The cholesterol metabolite 27-hydroxycholesterol increases the secretion of extracellular vesicles which promote breast cancer progression.

Cholesterol has been implicated in the clinical progression of breast cancer, a disease that continues to be the most commonly diagnosed cancer in women. Previous work has identified the cholesterol metabolite, 27-hydroxycholesterol (27HC), as a major mediator of the effects of cholesterol on breast tumor growth and progression. 27HC can act as an estrogen receptor (ER) modulator to promote the growth of ERα+ tumors, and a liver x receptor (LXR) ligand in myeloid immune cells to establish an immune-suppressive program. In fact, the metastatic properties of 27HC require the presence of myeloid cells, with neutrophils (PMNs) being essential for the increase in lung metastasis in murine models. In an effort to further elucidate the mechanisms by which 27HC alters breast cancer progression, we made the striking finding that 27HC promoted the secretion of extracellular vesicles (EVs), a diverse assortment of membrane bound particles that include exosomes. The resulting EVs had a size distribution that was skewed slightly larger, compared to EVs generated by treating cells with vehicle. The increase in EV secretion and size was consistent across three different subtypes: primary murine PMNs, RAW264.7 monocytic cells and 4T1 murine mammary cancer cells. Label-free analysis of 27HC-EVs indicated that they had a different metabolite composition to those from vehicle-treated cells. Importantly, 27HC-EVs from primary PMNs promoted tumor growth and metastasis in two different syngeneic models, demonstrating the potential role of 27HC induced EVs in the progression of breast cancer. EVs from PMNs were taken up by cancer cells, macrophages and PMNs, but not T cells. Since EVs did not alter proliferation of cancer cells, it is likely that their pro-tumor effects are mediated through interactions with myeloid cells. Interestingly, RNA-seq analysis of tumors from 27HC-EV treated mice do not display significantly altered transcriptomes, suggesting that the effects of 27HC-EVs occur early on in tumor establishment and growth. Future work will be required to elucidate the mechanisms by which 27HC increases EV secretion, and how these EVs promote breast cancer progression. Collectively however, our data indicate that EV secretion and content can be regulated by a cholesterol metabolite, which may have detrimental effects in terms of disease progression, important findings given the prevalence of both breast cancer and hypercholesterolemia.