br Methods br Results and discussion

Methods
Results and discussion
Conclusions
Conflict of interest
Acknowledgment The research is supported by the National Natural Science Foundation of China (Grant No. U1603285).
Overview of the MAPK pathway The mitogen-activated protein kinases (MAPK) pathway, often known as a cascade of protein kinases composed of RAS, RAF, mitogen-activated protein/extracellular signal-regulated kinase (MEK) and the extracellular signal-regulated kinase (ERK), is a highly conserved signal transduction pathway in all eukaryotic cells. The MAPK pathway is one of the best-characterized signaling cascades that regulates a variety of normal cellular functions, such as cell proliferation, differentiation, survival and apoptosis, by transmitting signals from upstream extracellular growth factors to diverse downstream effectors located in the nucleus. The activation of the MAPK pathway initiates from a conformational change of RAS. Stimulated by upstream receptors, guanosine diphosphate (GDP)-bound RAS (inactive) switches to guanosine triphosphate (GTP)-bound RAS (active), which causes the recruitment and activation of RAF. Activated RAF phosphorylates and actives MEK, whose activation directly leads to the phosphorylation of ERK. Activated ERK phosphorylates multiple substrates ranging from kinases to transcription factors, and is positioned as a key kinase that controls a large number of cellular processes due to its rather broad nature of substrate recognition.
Aberrations of the MAPK pathway in cancer It has been widely appreciated that aberrant activation of this pathway is closely linked to various kinds of cancers. Dysregulated MAPK signaling leads to the occurrence and progression of cancers via multiple mechanisms, particularly gentic alterations1, 26. RAS has been identified as an oncogene and is mutationally activated in approximately one-third of all cancers, with sr1 (90%), colon (50%), thyroid (50%), lung (30%) and melanoma (25%) with high-ranking prevalence. RAS mutants encode mutated proteins that harbor single amino-acid substitutions primarily at glycine 12 (G12) and glutamine 16 (Q16) in human cancers (Table 1). These mutated proteins are GAP-insensitive and constitutively GTP-bound, leading to stimulus-independent and persistent activation of the downstream effectors. Among the RAS family, KRAS is the most frequently mutated isoform and occurs in more than 20% of all human cancers, followed by NRAS (8%) and HRAS (3.3%), and no other RAS mutation has been found. The mutation types of RAS proteins may be associated with tumor types; the NRAS mutations have been identified in approximately 20% of melanomas, for example . BRAF mutations have been widely identified in tumors, with a significant percentage (7%) of all human cancers. This mutation is highly prevalent in hairy cell leukemia (100%), melanoma (50%–60%), papillary thyroid cancer (40%–60%), colorectal cancers (CRC, 5%–10%), pilocytic astrocytoma (10%–15%) and non-small cell lung cancer (NSCLC, 3%–5%). The most common mutation of BRAF refers to BRAF-V600E, which is a point mutation at valine 600 to yield glutamic acid. The BRAF-V600E mutation is notably prevalent in melanomas (63%) and papillary thyroid carcinomas (more than 50%). Oncogenic BRAF mutations lead to overactivity of its downstream effectors MEK and ERK. In terms of downstream kinases in the MAPK pathway, MEK mutations have been mainly identified in melanoma, and also in ovarian cancer cell lines and gliomas34, 35. Generally, all of the upstream mutations can lead to ERK protein hyperactivation, which is responsible for a series of ERK-signaling-regulated substrate activation and consequently related to a wide range of tumors. For instance, overexpression of ERK can induce modulation of anti-apoptotic molecules such as BCL-2, a protein that is linked to drug resistance in some types of breast cancer.
Inhibitors targeting the MAPK pathway