Introduction
Leptin is a small (16 kDa) protein produced and secreted by adipose tissue, which is involved in appetite regulation, bone formation and reproductive function. Recent studies have demonstrated that this hormone stimulates growth, migration, invasion and angiogenesis in tumour cell models, suggesting that leptin is capable of promoting an aggressive cancer phenotype [
1]. Epidemiological studies have indicated a positive correlation between obesity and an increased risk of several types of cancer [
2]. Serum leptin levels have been reported to be higher in overweight and obese women than in women with normal weight. In obese individuals, leptin levels can reach 40 ng/mL, which is up to ten times higher than in normal weight people [
3,
4]. Cancer risk is higher among overweight and obese people, with an increased risk of 16 and 30%, respectively [
5]. Moreover, leptin and its receptors are over-expressed in different human cancers [
1]. Leptin has been proposed as one of the six markers of ovarian cancer [
6]. Uddin et al. revealed a significant association between ObR overexpression and poor survival rates in 59.2% of epithelial ovarian cancer cases [
7].
Granulosa cell tumours constitute the second largest group of ovarian tumours (approximately, 25–30%), with tumours derived from epithelial cells accounting for approximately 70% [
8]. Granulosa cell tumours can be divided into two histopathological forms: a mature form, diagnosed most frequently in peri-menopausal women (95%), and a juvenile form, diagnosed in young women and girls who have aged prematurely (5% of cases) [
9]. Despite the fact that granulosa cell tumours can be successfully treated by surgery, relapses are often observed and further adjuvant treatment is still not possible.
To our knowledge, there are no data showing a direct correlation between obesity and folliculomas. Moreover, folliculoma is not well studied among the ovarian cancer types, although both the short (ObRa) and long (ObRb) forms of the leptin receptor are present in human granulosa cells [
10,
11]. Löffler et al. [
10] showed that, in polycystic ovaries, leptin-positive cells were noted both in the hypertrophied theca layer and in the luteinised granulosa layer. Taking into consideration that leptin at a supraphysiological concentration, as noted in obese women, has stimulatory effects on testosterone secretion characteristic of polycystic ovarian syndrome correlated with obesity [
12], we hypothesised that, as in the case of epithelial ovarian cancer, ObR overexpression in granulosa cell tumours could be correlated with the incidence of granulosa cell cancer, and leptin receptor blockers might be used as an adjuvant therapy.
Currently, several groups of scientists are working on the synthesis of molecules that block ObR. A number of leptin receptor antagonists have been synthesised for therapeutic use, with several completing pre-clinical testing [
13], indicating their possible use in anticancer therapy. In previous studies using different epithelial cancer cell lines, we showed that SHLA and quadruple leptin mutein, Lan2 (L39A/D40A/F41A/I42A), had no effect on non-cancerous HOSEpiC cell proliferation [
14]. However, both antagonists reversed the stimulatory effect of leptin on metastatic carcinoma CaOV-3 cell proliferation to control levels and even below control levels in chemoresistant OVCAR-3 cells. Leptin receptor antagonists have been investigated in breast and prostate cancers, which mainly are hormone dependent. It has been shown that Aca1, Allo-aca and D-ser can inhibit leptin-stimulated proliferation in MCF-7 breast cancer cells [
15]. Another leptin receptor antagonist, LDFI (Leu-Asp-Phe-Ile), is also able to inhibit the proliferation of MCF-7 cells in vitro and in vivo [
16]. The antagonist Lan1 is able to inhibit the phosphorylation of leptin-signalling proteins Jak2, ERK1/2 and Akt, in PC3 and DU145 prostate cancer cell lines [
17]. In a previously published study using epithelial ovarian cancer cell lines, we investigated the effect of SHLA and Lan2 on the JAK/Stat3, MAPK/ERK and PI3K/Akt pathways and showed an inhibitory effect of SHLA on all tested signalling proteins in OVCAR-3 cells and of Lan2 on Stat3 and ERK1/2 proteins in CaOV-3 cells [
14]. These data point to a similar signalling pathway in the antagonistic effects of leptin receptor blockers.
In the present study, we evaluated the effect of leptin and three of its receptor antagonists: Lan1 (L39A/D40A/F41A mutant), Lan2 (L39A/D40A/F41A/I42A mutant) and SHLA (D23L/L39A/D40A/F41A mutant) on leptin and oestradiol receptor gene and protein expression, cell proliferation including cell cycle protein expression, caspase-3 activity and oestradiol secretion in two granulosa tumour cell lines. The two cell lines were COV434, representing the juvenile form of granulosa tumour, and the steroidogenic human ovarian granulosa-like tumour (KGN), representing the adult type of this cancer (corresponding to peri- to post-menopausal age). The human immortalised non-luteinised granulosa cell line HGrC1 was used as the control.
