RNAi‐mediated MEK1 knock‐down prevents ERK1/2 activation and abolishes human hepatocarcinoma growth in vitro and in vivo

Abstract

The mitogen-activated protein kinases MEK/ERK pathway regulates fundamental processes in malignant cells and represents an attractive target in the development of new cancer treatments especially for human hepatocarcinoma highly resistant to chemotherapy. Although gene extinction experiments have suggested distinct roles for these proteins, the MEK/ERK cascade remains widely considered as exhibiting an overlap of functions. To investigate the functionality of each kinase in tumorigenesis, we have generated stably knock-down clones for MEK1/2 and ERK1/2 isoforms in the human hepatocellular carcinoma line HuH7. Our results have shown that RNAi strategy allows a specific disruption of the targeted kinases and argued for the critical function of MEK1 in liver tumor growth. Transient and stable extinction experiments demonstrated that MEK1 isoform acts as a major element in the signal transduction by phosphorylating ERK1 and ERK2 after growth factors stimulation, whereas oncogenic level of ERK1/2 phosphorylation appears to be MEK1 and MEK2 dependent in basal condition. In addition, silencing of MEK1 or ERK2 abolished cell proliferation and DNA replication in vitro as well as tumor growth in vivo after injection in rodent. In contrast, targeting MEK2 or ERK1 had no effect on hepatocarcinoma progression. These results strongly corroborate the relevance of targeting the MEK cascade as attested by pharmacologic drugs and support the potential application of RNAi in future development of more effective cancer therapies. Our study emphasizes the importance of the MEK/ERK pathway in human hepatocarcinoma cell growth and argues for a crucial role of MEK1 and ERK2 in this regulation.The canonical signaling pathway known as Ras-dependent mitogen-activated protein kinase (MAPK) is involved in several biological processes such as proliferation, survival, motility and cellular differentiation.1,2 This cascade is under the control of numerous factors leading to the successive activation of Raf-family protein, MEK1/2 (mitogen-activated protein/ERK kinase) and ERK1/2 (extracellular signal-regulated kinase) ensuring signaling continuity by phosphorylation from the membrane receptors to downstream cytoplasmic and nuclear substrates. 3,4 The MEK/ERK pathway acts as an essential element in cellular responses induced by mitogen factors and contains several protooncogenes constitutively activated in more than 35% of a large variety of human cancers. This includes hepatocellular carcinoma (HCC), which is one of the most chemoresistant tumors.5-7

MEK1 promotes YAP and their interaction is critical for tumorigenesis in liver cancer

“…ERK is a pivotal downstream effecter of MEK, as RNAi-mediated MEK knockdown prevents ERK activation and abolishes hepatocarcinoma growth [22]. However, in this study, we found MEK1 regulates YAP via an ERK independent manner; because ectopic expression of ERK has no effects on YAP (Fig.…”

Autophagy-related Gene 7 (ATG7) and Reactive Oxygen Species/Extracellular Signal-regulated Kinase Regulate Tetrandrine-induced Autophagy in Human Hepatocellular Carcinoma

“…The ERK signaling pathway can be activated in response to a diverse range of extracellular stimuli, including mitogens, growth factors, and cytokines, as well as immediate extracellular stresses, such as radiation or chemotherapeutic agents (53)(54)(55). We previously demonstrated that treatment with 30 M tetrandrine for 24 h repressed ERK activation and led to apoptosis in cancer cells (Fig.…”

Preferential inhibition of hepatocellular carcinoma by the flavonoid Baicalein through blocking MEK-ERK signaling

“…They are mostly associated with the mitogen-activated protein kinase (MAPK) pathway (9). Constitutively activated extracellular signal-regulated kinases (ERK) have been shown to increase proliferation of human HCC cells (10). So far there is no report that investigates the effects of Baicalein on ERK in HCC.…”