Metabolic Enzyme ME1 Emerges as Key to Unlocking Progestin Therapy in Endometrial Cancer

A metabolic enzyme long known simply as a cellular housekeeper for fat synthesis may hold the key to rescuing one of oncology’s most frustrating treatment failures. Researchers at Shinshu University School of Medicine in Japan report that malic enzyme 1, or ME1, is overproduced in endometrioid endometrial carcinoma and that patients whose tumors carry high levels of the protein face significantly worse outcomes. More strikingly, the team showed that blocking ME1 can restore the sensitivity of cancer cells to medroxyprogesterone acetate, the progestin drug at the heart of fertility-sparing therapy for young women with early-stage disease. The study, published in the Journal of Cancer Research and Clinical Oncology, weaves together tissue analysis, cell biology, and animal modeling into a case that ME1 deserves attention as both a prognostic marker and a druggable vulnerability.

Endometrial cancer is the most common gynecologic malignancy in developed countries, and its incidence is rising, driven in part by obesity and changing reproductive patterns. For younger patients who hope to preserve their fertility, the standard alternative to hysterectomy is progestin therapy, typically high-dose medroxyprogesterone acetate, which can induce tumor regression and allow pregnancy. The catch is resistance: a substantial fraction of tumors either fail to respond from the outset or relapse after an initial remission, forcing patients toward definitive surgery. Clinicians have long sought molecular clues that predict who will respond and interventions that convert non-responders into responders. The Shinshu team, led by Natsuki Uchiyama and corresponding author Ryoichi Asaka, approached both questions through the lens of tumor metabolism.

ME1 occupies a strategically important position in cellular biochemistry. The enzyme decarboxylates malate to pyruvate while reducing NADP+ to NADPH, a molecule that functions as the cell’s principal reducing currency. NADPH feeds fatty acid and cholesterol synthesis and, critically, powers antioxidant systems such as glutathione reductase that neutralize reactive oxygen species, the chemically aggressive byproducts of aerobic metabolism. Cancer cells, which run biosynthetic pathways at full throttle while enduring metabolic stress, often become dependent on NADPH-generating enzymes. That dependence creates a therapeutic window: inhibit the enzyme, and the tumor cell is caught between a shortage of building blocks and an inability to defuse its own oxidative waste. Until this study, however, ME1’s role in endometrial carcinoma was essentially unmapped.

To chart that map, the researchers performed immunohistochemistry on surgical specimens from 172 patients with endometrioid endometrial carcinoma, alongside six samples of normal endometrium. The staining revealed a clear pattern: ME1 protein was significantly more abundant in tumor tissue than in healthy endometrium. When the team stratified patients by expression intensity, using an H-score threshold of 75.66, they found that tumors with high ME1 were independently associated with shorter progression-free survival, with a hazard ratio of 1.89 and a 95 percent confidence interval of 1.07 to 3.34, reaching statistical significance at P equals 0.028. Because the analysis controlled for other clinical variables, the result suggests ME1 is not merely a passenger marker of aggressive disease but a feature that tracks with, and may contribute to, clinical behavior.

The next step was mechanistic. Working with endometrial cancer cell lines in vitro, the researchers suppressed ME1 two ways: genetically, using small interfering RNA to degrade the enzyme’s messenger RNA, and pharmacologically, using a small-molecule inhibitor. Both approaches curtailed proliferation across every cell line tested, confirming that these cells rely on ME1 for growth. But the most consequential findings emerged in Ishikawa cells, a line that expresses the progesterone receptor, the molecular gatekeeper through which medroxyprogesterone acetate exerts its antiproliferative effects. When ME1 was inhibited in these cells, reactive oxygen species accumulated, and, unexpectedly, the progesterone receptor itself was upregulated. The tumor was, in effect, being re-sensitized to the drug by two complementary routes: oxidative stress pushed it toward the brink, while increased receptor expression gave the progestin more targets to engage.

