Background
Ovarian cancer (OvCa) is the third most common gynecologic cancer globally with 313,959 new diagnoses and 207,252 deaths reported in 2020 [
1]. The age-standardized incidence rate is 6.6 per 100,000, representing a decreasing trend in global incidence, but with a substantial increase in reported incidence in younger women aged 15–40 [
1]. The majority of women with OvCa are diagnosed with metastatic disease, resulting in a poor 5-year survival of 31% due to painful complications resulting from widely disseminated intra-peritoneal (i.p.) metastases. Metastasis occurs from direct extension of the primary ovarian or fallopian tube tumors and exfoliation of single cells and multicellular aggregates (spheroids or tumorspheres) into the peritoneal cavity, wherein dissemination is facilitated by the accumulation of ascites fluid [
2,
3]. Metastatic cells and multicellular aggregates survive in ascites, adhere to the mesothelial cells of the peritoneal membrane that covers abdominal organs, induce mesothelial cell retraction, anchor in the collagen-rich sub-mesothelial extracellular matrix, and proliferate to form widely disseminated secondary lesions [
2‐
5].
Obesity is a recognized noninfectious pandemic [
6] that increases OvCa incidence, enhances metastatic success and reduces survival [
7‐
14]. Meta-analyses, including a new study evaluating 2.7 million women [
15], show a relationship between obesity and risk of OvCa incidence in women with tumors of serous, endometrioid, and mucinous histology. Importantly, obesity has an adverse effect on survival of women with OvCa, implicating a link between host obesity, metastatic success, and response to therapy [
6‐
16]. This survival effect is highlighted by a recent study that evaluated genomic and clinical data from a cohort of > 600 women diagnosed with high grade serous OvCa (HGSOC) having similar mutational profiles. The authors identified a “poor outcome” cohort associated with upregulated obesity- and lipid metabolism-related genes who have significantly reduced progression-free and overall survival relative to women with the same mutational profile but lacking this gene cluster [
17].
These epidemiologic data are consistent with pre-clinical data showing preferential homing of metastasizing ovarian tumors to the omental fat pad [
2,
3]. Data examining ex vivo human tumor-adipocyte interactions showed that adipocytes promote homing, migration, and invasion of OvCa cells and transfer fatty acids to promote tumor cell growth [
18]. These in vitro data are consistent with the increased tumor aggressiveness observed in a genetically engineered murine OvCa model when comparing mice on a low fat diet (LFD) to those on a high fat diet (HFD), showing increased tumor size and associated metabolic changes [
19]. We have previously demonstrated a positive link between obesity and OvCa metastatic success in multiple murine pre-clinical models of diet induced obesity and mutational obesity [
20]. Obesity was found to significantly enhance i.p. metastatic tumor burden and lipid transport into tumor cells and was associated with changes in the immune microenvironment reflected by a decreased M1/M2 macrophage ratio [
20]. The objective of the current study was to evaluate response to standard-of-care chemotherapy in pre-clinical models of diet induced obesity and to assess obesity-associated changes in the tumor immune landscape in human and murine tumors. Our results show a significantly diminished response to chemotherapy in pre-clinical studies using mice on a HFD. A corresponding decrease in M1/M2 macrophage ratio and enhanced fibrosis in both murine and human tumors suggests potential mechanisms by which host obesity contributes to poor outcomes in women with ovarian cancer.
