Background
Progestins generally suppress the growth of estrogen receptor (ER) and progesterone receptor (PR) positive breast cancers and are occasionally used as treatments. However, progestins also increase populations of therapy-resistant cancer stem cells. We speculated that the downstream effects of progestins on cell metabolism might help explain its unusual impact on cell phenotype. Thus, in this study we investigated how progestins, in the absence or presence of estrogens, affect cell metabolism in ER+PR+ breast cancer.
Methods
Metabolites were quantified and compared from ER+PR+ breast cancer cell lines T47D and UCD65 treated with vehicle, estrogen only, progestin only, or the combination using ultra-performance liquid chromatography coupled with mass spectrometry (UPLC-MS). Metabolic flux analysis was performed on cells given the same hormone treatments. Likewise, the influence of treatments on mitochondrial morphology was measured using transmission electron microscopy in cell lines and patient-derived xenograft tumors and mitochondrial-targeted GFP in cell lines. Mitochondrial biogenesis was measured via fluorescence shift in the photoconvertible MitoTimer reporter coupled with confocal microscopy, and biogenesis regulators were measured by qPCR. Select metabolites and ATP were measured using fluorometric assays.
Results
Estrogen plus progestin treatment largely reverses estrogen-stimulated metabolic activities in breast cancer cells including increased TCA cycle metabolites, amino acid metabolism, and glutathione metabolism. Addition of progestins to estrogen impaired mitochondrial oxygen consumption and ATP production of cancer cells. Moreover, while estrogen-treated cells had elongated mitochondrial morphology, progestin co-treatment resulted in a more aged and less elongated mitochondrial population. Notably, progestins blocked the estrogen-induced expression of mitochondrial biogenesis regulators PGC1α and PGC1β and their downstream targets. While progestin treatment reduced total intracellular amino acid and glutathione pools, a subpopulation of progestin-inducible cancer stem cells revealed a dependence on glutathione.
Conclusions
These findings indicate that progestins antagonize estrogen’s effects on cellular metabolism and shift cells to a more quiescent phenotype, with reduced mitochondrial functional capacity, more reliance on glycolysis, and increased cell survival traits. Our results have implications for current clinical studies testing selective PR modulators in ER+ breast cancers.