p38 MAPK signaling mediates mitochondrial apoptosis in cancer cells induced by oleanolic acid

p38 MAPK-dependent Nrf2 induction enhances the resistance of glioma cells against TMZ

Temozolomide (TMZ) is an effective agent for clinical glioma treatment, but the innate and acquired resistance of glioma always limits its application. Although some advances have been achieved to elucidate the molecular mechanism underlying TMZ resistance, the role of Nrf2 (a principle regulator of cellular defense against drugs and oxidative stress) has not been well established in the acquisition of this phenotype. Our data showed that TMZ treatment induces the activation of Nrf2 and p38 MAPK signaling in glioma cells, while p38 inhibition abolished the effect of TMZ on Nrf2. Further study revealed that Nrf2 silencing was able to enhance the response of glioma cells to TMZ. Additionally, Nrf2 overexpression overrides the effect of p38 MAPK activation on Temozolomide resistance. In conclusions, we identified a p38 MAPK/Nrf2 signaling as a key molecular network contributing to TMZ resistance of glioma, and provided evidence that suppressing this signaling may be a promising strategy to improve TMZ's therapeutic efficiency.

Oleanolic acid suppresses the proliferation of lung carcinoma cells by miR-122/Cyclin G1/MEF2D axis

Oleanolic acid (OA) is a natural compound from plants with anti-tumor activities. However, the mechanism of the inhibitory effect of OA on cell cycle progression has not been completely explored. We employed several lung carcinoma cell lines to investigate the cell cycle-related molecular pathway affected by OA. The data revealed that OA suppressed the proliferation of lung cancer cells in both dose- and time-dependent manners, along with an increase in miR-122 abundance. The suppression of miR-122 abolished the effect of OA on lung cancer cells. CCNG1 and MEF2D, two putative miR-122 targets, were found to be downregulated by OA treatment. Restoring their expression counteracted the effect of OA on lung carcinoma cells. OA was further shown to induce the expression of miR-122-regulating transcriptional factors in lung cancer cells. Collectively, OA induced cell cycle arrest in lung cancer cells through miR-122/Cyclin G1/MEF2D pathway. This finding may contribute to the understanding of the molecular mechanism of OA's anti-tumor activity.

MEF2 transcription factors promotes EMT and invasiveness of hepatocellular carcinoma through TGF-β1 autoregulation circuitry

Invasion and metastasis is the main causes leading to the death of hepatocellular carcinoma (HCC) patients. However, the underlying mechanism is still to be explored. Transforming growth factor β1 (TGF-β1) is a stronger inducer of HCC cell invasion. However, the downstream effector of TGF-β1 that promotes HCC invasion is still unknown. In this study, we found that PI3K/Akt activation takes place following the stimulation of TGF-β1. The inhibition of PI3K/Akt activation abolished epithelial-mesenchymal transition (EMT) and invasion of HCC cells induced by TGF-β1. Myocyte enhancer factors 2 (MEF2) family proteins were found to be overexpressed in HCC cells under the treatment of TGF-β1 in a PI3K/Akt-dependent way. Silencing the expression of MEF2s was able to prevent the effect of TGF-β1 on HCC EMT and invasion. Unexpectedly, MEF2 proteins were able to promote the expression of TGF-β1 in HCC cells, suggesting the existence of regulatory circuitry consisting of TGF-β1, PI3K/Akt, and MEF2. A natural compound, oleanolic acid, was demonstrated to suppress the invasion and EMT of HCC cells by downregulating MEF2, showing that targeting this pathway is an effective therapeutic strategy for HCC invasion. We believe that our findings can contribute to better understanding of the involved mechanism of HCC invasion and the development of preventive and therapeutic strategy.