Constitutively active Ras negatively regulates Erk MAP kinase through induction of MAP kinase phosphatase 3 (MKP3) in NIH3T3 cells

The effects of mutant Ras proteins on the cell signalome

The genetic alterations in cancer cells are tightly linked to signaling pathway dysregulation. Ras is a key molecule that controls several tumorigenesis-related processes, and mutations in RAS genes often lead to unbiased intensification of signaling networks that fuel cancer progression. In this article, we review recent studies that describe mutant Ras-regulated signaling routes and their cross-talk. In addition to the two main Ras-driven signaling pathways, i.e., the RAF/MEK/ERK and PI3K/AKT/mTOR pathways, we have also collected emerging data showing the importance of Ras in other signaling pathways, including the RAC/PAK, RalGDS/Ral, and PKC/PLC signaling pathways. Moreover, microRNA-regulated Ras-associated signaling pathways are also discussed to highlight the importance of Ras regulation in cancer. Finally, emerging data show that the signal alterations in specific cell types, such as cancer stem cells, could promote cancer development. Therefore, we also cover the up-to-date findings related to Ras-regulated signal transduction in cancer stem cells.

ΔNp63α modulates phosphorylation of p38 MAP kinase in regulation of cell cycle progression and cell growth

p53-related p63 plays a critical role in regulation of cell proliferation, survival and cell differentiation. Dysregulation of p63 functions results in a disruption of a variety of normal biological processes, including stem cell biology, embryonic development, aging and tumorigenesis. ΔNp63α, a predominantly expressed p63 protein isoform in epithelial cells, plays a crucial role in regulation of cell cycle progression and cell growth. p38 MAP kinases (p38MAPK) are the members of mitogen-activated protein kinases family and are critical in regulation of cell survival in response to stress signals. In this study, we show that ectopic expression of ΔNp63α inhibited phosphorylation of p38MAPK. Acute knockdown of p63 led to a significant upregulation of p38MAPK phosphorylation, resulting in increased p21^cip1/waf1 expression, reduced phosphorylation of retinoblastoma protein (RB), cell cycle G1 arrest and cell growth retardation. Restoration of ΔNp63α expression reversed cell cycle arrest and growth inhibition induced by p63 ablation. Pharmacological inhibition of p38MAPK significantly suppressed ΔNp63α ablation-induced cell cycle G1/S arrest. In addition, MAP Kinase Phosphatase 3 (MKP3) was responsible for ΔNp63α-mediated regulation of p38MAPK phosphorylation. Together, these results suggest that ΔNp63α-MPK3-p38MAPK signaling pathway plays an important role in cell cycle progression and cell growth.

Translational control of ERK signaling through miRNA/4EHP-directed silencing

MicroRNAs (miRNAs) exert a broad influence over gene expression by directing effector activities that impinge on translation and stability of mRNAs. We recently discovered that the cap-binding protein 4EHP is a key component of the mammalian miRNA-Induced Silencing Complex (miRISC), which mediates gene silencing. However, little is known about the mRNA repertoire that is controlled by the 4EHP/miRNA mechanism or its biological importance. Here, using ribosome profiling, we identify a subset of mRNAs that are translationally controlled by 4EHP. We show that the Dusp6 mRNA, which encodes an ERK1/2 phosphatase, is translationally repressed by 4EHP and a specific miRNA, miR-145. This promotes ERK1/2 phosphorylation, resulting in augmented cell growth and reduced apoptosis. Our findings thus empirically define the integral role of translational repression in miRNA-induced gene silencing and reveal a critical function for this process in the control of the ERK signaling cascade in mammalian cells.

Quercetin promotes the osteogenic differentiation of rat mesenchymal stem cells via mitogen-activated protein kinase signaling

The aim of the present study was to investigate the effects of quercetin on the mitogen-activated protein kinase (MAPK) signaling pathway in the osteogenic differentiation of rat mesenchymal stem cells (MSCs). A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and an alkaline phosphatase (ALP) assay were used to determine the effects of quercetin (concentrations of 0.1, 1 and 10 µmol/l) on the proliferation and differentiation of MSCs and the expression of ALP, respectively. In addition, through the introduction of inhibitors of p38 MAPK, extracellular signal-regulated kinase (ERK)1/2 and c-Jun NH₂-terminal kinase (JNK), the effects of quercetin on the proteins, ALP, collagen type I (COL I) and bone γ-carboxyglutamate protein (BGP), which are indicators of osteogenic differentiation, were investigated. Immunoblotting was performed to determine the phosphorylation levels of p38 MAPK, ERK1/2 and JNK, while fluorescent quantitative polymerase chain reaction was used to determine the mRNA expression levels of transforming growth factor (TGF)-β1, bone morphogenetic protein (BMP)-2 and core binding factor (CBF)α1. At all the concentrations tested, the concentrations of 10, 1 and 0.1 µmol/l quercetin were shown to promote the differentiation of MSCs and the expression of ALP, in which the concentration of 10 µmol/l was optimal. When compared with the control group, the phosphorylation levels of p38 MAPK, ERK1/2 and JNK, the protein expression levels of ALP, COL I and BGP, and the mNRA expression levels of TGF-β1, BMP-2 and Cbfα1 were increased in the quercetin-treated group. However, with the introduction of inhibitors, the levels of phosphorylated p38 MAPK, ERK1/2 and JNK, and the protein expression levels of ALP, COL I and BGP decreased. Furthermore, the mRNA expression levels of TGF-β1, BMP-2 and CBFα1 decreased in the quercetin + SP600125 (inhibitor of JNK) and quercetin + PD98059 (inhibitor of ERK1/2) groups. Therefore, quercetin was demonstrated to promote the osteogenic differentiation of MSCs by activating the MAPK signaling pathway. The ERK1/2 and JNK signaling pathways regulate the expression of TGF-β1, BMP-2 and CBFα1. Thus, activation of the ERK1/2 and JNK signaling pathways may play a leading role in the quercetin-promoted osteogenic proliferation and differentiation of MSCs.