Based on the fact that leptin exerts its activity not only through the leptin receptor (ObR), but also through cross talk with other signalling systems implicated in tumour genesis [
18,
19], in this study we focused our attention on the relationship between the leptin/ObR axis and oestrogen receptors (ERα/β).
Taking into consideration that oestradiol can modulate ObR expression in some oestrogen-responsive tissues, we hypothesised that blocking ObR expression could be a novel treatment for granulosa ovarian cancer.
Discussion
This study clearly demonstrates twofold higher leptin receptor gene and protein expression in cancer granulosa cell lines compared to non-cancer cell lines. Additionally, we found that leptin increased its own receptor gene expression only in cancer cell lines.
These results on both the short (ObRa) and long (ObRb) forms of the leptin receptor are in agreement with other studies demonstrating both forms of the leptin receptor in granulosa cells [
11,
24]. As described by us, higher ObR expression and a leptin-stimulatory effect on its own receptor in cancer granulosa cells correspond with our previously published data [
14] in epithelial ovarian cancer cells, suggesting similar effects in both epithelial ovarian cancer and folliculoma. Increased expression of ObR, corresponding to higher risk, has also been described in cases of breast [
1] and prostate cancer [
25], both hormone-dependent cancer types. In ovarian cancer patients, high leptin levels are associated with poor treatment prognosis [
26].
Cross talk between leptin and oestrogen receptors should also be taken into consideration. It was apparent that, in cancer cells, elevated ObR expression correlated with elevated ERα expression. The direct relationship between ObR and ER is still unresolved; however, the existence of cross talk between these receptors was examined. Fusco et al. [
19] described a threefold increase in ERα expression in the presence of 100 ng/mL leptin. Different effects, dependent on the type of ER expression, have been described by Ray et al. [
27] and Ozbay et al. [
28], who showed that leptin increased the proliferation level to a greater extent in the ER+ breast cancer cell line T47-D than in ER− MDA-MB231 cells. This finding is in line with our results, as all investigated granulosa cell lines demonstrated a higher degree of proliferation than in previously examined epithelial ovarian cell lines [
14]. Physiologically, the primary sites of oestrogen receptors include the same areas where ObR are located [
29]. Leptin can enhance aromatase activity, promoting oestrogen production from androstenedione in adipose tissue and hence stimulate the progression of oestrogen-dependent breast cancer [
30]. Furthermore, leptin enhances the activation of oestrogen receptor alpha (ERα) through the MAPK pathway in MCF-7 and HeLa cells [
31]. In ER+ MCF-7 cells, chronic exposure to leptin has been found to result in a higher ERα/ERβ ratio, enhanced oestrogen transcriptional activity, greater cell growth and resistance to the anti-oestrogen compound tamoxifen [
32].
It is well known that leptin can directly affect ovarian function by its action on oestradiol secretion [
33]. There have been a few studies on blocking leptin activity in the ovary and interactions with oestradiol secretion. Our results show that the leptin receptor antagonists SHLA and Lan1, at concentrations of 100 ng/mL, increased oestradiol (E2) secretion by the HGrC1 cell line, but had no effect on oestradiol secretion in granulosa cancer cells (except for Lan2, which decreased E2 secretion in KGN cells). The effect on ERα and ERβ expression was variable in this study. In HGrC1 cells, all antagonists decreased ERβ gene expression, but only Lan1 and Lan2 decreased protein levels. The blockers SHLA and Lan1 decreased ERα in juvenile COV434 cells, while Lan1 and Lan2 had an effect in adult form KGN cells. These results suggest that, independently of the magnitude of ERα expression, Lan1 was sufficient in both types of cells. Our data are in agreement with Fusco et al. [
19], who showed that leptin receptor silencing in MCF-7 breast cancer cells results in decreased ERα expression. The same authors reported that higher leptin levels are more strongly correlated with ER+ breast cancer rather than ER−. In another study, Dupuis et al. [
11] used the leptin receptor antagonist PEG-SMLA to demonstrate that inhibition of ObR impaired follicle rupture without affecting meiotic maturation of oocytes in ovarian follicles.
Leptin, via direct action through ObR and additionally by enhancing the activation of oestrogen receptor alpha (ERα) through the MAPK pathway, induces cell proliferation. The data presented here show that leptin at a supraphysiological level induced cell proliferation in all investigated cell types, with the greatest effect observed in the normal granulosa cell line HGrC1 where proliferation reached 180% of the control. The mutagenic effect of leptin has been observed in various cell types, including breast cancer [
33,
34], endometrial cancer [
35] and prostate cancer cells [
25]. Kato et al. [
36] showed that higher leptin levels (above 100 ng/mL) or longer incubation times are required to have an effect on proliferation. However, Fiedor and Gregoraszczuk [
14] and Ptak et al. [
37], using the same leptin concentration, described a similar effect on the proliferation of epithelial ovarian cancer cells.
