Regulation of Intracellular Calcium in Uterine Leiomyomas

Background Uterine leiomyomas (fibroids) are benign smooth muscle cell tumors of the uterus that affect a large percentage of reproductive aged women, contribute to infertility, excessive menstrual bleeding, and represent the leading cause of hysterectomy (~70%). Despite their importance, the mechanisms that promote fibroid growth are still not well‐understood. Fibroids are comprised of smooth muscle cells (SMCs) and fibroblasts and their growth is modulated by hormones including estrogen and progesterone. Intracellular calcium [Ca 2+ ] i signaling is a key aspect of SMC physiology in promoting contractility, cell proliferation, and fibrosis: aspects relevant to fibroid biology. While [Ca 2+ ] i regulation in normal myometrium is well characterized, this has not been explored in fibroids. The goal of this study was to explore [Ca 2+ ] i regulation in fibroid SMCs and the effect of sex steroids towards obtaining insights into tumor progression. Methods Cells were enzymatically isolated from normal myometrium and select fibroids of different sizes from patients undergoing myomectomy at Mayo Clinic, Rochester, MN (IRB approved). [Ca 2+ ] i was measured using fura‐2/AM and real‐time fluorescence microscopy. The contribution of Ca 2+ influx vs. intracellular pathways in the responses to 1uM oxytocin were assessed using pharmacological inhibitors/activators. Cells were pretreated with vehicle, 1nM estrogen, or 1nM progesterone to assess the effect of sex steroids. Results Fibroid‐derived cells showed a greater contribution of Ca 2+ influx pathways in their responses to oxytocin than normal myometrium. Here, the Ca 2+ ‐activated chloride channel TMEM16A (Ano1) was found to be contributory, with Western analysis showing higher levels of Ano1 in fibroids. Exposure to estrogen resulted in significantly higher [Ca 2+ ] i responses compared to cells treated with vehicle or progesterone, although progesterone did enhance [Ca 2+ ] i responses as well. Estrogen increased Ano1 expression. Cell proliferation was also increased in fibroid cells. Conclusions Exposure to estrogen and progesterone enhances oxytocin induced [Ca 2+ ] i responses of fibroid‐derived cells, potentially via increased TMEM16A (Ano1). Such increases may further contribute to the observed hormone‐induced enhancement of fibroid cell proliferation. Menstrual cycle fluctuations in reproductive aged women may thus contribute to the progression of fibroids. Greater understanding of calcium signaling pathways such as Ano1 in fibroid tissue may allow for novel therapeutic targets in uterine leiomyomas. Support or Funding Information Supported by NIH grants R25 GMO75148 (JLR), R01 HL088029 (YSP), and R01 HL56470 (YSP). This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .