The MAPK pathway across different malignancies: a new perspective

Abstract

The mitogen-activated protein kinase/extracellular signal-regulated (MAPK/ERK) pathway is activated by upstream genomic events and/or activation of multiple signaling events in which information coalesces at this important nodal pathway point. This pathway is tightly regulated under normal conditions by phosphatases and bidirectional communication with other pathways, like the protein kinase B/mammalian target of rapamycin (AKT/m-TOR) pathway. Recent evidence indicates that the MAPK/ERK signaling node can function as a tumor suppressor as well as the more common pro-oncogenic signal. The effect that predominates depends on the intensity of the signal and the context or tissue in which the signal is aberrantly activated. Genomic profiling of tumors has revealed common mutations in MAPK/ERK pathway components, such as v-raf murine sarcoma viral oncogene homolog B1 (BRAF). Currently approved for the treatment of melanoma, inhibitors of BRAF kinase are being studied alone and in combination with inhibitors of the MAPK and other pathways to optimize the treatment of many tumor types. Therapies targeted toward MAPK/ERK components have various response rates when used in different solid tumors, such as colorectal cancer and ovarian cancer. Understanding the differential nature of activation of the MAPK/ERK pathway in each tumor type is critical in developing single and combination regimens, because different tumors have unique mechanisms of primary and secondary signaling and subsequent sensitivity to drugs.

Keywords: colorectal cancer; extracellular signal-regulated kinase (ERK); melanoma; mitogen-activated protein kinase (MAPK); mitogen-activated protein kinase-kinase (MEK); ovarian cancer; signaling; v-raf murine sarcoma viral oncogene homolog (BRAF).

Figure 1

A model of the MAPK/ERK pathway. After membrane receptor activation, adaptor proteins recruit RAS proteins to activate steps concluding with ERK activation. Successive steps of phosphorylation amplify the signal, Raf→MEK→ERK, until ERK activates its cytoplasmic and/or nuclear targets. Regulatory phosphatases, Sprouty and Spred, modulate the intensity of the signal. The PI3K-AKT pathway interacts with the MAPK/ERK node under normal conditions and in the cancer cell. Target cytoplasmic proteins include RSK, ribosomal S6 kinases; GSK3, glycogen synthase kinase 3; L1, adhesion molecule L1. Additional proteins in nucleus include CPS2, p90Rsk.

Figure 2

Mechanisms of resistance to BRAF inhibitors. A) In melanoma mechanisms of secondary resistance to BRAF inhibitors include BRAF splice variants expression, CRAF activation (all of which activate MAPK/ERK) or signaling trough alternatives pathways AKT/m-TOR etc. (see table 2) B) In Colorectal cancer, primary resistance to BRAF inhibitors is caused by direct activation of EGFR and AKT/m-TOR pathway.