Discussion
The World Health Organization has identified obesity as a non-infectious and non-communicable pandemic that significantly impacts disease-related morbidity and death [
6]. The most recent National Health and Nutrition Examination Survey data show the prevalence of obesity (BMI = 30 kg/m
2 or above) in U.S. women over age 20 as 41.8%, with 11.7% exhibiting severe obesity (BMI = 40 kg/m
2 and above) [
29]. Evaluation of data from 10 countries with an obesity prevalence over 25% show that mortality rates for women with ovarian cancer have not decreased, but rather have remained stable or are increased [
6]. Another global study showed increasing incidence in younger women (aged 15–40) in some countries that was correlated with high rates of obesity and metabolic syndrome [
1]. Obesity and overweight can result in post-operative complications which can delay adjuvant therapies, thereby affecting overall prognosis [
30]. In addition, obesity may also impact metastatic success and response to therapy. This is supported by a study characterizing a “poor outcome” cohort of HGSOC patients whose tumors express an upregulated cluster of obesity- and lipid metabolism-related genes. These women have significantly reduced progression-free and overall survival relative to women with the same mutational profile but lacking this gene cluster [
17]. Moreover, in a study of women undergoing secondary cytoreductive surgery for recurrent ovarian cancer, BMI was an independent predictor of poor survival, further suggesting an effect of host weight on tumor biology and/or treatment response [
31]. These epidemiologic data are supported by the findings of the current pre-clinical study, showing a poor response to standard-of-care chemotherapy in mice on a high fat diet. Our data show substantial remaining tumor burden in HFD mice following 6–9 cycles of weight-adjusted standard of care paclitaxel and carboplatin chemotherapy. These tumors also exhibited a significant decrease in the M1/M2 macrophage ratio and enhanced tumor fibrosis. A similar decrease in M1/M2 macrophage staining and enhanced fibrosis was also observed in tumors from HGSOC patients with high BMI. While a direct link between tumor-associated macrophages and fibrosis was not evaluated in this study, it is interesting to note that recent studies have demonstrated that macrophages, in addition to cancer-associated fibroblasts, contribute to collagen deposition and tumor fibrosis in ovarian and colorectal cancers [
32,
33]. Specifically, TAMs have been shown to directly participate in collagen deposition, cross-linking, and linearization [
33].
Obesity has been shown to alter extracellular matrix deposition, and this desmoplasia can hamper the response to chemotherapy [
34]. In the current study, enhanced tumor-associated fibrosis was observed both in human tumor tissues from high BMI patients and in tumors from mice fed a HFD. Therapeutic targeting of tumor-associated fibrosis was recently shown to enhance the response to chemotherapy in a murine model of ovarian cancer [
35]. In this study, mice treated with the anti-hypertensive agent Losartan in combination with pegylated liposomal doxorubicin demonstrated reduced intratumoral collagen and hyaluronan content relative to single agent controls. Moreover, a retrospective analysis of ovarian cancer patients taking similar agents in addition to standard of care chemotherapy showed a significant increase in overall survival (63 mo relative to 33 mo in controls) [
35]. Another pre-clinical report showed that antibody targeting of the protein MFAP5 reduced fibrosis in ovarian tumors in mice and enhanced response to the chemotherapeutic agent paclitaxel. These investigators also identified a “fibrotic gene signature” in human tumors predictive of significantly reduced survival relative to patients whose tumors had low expression of this gene signature (19
vs 33 mo, respectively) [
36]. A third study investigated age-associated ovarian fibrosis both in murine models and in human ovaries, demonstrating enhanced deposition of anisotropic collagen in aged ovaries [
37]. In the human cohort the use of metformin, which has been shown to reduce or prevent fibrosis in pre-clinical models, was strongly associated with reduced fibrosis. Interestingly, fibrotic ovaries showed increased M2-macrophages relative to non-fibrotic ovaries or ovaries of metformin users, indicating a correlation between ovarian fibrosis and M2-polarization of macrophages. While the above studies did not evaluate the impact of obesity in pre-clinical murine models or retrospective analyses of patient data, these collective results suggest that an anti-fibrosis therapeutic strategy may enhance chemotherapeutic efficacy in patients with high BMI or in pre-clinical models of diet-induced obesity.
To evaluate potential factors that may influence macrophage polarization, adipokine expression was evaluated. FGF21 was the only adipokine, expression of which was elevated in the ascites of mice on a HFD. A predominant function of FGF21 is to stimulate insulin-independent glucose uptake. Little is known about the role of FGF21 in ovarian cancer; however a recent report identified FGF21 as a highly significantly upregulated gene when comparing cisplatin-resistant human ovarian cancer cells (A2780CP) to the parental cell line (A2780) [
38]. Modulation of FGF21 levels by overexpression or siRNA silencing confirmed that FGF21 overexpression induces chemoresistance in ovarian cancer cells [
38]. These data suggest that FGF21-mediated chemoresistance may contribute mechanistically to the observed tumor burden remaining in HFD mice after chemotherapy treatment. Additional murine studies showed that FGF21 expression alters macrophage polarization in liver and adipose tissue, resulting in a decreased M1/M2 ratio [
39,
40]. Together these data suggest that additional studies on expression of FGF21 in ovarian cancer and its role in therapeutic response are warranted.
