An "Iron-Clad" Link Between Obesity and Accelerated Depletion of the Ovarian Reserve.

Nonsyndromic obesity is predominantly driven by excess caloric intake, particularly so by excess calories from highly processed foods. The pathophysiological condition includes altered overall body composition with elevated adipose tissue and a metabolic profile(s) that often includes insulin resistance. Obesity has been shown to negatively impact ovarian physiology; however, women with obesity do not appear to have an earlier menopause, and the relationship of obesity with ovarian aging and the metrics of ovarian reserve is still poorly understood. An exciting article published recently in Fertility and Sterility by Zhou et al. (1Zhou J. Lin L. Liu L. Wang J. Xia G. Wang C. The transcriptome reveals the molecular regulatory network of primordial follicle depletion in obese mice.Fertil Steril. 2023; 120: 899-910Abstract Full Text Full Text PDF Scopus (3) Google Scholar) of the China Agricultural University provides significant new insights into obesity’s impact on the reserve of primordial ovarian follicles in a murine high-fat diet (HFD) model. Titled “The transcriptome reveals the molecular regulatory network of primordial follicle depletion in obese mice,” this work effectively used several complementary approaches. First, the HFD-induced obese state resulted in increased ovarian lipid content as well as accelerated primordial follicle growth activation (PFGA; referred to in the manuscript as the primordial to primary follicle transition) and increased follicle atresia as revealed by histomorphometric evaluation of serial ovary sections. A detailed bromodeoxyuridine uptake study revealed that granulosa cell proliferation was greatly enhanced in the primary follicles of HFD-treated animals compared with controls as well. The enhancement of both follicle growth and death “simultaneously” can seem paradoxical, but an increased rate of primordial follicle recruitment, followed by increased follicle atresia after they begin to grow, is easy to understand. As we see in this study, this type of response to insult has been identified previously across populations of ovarian follicles. Next, precise laser-capture microdissection was used to isolate primordial and primary follicles for the comparison of gene expression between the follicle developmental stages and between obese and normal diet controls. Reproducible significant changes in the immature follicle transcriptome(s) were detected where gene expression changes and pathway analysis provided interpretable clues about accelerated follicle loss due to PFGA and atresia in the HFD and obese groups. The laser-capture microdissection transcriptomic data were used to “[reveal] the specific gene signatures during the primary follicle transition…” and also transcription factor network changes that were interpreted to contribute to follicle loss in response to HFD. The investigators provided the principal component analysis comparison of the clustering of follicle gene expression, and it was notable that HFD induced a greater change in the overall transcriptome in primary follicles compared with controls than seen in primordial follicles (HFD vs. controls). Of several gene sets identified that showed significant enrichment in obese animals (including, among others, oxidative stress, VEGF, and mTOR pathways), the ferroptosis cell death pathway (2Dixon S.J. Lemberg K.M. Lamprecht M.R. Skouta R. Zaitsev E.M. Gleason C.E. et al.Ferroptosis: an iron-dependent form of nonapoptotic cell death.Cell. 2012; 149: 1060-1072Abstract Full Text Full Text PDF PubMed Scopus (7474) Google Scholar, 3Li J. Cao F. Yin H.L. Huang Z.J. Lin Z.T. Mao N. et al.Ferroptosis: past, present and future.Cell Death Dis. 2020; 11: 88Crossref PubMed Scopus (1380) Google Scholar) was identified and validated as a potentially central mechanism of primordial follicle loss in the obese mouse. Ferroptosis is a relatively recently identified (2Dixon S.J. Lemberg K.M. Lamprecht M.R. Skouta R. Zaitsev E.M. Gleason C.E. et al.Ferroptosis: an iron-dependent form of nonapoptotic cell death.Cell. 2012; 149: 1060-1072Abstract Full Text Full Text PDF PubMed Scopus (7474) Google Scholar) cell death mechanism that involves ferrous iron–induced oxidation of cellular lipids; see the article by Li et al. (3Li J. Cao F. Yin H.L. Huang Z.J. Lin Z.T. Mao N. et al.Ferroptosis: past, present and future.Cell Death Dis. 2020; 11: 88Crossref PubMed Scopus (1380) Google Scholar) for a review. Upstream events that activate ferroptosis have tended