The Function Of GPR48in Breast Cancer Bone Metastasis And Osteolysis

Breast cancer is a common malignant tumor that threats women’s health. The morbidity and lethality of this disease remains high all over the world. Breast cancer metastasis is the main reason that leads to death in most patients. Patients with advanced breast cancer always develop into bone pain symptom and suffer dreadful pain, thus the quality of their life is severely affected.Breast cancer metastasis consists of several procedures including primary tumor cell aggravation, cellular adhesions loss, cell mobility enhancement, extracellular matrix degradation, invasion, angiogenesis, intravasation, survival in recirculating system, extravasation and metastatic colonization. A plenty of genetic alterations accumulate in metastatic cells so as to change their original cellular characters and make them evolve into more aggressive cells.G protein-coupled receptors (GPCR) are the largest family of cell-surface receptors. Over1000genes encode GPCR in the human genome. GPCRs involve into lots of diseases and>50%modern drugs were designed to modulate their activity. Therefore, GPCRs are important targets in modern pharmaceutical industry.GPR48, a Leucine-rich repeat-containing G-protein-coupled receptor, belongs to orphan receptors. Previous studies indicate GPR48participates in several organs development and GPR48deficiency would lead to kidney, liver and bone development defect. GPR48plays crucial role in regulating normal physiologic function.In previous study, we found mammary gland dysplasia in GPR48knockout mice. In GPR48/MMTV-PyMT hybrid mice, GPR48deletion could inhibit PyMT induced mammary gland tumorigenesis and lung metastasis. In this study, GPR48expression level was detected in human breast cancer cells lines and higher level of GPR48was found in highly aggressive cell lines. It was found GPR48expression level had positive correlation with breast cancer relapse and bone metastasis after analysis two published microarray dataset of breast cancer. All these result demonstrated that GPR48plays important role in breast cancer progression. To reveal the mechanism of how GPR48regulates breast cancer metastasis, a GPR48stable knockdown MDA-MB-231cell line was constructed in this study. We found shGPR48-231cells failed to metastasize to bone in intracardiac injection model. The ability of migration and invasion of shGPR48-231cells were restricted in vitro. GPR48modulates cells migration and invasion via FAK-Src pathway. GPR48deficiency would decrease FAK-Src activity in MDA-MB-231cells, slow down the turnover rate of stress fiber and focal adhesion and inhibit matrix metalloproteinase expression, thereby inhibiting cells migration and invasion ability.Breast cancer metastasis always leads to osteolysis. In contrast to shNC-231, xenografting shGPR48-231cells into mice tibia would significantly relief bone osteolysis in intratibia injection model. GPR48deletion could decrease the secretion of IL-6, IL-11and PTHrP by blocking PKA-CREB pathway. Decreasing of these cytokines could relief the stimulation effect to osteoblasts so as to inhibit the expression of Rankl by them, which would lead to osteoclasts maturation and bone resorption. GPR48deletion could also inhibit inflammatory factors secretion by osteoblasts, thereby decreasing the chemoattractive effect to other tumor cells.Furthermore, knockdown GPR48in MDA-MB-231could dramatically inhibit its tumorigenesis in vivo. This phenotype may involed into the activation of GPR48by its ligand in vivo. However, the mechanism is still not known.In a word, this study found GPR48expression had positive correlation with breast cancer relapse and bone metastasis. GPR48could promote breast cancer metastasis via modulating cell cystoskeleton, focal adhesion turnover and matrix metalloproteinase expression. At the same time, GPR48could control the secretion of several cytokines such as IL-6, IL-11, PTHrP to alter bone microenvironment, stimulating osteoclasts maturation and bone resorption. GPR48plays important role in both bone metastasis and osteolysis, which makes it a potential target for breast cancer treatment in the future.