High salt diet may promote progression of breast tumor through eliciting immune response

Dietary patterns are believed to regulate tumor progression by altering the tumor microenvironment. Of note, a high salt diet is a risk factor for various diseases. However, the role of high salt intake in the progression of cancers remains unknown.

We constructed an in vivo high salt diet model in MMTV-PyVT mice with spontaneous tumor-forming properties to explore the role of a high salt diet in the progression of breast cancer as well as the modulation of the tumor microenvironment. Also, in vitro experiments were performed to understand the mechanism.

High salt diet accelerated the development (P < 0.05) and lung metastasis (P < 0.05) of breast cancer in MMTV-PyVT mice, compared to the normal diet model. Moreover, higher frequency of Th17 cells in circulation, tumor tissue and draining lymph node tissue were observed in the high salt diet model (P < 0.05 for all). In vitro, co-culture with Th17 cells facilitated the proliferation, migration and invasion of MCF-7 human breast cancer cells, while these enhanced aggressive behaviors could be reversed by application of 1,25-vitamin D₃ which could inhibit the differentiation of Th17 cells (P < 0.001 for all). In vitro, co-culture with Th17 cells activated MAPK signaling in MCF-7 cells (P < 0.001 for all). Consistently, activated MAPK/ERK signaling was observed by immunohistochemistry in breast cancer cell nodes in the high salt diet model (P < 0.05 for all). Mechanistically, higher level of IL-17F could be detected in breast tumors and serum from the high salt diet model through qRT-PCR and ELISA (P < 0.05 for all). IL-17F treatment facilitated the proliferation, migration and invasion of MCF-7 human breast cancer cells and activated MAPK/ERK signaling in MCF-7 cells (P < 0.001 for all). Moreover, the tumor-promoting function induced by Th17 cells and IL-17F could be inhibited by the administration of ERK inhibitor (sch772894) (P < 0.001 for all). Lastly, high concentration NaCl-induced Th17 cells promoted the proliferation, migration and invasion of MCF-7 human breast cancer cells and activated MAPK/ERK signaling in MCF-7 cells which could be inhibited by neutralizing anti-IL-17F (P < 0.001 for all).

High salt intake accelerates the growth of breast cancer and facilitates lung metastasis, as well as increases the level of Th17 cells. Increased Th17 cells might promote the growth of breast cancer via the secretion of IL-17F to activate the MAPK signaling pathway in breast cancer cells.