All ObR antagonists used in this study reversed the leptin-stimulatory effects on cell proliferation, although with varying degrees of success. In non-cancerous HGrC1 cells, the most potent antagonist was SHLA. In KGN cells, all three antagonists at all concentrations reversed leptin-stimulated proliferation. Surprisingly, in the COV434 cell line, the effects of blockers were negligible. The use of leptin receptor antagonists has been well studied in breast cancer. There are reports of the inhibitory action of Aca-1 and Allo-aca on leptin-stimulated proliferation of MCF-7 and MDA-MB231 breast cancer cells [
15], and D-Ser and DDD on the proliferation of MCF-7 cells [
38]. Catalano et al. [
16], using LDFI (leptin binding site I), showed the inhibitory effects on the leptin-induced growth of ERα-positive (MCF-7) and ERα-negative (SKBR3) breast cancer cells. Fusco et al. [
19] demonstrated the inhibitory effects of a neutralising monoclonal antibody (9F8) on cell proliferation in the ER-positive MCF-7 cell line, but not in MDA-MB231 ER-negative cells. We previously described SHLA and Lan2 as a promising treatment for epithelial ovarian cell tumours [
14]. To our knowledge, these are the first findings to indicate the possible use of ObR blockers in the treatment of folliculoma cancer.
Our results reveal that leptin at 40 ng/mL does not affect caspase-3 activity in non-luteinising cells HGrC1, but could decrease it in COV434 cells and to a small extent in KGN cells. Using granulosa cells, Sirotkin et al. [
39] showed a stimulatory effect of leptin at a dose of 100 ng/mL on Bax protein in human granulosa cells, suggesting that leptin can modulate apoptosis in human ovaries. The discrepancy in the findings may be due to the different concentrations of leptin. All the antagonists investigated here had no effect on caspase-3 activity in HGrC1 or KGN cells. Only Lan1 and Lan2, at the highest concentrations, restored caspase-3 activity close to the control level in COV434 cells. Previous experiments conducted in our laboratory have shown that, in epithelial ovarian tumours, leptin at 40 ng/mL alone or in combination of SHLA or Lan2 does not affect the activity of caspase-3, -8 or -9 (unpublished data).
With regard to the mechanism of action of the ObR antagonists, we showed that, in non-cancer HGrC1 cells, Lan1 and Lan2 decreased the expression of cyclin A2 and cdk4. In COV434 cells, all tested antagonists decreased cyclin D1 and cdk4 protein expression, while Lan1 also decreased cyclin A2 and cdk2, SHLA had an inhibitory effect on cyclin A, and Lan2 inhibited cdk2 expression. It is generally believed that the critical function of the cyclin A–Cdk2 complex is the phosphorylation of substrates that start DNA replication and co-ordinate the end of S-phase [
40]. In KGN cells, only cyclin D1 and cdk2 were decreased by all leptin receptor blockers. Additionally, we observed an inhibitory effect of Lan1 and Lan2 on the expression of the transcription factor E2F1, suggesting that the anti-proliferative effect of this antagonist in ovarian cancer may be mediated, in part, by the down-regulation of E2F1. Our previously published results [
14] concerning the action of leptin receptor antagonists on epithelial ovarian cancer cells showed that both antagonists studied decreased cdk2 and cdk4 protein expression in CaOV-3 and OVCAR-3 cells. Additionally, in CaOV-3 cells, cyclin D1 expression decreased under the influence of SHLA and Lan2.
In summary, (1) in juvenile form of folliculoma, SHLA and Lan-1 increased ERβ expression, while in the adult form all blockers decreased ERα expression. As a consequence, the ratio moved towards greater expression of ERβ, characteristic of non-cancer granulosa cells. (2) In both types of folliculoma, Lan1 and Lan2 acted as inhibitors of cyclinD/cdk4, cdk2 and E2F. The ability of these cyclins to activate the cyclin-dependent kinase CDK4 is the most extensively documented mechanism for their oncogenic actions and provides an attractive therapeutic target.
In conclusion, taking into consideration that these results are based on experiments performed on cell lines, and did not consider the tumour microenvironment, further studies should be performed using explants of granulosa cancer from patients with folliculoma or the coculture of granulosa cancer cell line with fibroblasts, epithelial cells and other components of the tumour environment.