(277) Reprogrammed Steroidogenic Stroma Helps Maintain Steroid Production in Follicle-Depleted Ovaries

Alana Mermin-Bunnell^1,2,3; Josephine Cleverdon^2,3; Philippe Godin^2,3; Monica M. Laronda⁴; Marie-Charlotte Meinsohn^2,3; David Pepin^2,3

1. Harvard-MIT Program in Health Sciences and Technology, Institute for Medial Engineering and Science, Massachusetts Institute of Technology, Cambridge, MA, USA

2. Pediatric Surgical Research Laboratories, Massachusetts General Hospital, Boston, MA, USA

3. Department of Surgery, Harvard Medical School, Boston, MA, USA

4. Stanley Manne Children's Research Institute, Ann and Robert H. Lurie Children's Hospital of Chicago, Chicago, IL, USA; Department of Pediatrics and Department of Obstetrics and Gynecology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA

Abstract Text:

In premenopausal females, activated ovarian follicles are primarily responsible for estrogen and testosterone production. These sex steroids are synthesized through a coordinated process between theca and granulosa cells, where a key player is the CYP17A1 enzyme that converts progestogens into androgens in theca cells. CYP17A1-expressing theca cells have been shown to originate from a progenitor in the ovarian stroma and are recruited to a nearby follicle by oocyte-secreted signals. Follicles possess two layers of theca cells: an inner layer of steroidogenic theca interna cells and an outer layer of structural theca externa cells. When the ovarian reserve is depleted by menopause or conditions like premature ovarian insufficiency (POI), often caused by harmful chemotherapeutic agents, steroid hormone production in human females drops dramatically. This loss of endogenous hormone production leads to health consequences including osteoporosis, vasomotor symptoms, and increased cardiovascular risk. Interestingly, in aged mice with depleted ovarian reserve, we did not observe a similar drop in hormone production. RNA in situ hybridization identified a Cyp17a1+ stromal population in their follicle-depleted ovaries. We therefore hypothesize that in environments lacking activated follicles, stromal progenitors can differentiate into Cyp17a1+ steroidogenic stroma, compensating for the hormonal function of follicles. To explore this compensatory mechanism, we utilized a mouse model of POI by injecting eight-week-old mice with 4-vinylcyclohexene diepoxide (VCD, 160mg/kg daily) for 15 days. VCD eliminates pre-antral follicles by oxidative stress and pro-apoptotic signaling, thus accelerating ovarian aging to postmenopause. Despite complete follicular depletion, VCD-treated mice also maintained a normal steroidal hormone profile. We hypothesized that high LH levels in the environment of an exhausted ovarian reserve trigger stromal differentiation and promote steroidogenesis, enabling this new population to replace theca cells in the ovarian–pituitary–hypothalamic axis. When treated with hCG (an LH analog, 1IU/mL), AMH (10μg/mL), and FSH (100ng/mL), stromal cells isolated from the ovaries of VCD-treated mice significantly upregulated expression of Cyp17a1 by qPCR (P=0.0030), along with other steroidogenic genes. Additionally, using RNA in situ hybridization, we detected the presence of these CYP17A1+ endocrine stromal cells in human postmenopausal ovaries. Moreover, ovarian sections from polycystic ovarian syndrome (PCOS) patients showed significantly increased stromal CYP17A1 mRNA staining than in non-PCOS patients (t=5.26, P=0.0134), suggesting a potential new mechanism underlying the hyperandrogenism characteristic of PCOS. Notably, PCOS patients commonly exhibit a high LH to FSH ratio along with elevated testosterone, which aligns with the concept that CYP17A1+ stromal cells are driven in part by high LH levels. To conclude, this steroidogenic stromal population could contribute to future innovative treatments to maintain endogenous hormone production in follicle-depleted and perimenopausal patients. This study also provides insights into PCOS-associated hormonal dysregulation and enhances our understanding of the progenitor potential of ovarian stromal cells.