patients, the lower expression could be associated with a shorter time to death. This tendency, which needs to be confirmed, supports the notion of a context-dependent protective function of PGC1α (17). A recent example of the same is the finding that decreased PGC1α expression may contribute to tumour invasion and metastasis (28).

VDAC expression is often used as a marker of mitochondrial content. In line with downregulation of PGC1α and a putative downregulation of mitobiogenesis, VDAC expression was also decreased in both type I and II EC tumour cells compared to benign tissue. However, its expression has been reported to vary in cancer cells (20,29). Likewise, we have noted VDAC upregulation also in the absence of PGC1α expression in ovarian clear cell carcinoma (14), a subtype that is notoriously resistant to treatment. Altogether, this supports the idea that VDAC is not necessarily a ‘housekeeping’ indicator of mitochondrial content, and we therefore suggest that the roles and functions of VDAC in tumour cells depend on cellular context.

Our data are based on a large and representative group of patients. Importantly, each patient is her own control, as we used paired tumour and benign tissue samples from the same patients, and thus did not have to create a matching control group. On the other hand, it is impossible to exclude the risk that the healthy tissue adjacent to cancer tissue does not harbour some pre-cancerous molecular changes.

Further investigation of the correlation between lower PGC1α expression and shorter time-to-death, in particular in the FIGO Grade 1 group, could be of clinical significance. If expression of PGC1α at early cancer stages is correlated with a higher risk of recurrence it might thus signal that these patients be treated more aggressively than is generally the case today.

In summary, we have shown downregulation of PGC1α as well as VDAC protein levels in EC of both types, indicating altered mitochondrial functions in EC compared to benign tissue. The results also indicate that PGC1α and VDAC levels are not of major prognostic value in EC.