Adjuvant tamoxifen therapy improves survival in breast cancer patients. Unfortunately, long-term treatment comes with side effects that impact health and quality of life, including hot flashes, changes in bone density, and fatigue. Partly due to a lack of proven animal models, the tissues and cells that mediate these negative side effects are unclear. Here, we show that mice undergoing tamoxifen treatment experience changes in temperature, bone, and movement. Single-cell RNA sequencing reveals that tamoxifen treatment induces widespread gene expression changes in the hypothalamus and preoptic area (hypothalamus-POA). These expression changes are dependent on estrogen receptor alpha (ERα), as conditional knockout of ERα in the hypothalamus-POA ablates or reverses tamoxifen-induced gene expression. Accordingly, ERα-deficient mice do not exhibit tamoxifen-induced changes in temperature, bone, or movement. These findings provide mechanistic insight into the effects of tamoxifen on the hypothalamus-POA and indicate that ERα mediates several physiological effects of tamoxifen treatment in mice.
Keywords: bone; cancer biology; estrogen receptor alpha; hypothalamus; mouse; neuroscience; physical activity; tamoxifen; thermoregulation.
Estrogen is a hormone often known for its role in female development and reproduction. Yet, it also has an impact on many biological processes such as immunity and the health of bones, the heart, or the brain. It usually works by attaching to receptor proteins in specific cells. For instance, estrogen-responsive cells are present in the hypothalamus, the brain area that controls energy levels as well as the body’s temperature and internal clock. Breast cancer cells are also often sensitive to estrogen, with the hormone fuelling the growth of tumors. The drug tamoxifen blocks estrogen receptors, stopping cells from responding to the hormone. As such, it is often used to reduce the likelihood that estrogen-dependent breast cancer will come back after treatment. However, its use can induce hot flashes, changes in bone density, fatigue and other life-altering side effects. Here, Zhang et al. investigated how estrogen receptors in the hypothalamus and a related region known as the preoptic area could be responsible for these side effects in mice. When the rodents were given tamoxifen for 28 days, they experienced changes in temperature, bone density and movement similar to those found in humans. In fact, genetic analyses revealed that the drug altered the way genes were turned on and off in certain cells types in the hypothalamus. Crucially, mice whose hypothalamus and preoptic area lacked estrogen receptors did not experience these behavioral and biological alterations. The findings by Zhang et al. help to understand how the side effects of tamoxifen emerge, singling out estrogen receptors in particular brain regions. This result could help to develop new therapies so that breast cancer can be treated with a better quality of life.
© 2021, Zhang et al.