p90RSK Activation Promotes Epithelial-Mesenchymal Transition in Cisplatin-Treated Triple-Negative Breast Cancer Cells

p90 ribosomal S6 kinase (p90RSK), one of the downstream effectors in ERK1/2 pathways, shows high expression in human breast cancer tissues. However, its role in breast cancer development and drug resistance is not fully understood. Here, we demonstrate that Cis-DDP treatment failed to increase cytotoxicity in MDA-MB-231 cells compared to MCF-7 cells and p90RSK activation was involved in Cis-DDP-resistance to MDA-MB-231 cells. In the study, we found that inhibition of p90RSK expression or activation using a small interfering RNA (siRNA) or dominant-negative kinase mutant (DN-p90RSK) plasmid overexpression increased Cis-DDP-induced cytotoxicity of MDA-MB-231 cells, respectively. Mechanistically, we found that Cis-DDP resistance was associated with up-regulation of epithelial growth factor (EGF) expression and EGF treatment induced cancer survival signaling pathway including activation of ERK1/2, p90RSK, and Akt. We also examined the expression of epithelial-mesenchymal transition (EMT)-associated proteins using a reverse transition-quantitative PCR analysis. Cis-DDP treatment induced EMT by increasing the expression levels of N-cadherin, Snail, and Twist, while decreasing the expression levels of E-cadherin. Furthermore, we examined the epithelial marker, Zonula occludens-1 (ZO-1) using immunofluorescence analysis and found that Cis-DDP-inhibited ZO-1 expression was recovered by p90RSK deactivated condition. Therefore, we conclude that Cis-DDP resistance is involved in EMT via regulating the EGF-mediated p90RSK signaling pathway in MDA-MB-231 cells.

LPS-stimulated Macrophage Activation Affects Endothelial Dysfunction

Intestinal microbiota is involved in the atherosclerotic process by development of an atheromatous core with foam cells in carotid arteries. It has reported that lipopolysaccharide (LPS) from Escherichia coli localizes in human atherosclerotic plaque and causes inflammation via interaction with toll like receptor 4. However, there is no evidence that whether LPS-activated macrophages regulate endothelial cell (EC) function. We evaluated whether LPS-activated macrophage acts as one of the stimulants activating EC and its underlying signaling pathways. Using Western blotting and quantitative reverse transcription-polymerase chain reaction (qRT-PCR), we confirmed that intraperitoneal injection with LPS increases iNOS protein and inflammatory cytokine, TNF-α and IL-6 mRNA expressions. To determine whether LPS-mediated macrophage inflammatory condition affects EC activation and inflammation, human umbilical vein endothelial cells (HUVECs) were incubated with isolated peritoneal macrophages from LPS-injected mice. Interestingly, p90RSK Serine 380 phosphorylation and protein expression were significantly increased by macrophage treatment in EC. Messenger RNA levels of vascular cell adhesion molecule 1 and p90RSK was increased, but endothelial nitric oxide synthase was decreased. In addition, NF-κB promoter activity, which plays an important role in the pathogenesis of inflammation, was strongly enhanced by the macrophage treatment in EC. We further evaluated the effects of LPS on EC function in the mouse aorta using en face staining. In agreement with in vitro result, p90RSK expression was strongly increased in the steady laminar flow region of the mouse aorta in mice injected with LPS. Together, our study demonstrates that p90RSK might be a one of the major therapeutic candidates for the prevention of vascular diseases mediated by LPS.

The p90rsk-mediated signaling of ethanol-induced cell proliferation in HepG2 cell line

Ribosomal S6 kinase is a family of serine/threonine protein kinases involved in the regulation of cell viability. There are two subfamilies of ribosomal s6 kinase, (p90rsk, p70rsk). Especially, p90rsk is known to be an important downstream kinase of p44/42 MAPK. We investigated the role of p90rsk on ethanol-induced cell proliferation of HepG2 cells. HepG2 cells were treated with 10~50 mM of ethanol with or without ERK and p90rsk inhibitors. Cell viability was measured by MTT assay. The expression of pERK1, NHE1 was measured by Western blots. The phosphorylation of p90rsk was measured by ELISA kits. The expression of Bcl-2 was measured by qRT-PCR. When the cells were treated with 10~30 mM of ethanol for 24 hour, it showed significant increase in cell viability versus control group. Besides, 10~30 mM of ethanol induced increased expression of pERK1, p-p90rsk, NHE1 and Bcl-2. Moreover treatment of p90rsk inhibitor attenuated the ethanol-induced increase in cell viability and NHE1 and Bcl-2 expression. In summary, these results suggest that p90rsk, a downstream kinase of ERK, plays a stimulatory role on ethanol-induced hepatocellular carcinoma progression by activating anti-apoptotic factor Bcl-2 and NHE1 known to regulate cell survival.