The Effect Of Gut-derived Serotonnin Induced By Depression On Breast Cancer Metastasis And Its Mechanism

Breast cancer is one of the most common cancers worldwide and tends to metastasize to bone, the mechanisms underlying this metastasis are complex and involve both particular characteristics of the breast cancer cells and the bone matrix(soil and seed concept). During metastasis, breast cancer cells disrupt the normal physiological process(bone formation and bone resorption) and generate factors that directly or indirectly induce the formation of osteoclasts in the bone microenvironment, which in turn releases growth factors from the bone matrix(e.g., TGF- β) that stimulate tumor growth and further osteolysis. This reciprocal interaction between breast cancer cells and the bone microenvironment results in a ‘vicious cycle’ that increases both bone destruction and the tumor burden.Depression is a serious complication in women with breast cancer which is often underestimated with a prevalence varying between 10 and 25%. Depressed individuals display lower bone mineral density(BMD) and higher bone resorption than non-depressed subjects and the underlying mechanism is yet to be elucidated. Research has shown that in depression, a defect in the uptake of serotonin in platelets and neurons increases circulating serotonin, which influences bone mass and induces osteoporosis. A recent study found that four 5-HT receptor(5-HTR) subtypes(1A, 1B, 2B and 4) were differentially expressed in human breast cancer. In human breast cancer cells, 5-HT induced both parathyroid hormone-related peptide(PTHrP) and the metastasis-associated transcription factor Runx2/Cbfa1. The mammary epithelium possesses a local serotonin signaling system which drives PTHrP expression during lactation and in breast cancer cells which may be a passage of breast communication to bone through serotonergic control of PTHrP. This finding that serotonin regulates the behavior of breast cancer cells and bone remodeling balance led us to investigate the possible involvement of depression in the control of breast cancer cell metastasis to the bone.MDA-MB-231 breast cancer cells were used to establish a bone metastasis model by using intracardiac injection in nude mice. Chronic mild stress(CMS) was chosen as a model of depression in mice before and after inoculation of the cells. Knockdown of the RUNX2 gene was performed by transfection of the cells with sh RNA silencing vectors against human RUNX2. A co-culture system was used to test the effect of the MDA-MB-231 cells on osteoclasts and osteoblasts. RT-PCR and western blotting were used to test gene and protein expression, respectively.We confirmed that depression induced bone metastasis by promoting osteoclast activity while inhibiting osteoblast differentiation. Free serotonin led to an increase in the expression of RUNX2 in breast cancer cells(MDA-MB-231), which directly inhibited osteoblast differentiation and stimulated osteoclast differentiation by the PTHrP/RANKL pathway, which caused bone destruction and formed osteolytic bone lesions. Additionally, the interaction between depression and breast cancer cells was interrupted by LP533401 or RUNX2 knockdown. In conclusion, depression promotes breast cancer bone metastasis partly through increasing levels of gut? derived serotonin. Activation of RUNX2 in breast cancer cells by circulating serotonin appears to dissociate coupling between osteoblasts and osteoclasts, suggesting that the suppression of gut? derived serotonin decreases the rate of breast cancer bone metastasis induced by depression.