Adipokine array data also showed a significant decrease in adiponectin in ascites from HFD mice relative to LFD mice. A role for adiponectin in macrophage polarization has been reported, although effects appear to differ based on the specific cancer type and animal model under investigation [
41]. However previous studies in mice and humans have shown that decreased serum adiponectin levels are associated with obesity, leading to hyperinsulinemia and insulin resistance [
41,
42]. As a consequence, levels of IGFPB-1 and -2 are decreased, thereby increasing IGF1 bioavailability. Together insulin and IGF1 upregulate VEGF and vascularity [
41]. These observations are of interest in light of the current study showing decreased levels of multiple IGFBPs (IGFBP-2, -3, -5, and -6) in HFD mice. This is consistent with an observation from our previous study showing enhanced tumor vascularity in mice fed on a western diet (40% fat) relative to control diet mice [
20]. A more detailed mechanistic analysis of the adiponectin/IGFBP/VEGF axis in regulation of vessel density in tumors of HFD mice is underway.
Immunotherapy incorporating immune checkpoint inhibitors (ICIs) has proven highly effective in treating many cancers [
43]. However, despite relatively high levels of tumor infiltrating lymphocytes, a number of clinical trials have shown a therapeutic efficacy of only 10–15% in trials using ICIs in ovarian cancer patients, suggesting that other components of the tumor microenvironment contribute to this treatment failure [
43,
44]. While obesity is typically associated with dysregulated immune responses, termed “inflammaging” [
45], an interesting recent study [
46] reported that obesity enhanced efficacy of ICI blockade in both pre-clinical murine models of obesity and melanoma and in high BMI human melanoma patients. In this study, obesity was found to induce T cell aging, characterized by higher PD-1 expression which rendered tumors more responsive to checkpoint blockade therapies [
43]. Data in the current study show no changes in PD-1 expression levels on either helper T cells or cytotoxic T cells when comparing ovarian cancer patients of high BMI to those of low BMI (
Supplemental data), suggesting that targeting other aspects of the tumor immune microenvironment may be preferential.
Tumor-associated macrophages (TAMs) are the most abundant immune cell type in the ovarian tumor microenvironment and can be broadly characterized as classically activated pro-inflammatory (M1) or alternatively activated anti-inflammatory (M2) based on surface marker expression and cytokine and chemokine profiles [
44,
47]. It has previously been reported that, while TAM density increases with increasing stage and grade, an overall decrease in M1/M2 ratio was observed with increasing cancer stage [
48]. This finding was confirmed by additional studies including a meta-analysis of 794 patients, showing that a high M1/M2 ratio was associated with more favorable overall survival and was predictive of better progression-free survival [
49]. Moreover, a high M1/M2 ratio also associated with an improvement in platinum-free interval [
50]. Given these data and the current findings, of interest are potential therapeutic strategies that target TAM survival or induce repolarization of M2 to M1 TAMs. One such candidate is trabectedin, the lead compound of ecteinascidins, that was approved by the U.S. Food and Drug Administration in 2015 (Yondelis) for treatment of unresectable or metastatic liposarcoma or leiomyosarcoma and by the European Union for treatment of relapsed platinum-sensitive ovarian cancer in 2009 [
49]. This drug has a unique dual mechanism of action wherein it induces cytotoxicity by forming adducts with minor groove DNA, inducing single-strand and double-strand breaks, cell cycle arrest and apoptosis [
51‐
53]. Additionally, trabectedin modulates the tumor immune microenvironment by selectively inducing apoptosis of monocytes/macrophages via caspase-8 activation, with an associated reduction in inflammatory cytokines/chemokines and angiogenesis [
52]. Interestingly, trabectedin retained anti-tumor efficacy in a pre-clinical model in which trabectedin-resistant IGROV cells were developed in vitro and xenografted in vivo [
52]. Even though these tumor cells were unresponsive to trabectedin in vitro
, neoplastic growth in vivo was significantly reduced, providing further evidence that targeting TAMS in the tumor microenvironment represents an important component of the anti-tumor activity of this drug. Of interest would be a retrospective evaluation of data from completed clinical trials of ovarian cancer patients treated with trabectedin to assess whether BMI is statistically associated with therapeutic efficacy, as well as the targeted recruitment of high BMI patients to future trials with this agent.
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