to be exposure to specific pharmaceutical agents, and its activation by HFD or the condition of obesity in ovarian follicles was perhaps unexpected and thus identified in an unbiased manner. Ferroptosis-related on-apoptotic cell death occurs after decreased cellular reactive oxygen species scavenger activity is established in cells, including decreased glutathione and glutathione peroxidase action. Reactive oxygen species levels can then increase rapidly, favoring cellular damage and an irreversible commitment to death. Ferroptosis is also characterized by mitochondrial depletion and depletion of the cristae of those that remain. In contrast to other well-known mechanisms of cell death, chromatin does not become concentrated or fragmented, and nuclei tend to maintain their normal size. Consistent with this, all characteristic ferroptosis stages occur in the absence of caspase activation (3Li J. Cao F. Yin H.L. Huang Z.J. Lin Z.T. Mao N. et al.Ferroptosis: past, present and future.Cell Death Dis. 2020; 11: 88Crossref PubMed Scopus (1380) Google Scholar). Having identified elevated ferroptosis pathway transcripts in the immature follicles of HFD-fed mice, the investigators performed validating experiments and showed that central pathway proteins (ferritin, Acyl CoA synthetase long-chain family member 4, solute carrier family 7 member 11, and glutathione peroxidase 1) are expressed at the protein level in immature follicles, and in all but one case (glutathione peroxidase 1), those proteins are significantly elevated in the follicles in the obese mice in the HFD group. These data are compelling supportive evidence that the accelerated follicle loss in that HFD group occurs in a way that involves the up-regulation and activation of ferroptosis. Identification of ferroptosis as a new player in accelerated follicle loss opens new avenues of investigation and fits well within the available literature in the area. Increased cellular stress favoring increased PFGA and also follicle atresia is reminiscent of a recognized response to chemotherapeutic insult. That phenomenon is referred to as follicular “burnout” (4Roness H. Gavish Z. Cohen Y. Meirow D. Ovarian follicle burnout: a universal phenomenon?.Cell Cycle. 2013; 12: 3245-3246Crossref PubMed Scopus (96) Google Scholar), but for now it remains unclear how and whether the pathophysiological insult that occurs in the context of HFD and results in ferroptosis up-regulation relates to the response to chemotherapy. Considered more generally, cellular stress and damage in the presence of the HFD could plausibly be activating the integrated stress response pathway. Acute stress- and-damage-induced activation of the integrated stress response has also been shown to correspond to increased granulosa cell proliferation (5Llerena Cari E. Hagen-Lillevik S. Giornazi A. Post M. Komar A.A. Appiah L. et al.Integrated stress response control of granulosa cell translation and proliferation during normal ovarian follicle development.Mol Hum Reprod. 2021; 27gaab050PubMed Google Scholar), and its hypothesized role in regulating the rate of PFGA may also be related to accelerated primordial follicle loss during HFD exposure and ongoing obesity. Future studies are now needed to determine how HFD and obesity activate the ferroptosis transcriptional and posttranslational signaling program(s) and, crucially, how the response of enhanced growth and enhanced death a la “burnout” is achieved. In summary, this study has identified and preliminarily validated ferroptosis as a potentially central mechanism of accelerated ovarian reserve depletion in response to HFD. The work is highly relevant to the human condition of obesity and provides preclinical clues about how the same process can be evaluated in patients with high body mass index. Future rescue experiments targeting ferroptosis could reveal that blocking ferroptosis induction using pharmacologic or other means could block accelerated ovarian reserve depletion in models of obesity and perhaps one day in obese patients. This very well-done study provides a solid platform for these types of future studies so that the pathway’s relevance and “targetability” in women and other species can be determined. The transcriptome reveals the molecular regulatory network of primordial follicle depletion in obese miceFertility and SterilityVol. 120Issue 4PreviewTo explore the dynamic transcriptional regulatory network of primordial follicle fate in obese mice to elucidate the potential mechanism of primordial follicle depletion. Full-Text PDF Open Access