LY 379268 Binding of RANKL secreted from
Binding of RANKL secreted from breast cancer cells to its cognate receptor RANK, on the surface of osteoclast precursor cells, leads to the recruitment of TRAF6 and consequently activates various downstream effector pathways including NF-κB and MAPKs (Erk1/2, p38, and JNK). These pathways culminate in the induction of key osteoclast transcriptional factor NFATc1 [12,13]. NFATc1 is considered the terminal switch that regulates osteoclast differentiation by promoting the expression of numerous osteoclast-specific genes involved in osteoclast fusion such as dendritic cell-specific transmembrane protein (DC-STAMP), V-ATPase V0 domain subunit d2 (ATP6V0d2), and bone resorption such as tartrate-resistant alkaline phosphatase (TRAP), and cathepsin K (CTSK) [14,15].
Sterol regulatory element-binding protein 2 (SREBP-2) has been studied extensively in the context of cholesterol homeostasis by transcriptionally regulating the expression of target genes. These include gene for the low-density lipoprotein receptor (LDLR), 3-hydroxy-3-methyl-glutaryl-coenzyme A (HMG-CoA) synthase and HMG-CoA reductase, which are involved in regulating fatty LY 379268 and cholesterol synthesis . Interestingly, recent genome-wide DNase-Seq analysis identified SREBP-2 as a novel transcriptional regulator involved in osteoclast differentiation . In addition, the SREBP inhibitor, Fatostatin, has been reported to prevent RANKL-induced bone loss by suppressing osteoclast differentiation . Furthermore, SREBP inhibition was recently found to exert multiple anti-tumor effects in various tumors [, , ]. However, the underlying mechanism by which SREBP-2 regulates osteoclast differentiation and the role of SREBP-2 in breast cancer remains ill-defined.
In this study, we showed that SREBP-2 is induced in a RANKL-CREB dependent pathway and plays a positive role in the regulation of osteoclast formation. SREBP-2 was found to transcriptionally control the expression of NFATc1 by binding to SRE in the promoter of NFATc1. In addition, we found elevated expression of SREBP-2 in breast cancer tissue samples, and the high SREBP-2 expression was positively correlated with invasive breast carcinomas and predictor of poor prognosis. Targeted inhibition of SREBP-2 exerts anti-migration and anti-invasion effects on breast cancer cells in vitro and protected against breast cancer-induced osteolysis in vivo. Thus, SREBP-2 represents a novel molecular target for the development of specific therapeutic agents for the treatment of osteolytic bone lesions induced by breast cancer metastases.
Materials and methods
Discussion The vicious cycle established between breast cancer cells and osteoclast, can lead to cancer-induced bone destruction which seriously impacts the quality of life of patients [4,5]. Although therapeutic interventions have been developed that aims to disrupt this vicious cycle of positive regulation their efficacy remains limited [9,27]. In this study, we have shown that SREBP-2 serves as an important molecular regulator in the pathogenesis of breast cancer-induced osteolysis in vivo, and that therapeutic targeting of SREBP-2 may be effective for treatment option for osteolytic bone lesions in breast cancer patients. In vitro assessments found SREBP-2 to be upregulated during osteoclast formation under the regulation of the RANKL-CREB signaling pathway. RANKL-induced activation of CREB stimulated the transcription and activation of SREBP-2, which then translocates into the nucleus where it participate in the induction of NFATc1 transcription during osteoclast formation. Suppression of SREBP2 expression by gene knockdown or CREB inhibition with KG-501, or the inhibition of SREBP-2 activation with Fatostatin, potently inhibited osteoclast formation and bone resorption. Similarly in MDA-MB-231 human breast cancer cells, CREB activation regulated the expression of SREBP-2 which was found to be involved in the transcriptional regulation of MMP-2 and MMP-9 expression. Furthermore, elevated levels of SREBP-2 expression was highly correlated with the invasive metastatic breast carcinomas and was found to be a predictor of poor prognosis for breast cancer patients. Thus our data suggested that SREBP-2 is intimately involved in the vicious cycle of breast cancer-induced osteolysis by promoting breast cancer metastasis and inducing osteoclast differentiation and bone resorption.