Our results demonstrated that platinum-sensitive tumors were associated with a stronger PR expression both in clinical specimens and cell line models. However, PR status was not significantly associated with PFS or OS in the patients with OCCC.
Previous studies analyzing the expression rates of sex hormone receptors in patients with OCCC have reported a wide range of PR expressions from 0% [17] to 60% [18], with most demonstrating < 10% positivity [19–22]. The percentage of PR-positive patients in the present study was relatively high at 37.5%. The differences among various studies may be due to differences in the PR antibodies used in IHC staining. The most common commercially available PR antibodies include clone 16, clone 636, clone 1A6, clone alpha PR6, and clone 1E2. Although all of these antibodies are thought to be able to recognize both PR-A and PR-B isoforms, a previous study demonstrated that some antibodies failed to detect PR-B in tissue sections when using IHC techniques despite their ability to do so with immunoblot analysis [23]. A possible reason is that some epitopes on PR-B may be masked during heat treatment of formalin-fixed tissue, which results in a certain degree of protein tertiary structural change. Therefore, it is possible that PR expression will not be detected or at least will be underestimated in tissues where PR-B is predominant.
To date, no previous study has evaluated PR expression only in OCCC patients. The largest collaborative study from Sieh et al. reported that PR expression was associated with better high-grade serous and endometrioid carcinoma survival but not mucinous or OCCC [12]. Another meta-analysis of 5685 EOC patients from 28 studies found that PR expression was related to PFS and OS, but that when focusing on studies of serous carcinoma the prognostic role of PR disappeared, which is in contrast to Sieh et al.’s study [13]. This discrepancy may be related to differences in the methodologies of measuring PR and interpreting IHC. For example, we quantified PR status using H-score, which is quite different to Sieh et al. Most previous studies have used a three-tier system: negative, weak, and strong, defined as positive staining in < 1%, 1–50%, and ≥ 50% of tumor cell nuclei, respectively. In the present study, we found a stepwise reduction in PR expression from FIGO stage I to IV. Previous studies have reported different expressions of PR, ranging from 100% in atypical ovarian endometriosis to 35% in OCCC, suggesting that a loss of PR may be involved in the etiology and progression of OCCC [14, 15]. As the critical point of PR level change in relation to chemosensitivity or survival is unknown, we quantified PR status and identified an optimal cut-off value. However, even when using this cut-off value, PR was still not a significant prognosticator. There are some possible explanations for this. First, we enrolled many FIGO stage I/II patients. Early-stage OCCC has an excellent prognosis, and even with recurrence the time to relapse may be more than 6 months after completing chemotherapy. Whether these tumors were actually platinum-sensitive remains unclear. Second, although we quantified PR status, the H-score values were not normally distributed. We attempted to stratify the patients by early stage versus advanced stage or with concurrent endometriosis versus without, and we even transformed the values to a certain exponent, however we still failed to overcome this problem. Third, the limited number of study patients after excluding others due to various reasons may also have biased the outcomes.
PR has two isoforms, A and B, and the role of each receptor isoform remains unclear with regards to hormone treatment responsiveness and prognosis. In breast cancer, PR-A-predominant breast tumors are more sensitive to anti-progestin treatment, while PR-B-predominant tumors are associated with advanced disease and poor prognosis [24]. In endometrial cancer, both PR-A and PR-B are associated with less aggressive tumors, however only PR-B predicts a better prognosis [25]. An interesting finding in our study was that the patients with either the A or B PR isoform appeared to be more sensitive to cisplatin, and that the patients with a high total PR expression had a trend towards better survival. The association between hormone receptor status and platinum sensitivity has seldom been investigated in EOC. In one preclinical study using OVCAR-3 cells, Preluso et al. reported that a high PR expression in ovarian cancer cells was associated with decreased PR membrane component-1 (PRMC-1) expression, which in turn increased the effectiveness of cisplatin [26]. The same group later transplanted SKOV-3 cells into nude mice, resulting in the development of an OCCC animal model. They demonstrated that PGRMC1 regulated not only the growth and platinum sensitivity of cultured SKOV-3 cells, but also ovarian tumors derived from these cells [27]. In an endometrial tumor model, they also found that PGRMC1-depleted tumors were more responsive to chemotherapeutic stress compared with PGRMC1-intact control tumors [28]. Taken together, these data support the close relationship between PGRMC-1 expression and chemosensitivity. Given the reported inverse relationship between PGRMC-1 and PR expression [26], our results are consistent with Preluso et al., although further studies investigating the relationship between PGRMC-1 and PR with different expressions of the isoforms are needed.
OCCC is thought to be a platinum-resistant malignant disease. Recent studies have identified specific immune-related molecular profiles such as PD-L1 expression and mismatch repair protein defects in OCCC [29, 30], which in turn has led to the development of a novel therapeutic approach focusing on immunotherapy. Although recent phase I/II results of using pembrolizumab were promising [31, 32], the sample size was small and further phase II/III verification is needed. Until then, clinicians still need to identify which patients may really benefit from traditional chemotherapy.