To quantify how well the two agents worked together, the team applied the Bliss independence model, a standard framework for drug synergy in which a score above zero indicates that the combination outperforms what the individual drugs would achieve additively. The maximum delta-Bliss score reached 0.51, a value indicating strong synergy between ME1 inhibition and medroxyprogesterone acetate. In practical terms, the combination suppressed cell proliferation far more effectively than either treatment alone, and the effect was most pronounced in the progesterone receptor-positive setting where the progestin has a pathway to act. This dual mechanism, ROS accumulation paired with receptor upregulation, offers a coherent explanation for a phenomenon that has puzzled clinicians: why some progesterone receptor-low tumors resist progestin therapy despite otherwise treatable disease.

Cell culture, however, is a simplified world, so the investigators moved to a xenograft model, transplanting endometrial cancer cells into mice and treating the animals with the ME1 inhibitor, medroxyprogesterone acetate, or both. The combination suppressed tumor growth more effectively than either single agent, and, importantly, the treated animals showed no apparent toxicity. That safety signal matters because progestin therapy is chosen precisely to spare young patients the reproductive and surgical consequences of hysterectomy; an add-on drug that imposed heavy side effects would undermine the strategy’s purpose. The in vivo result, while preliminary, suggests that metabolic sensitization could be layered onto existing hormonal regimens without unacceptable cost to the patient.

The study’s implications extend in two directions. As a prognostic tool, ME1 immunohistochemistry could eventually help clinicians stratify patients at diagnosis, identifying those with high-expression tumors who might need closer monitoring or earlier escalation rather than extended courses of progestin that are destined to fail. As a therapeutic target, ME1 inhibition offers a rational combination partner for progestin therapy, potentially converting resistant tumors into responsive ones. The concept fits within a broader movement in oncology that treats metabolic enzymes, once considered housekeeping proteins, as actionable dependencies, a strategy that has already yielded drugs against enzymes in nucleotide synthesis and one-carbon metabolism. ME1’s role in supplying NADPH makes it part of the same family of vulnerabilities.

Caution is warranted before clinical translation. The pharmacological inhibitor used in the study is an experimental tool, not an approved drug, and the xenograft model, though informative, cannot fully reproduce the hormonal and immune environment of the human endometrium. The patient cohort was retrospective, and the survival association, while statistically significant, will need validation in independent and prospective cohorts. Dosing, scheduling, and patient selection for any future combination trial remain open questions, as does the question of whether ME1 inhibition would benefit tumors with low progesterone receptor expression, where the progestin sensitization mechanism may not apply. The authors received no external funding for the work and declare no conflicts of interest, and the study was approved by the ethics committee of Shinshu University School of Medicine.

Even with those caveats, the research delivers a satisfying arc from bench to bedside logic: an enzyme is overexpressed in tumors, its abundance predicts worse outcomes, its suppression stresses cancer cells and raises the very receptor that hormonal therapy depends on, and the combination outperforms either approach alone in living animals. For the growing population of young women facing endometrial cancer who want to preserve their fertility, the prospect of a metabolic drug that makes progestin therapy work more reliably is more than an incremental finding. It reframes treatment resistance not as an immutable property of the tumor but as a metabolic state that can be chemically reversed. Larger trials will decide whether ME1 inhibitors can fulfill that promise, but the Shinshu study has supplied the biological rationale, the prognostic evidence, and the preclinical proof of concept in a single, coherent package.

Subject of Research: Malic enzyme 1 as a prognostic marker and therapeutic target for enhancing progestin sensitivity in endometrioid endometrial carcinoma

Article Title: Malic enzyme 1 is a prognostic factor and a therapeutic target to enhance progestin sensitivity in endometrial cancer

Article References: Uchiyama, N., Asaka, R., Fujioka, M., Kamijo, K., Yokokawa, Y., Shinagawa, M., Nakajima, M., Takeuchi, H., Ando, H., & Miyamoto, T. (2026). Malic enzyme 1 is a prognostic factor and a therapeutic target to enhance progestin sensitivity in endometrial cancer. Journal of Cancer Research and Clinical Oncology. https://doi.org/10.1007/s00432-026-06633-3

Image Credits: AI Generated

DOI: 10.1007/s00432-026-06633-3

Keywords: endometrial cancer, malic enzyme 1, ME1, progestin therapy, medroxyprogesterone acetate, progesterone receptor, reactive oxygen species, NADPH, fertility-sparing therapy, tumor metabolism, drug synergy, prognostic marker