Identification of a c-Jun N-terminal kinase-2-dependent signal amplification cascade that regulates c-Myc levels in ras transformation

Abstract

Ras is one of the most frequently activated oncogenes in cancer. Two mitogen-activated protein kinases (MAPKs) are important for ras transformation: extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase 2 (JNK2). Here we present a downstream signal amplification cascade that is critical for ras transformation in murine embryonic fibroblasts. This cascade is coordinated by ERK and JNK2 MAPKs, whose Rasmediated activation leads to the enhanced levels of three oncogenic transcription factors, namely, c-Myc, activating transcription factor 2 (ATF2) and ATF3, all of which are essential for ras transformation. Previous studies show that ERK-mediated serine 62 phosphorylation protects c-Myc from proteasomal degradation. ERK is, however, not alone sufficient to stabilize c-Myc but requires the cooperation of cancerous inhibitor of protein phosphatase 2A (CIP2A), an oncogene that counteracts protein phosphatase 2A-mediated dephosphorylation of c-Myc. Here we show that JNK2 regulates Cip2a transcription via ATF2. ATF2 and c-Myc cooperate to activate the transcription of ATF3. Remarkably, not only ectopic JNK2, but also ectopic ATF2, CIP2A, c-Myc and ATF3 are sufficient to rescue the defective ras transformation of JNK2-deficient cells. Thus, these data identify the key signal converging point of JNK2 and ERK pathways and underline the central role of CIP2A in ras transformation.

CIP2A signature reveals the MYC dependency of CIP2A-regulated phenotypes and its clinical association with breast cancer subtypes

“…JNK2 has also previously been implicated as the cancer-promoting isoform of the JNK family of mitogen-activated protein kinases (Mathiasen et al, 2011). CIP2A depletionelicited downregulation of JNK2 mRNA and protein levels was verified by RT-qPCR and western blot analysis, respectively (Figures 3c and d).…”

Cancerous Inhibitor of Protein Phosphatase 2A, an Emerging Human Oncoprotein and a Potential Cancer Therapy Target

“…In addition to CIP2A promoting MYC expression, MYC is also able to stimulate CIP2A mRNA and protein expression (13), establishing a positive feedback loop between these 2 human oncoproteins. The existence of the positive feedback loop between MYC and CIP2A has been verified by 2 additional studies (9,18). This bistable relationship between CIP2A and MYC may in part account for the prevalent MYC protein expression and activity in different human cancers.…”

“…Moreover, CIP2A expression induces the transformation of JNK2 À/À mouse embryo fibroblasts (MEF), which are otherwise defective for RAS-driven transformation (9). In addition to evidence indicating that CIP2A directly promotes malignant transformation, several recent studies have shown that CIP2A inhibition in fully malignant cancer cells results in decreased cell viability and anchorage-independent growth (4,(6)(7)(8)(10)(11)(12)(13)(14)(15).…”

“…Interestingly, CIP2A and MYC proteins are coexpressed in human gastric cancer samples, and CIP2A depletion in gastric cancer cells results in decreased expression of serine 62 phosphorylated MYC and destabilization of MYC (13). Furthermore, CIP2A and MYC are coexpressed in hepatocellular, colon, and NSCLC tissue samples (9,12,13). In addition to CIP2A promoting MYC expression, MYC is also able to stimulate CIP2A mRNA and protein expression (13), establishing a positive feedback loop between these 2 human oncoproteins.…”

“…This bistable relationship between CIP2A and MYC may in part account for the prevalent MYC protein expression and activity in different human cancers. Convincing evidence for a functional relationship between these proteins was provided by experiments in which ectopic expression of CIP2A rescued both MYC-and RAS-signaling defects in JNK2-null MEFs, which were similarly rescued by ectopic MYC expression (9). Another study demonstrated that both MYC and CIP2A induce the expression of the other protein and promote the selfrenewal and proliferation capacity of neural progenitor cells (18).…”