Reciprocal signaling between mTORC1 and MNK2 controls cell growth and oncogenesis.

eIF4E plays key roles in protein synthesis and tumorigenesis. It is phosphorylated by the kinases MNK1 and MNK2. Binding of MNKs to eIF4G enhances their ability to phosphorylate eIF4E. Here, we show that mTORC1, a key regulator of mRNA translation and oncogenesis, directly phosphorylates MNK2 on Ser74. This suppresses MNK2 activity and impairs binding of MNK2 to eIF4G. These effects provide a novel mechanism by which mTORC1 signaling impairs the function of MNK2 and thereby decreases eIF4E phosphorylation. MNK2[S74A] knock-in cells show enhanced phosphorylation of eIF4E and S6K1 (i.e., increased mTORC1 signaling), enlarged cell size, and increased invasive and transformative capacities. MNK2[Ser74] phosphorylation was inversely correlated with disease progression in human prostate tumors. MNK inhibition exerted anti-proliferative effects in prostate cancer cells in vitro. These findings define a novel feedback loop whereby mTORC1 represses MNK2 activity and oncogenic signaling through eIF4E phosphorylation, allowing reciprocal regulation of these two oncogenic pathways.

MAPK-interacting kinase 2 (MNK2) regulates adipocyte metabolism independently of its catalytic activity.

The mitogen-activated protein kinase (MAPK)-interacting kinases (MNKs) are serine/threonine protein kinases that are activated by the ERK1/2 (extracellular regulated kinase) and p38α/ß MAPK pathways. The MNKs have previously been implicated in metabolic disease and shown to mediate diet-induced obesity. In particular, knockout of MNK2 in mice protects from the weight gain induced by a high-fat diet. These and other data suggest that MNK2 regulates the expansion of adipose tissue (AT), a stable, long-term energy reserve that plays an important role in regulating whole-body energy homeostasis. Using the well-established mouse 3T3-L1 in vitro model of adipogenesis, the role of the MNKs in adipocyte differentiation and lipid storage was investigated. Inhibition of MNK activity using specific inhibitors failed to impair adipogenesis or lipid accumulation, suggesting that MNK activity is not required for adipocyte differentiation and does not regulate lipid storage. However, small-interfering RNA (siRNA) knock-down of MNK2 did reduce lipid accumulation and regulated the levels of two major lipogenic transcriptional regulators, ChREBP (carbohydrate response element-binding protein) and LPIN1 (Lipin-1). These factors are responsible for controlling the expression of genes for proteins involved in de novo lipogenesis and triglyceride synthesis. The knock-down of MNK2 also increased the expression of hormone-sensitive lipase which catalyses the breakdown of triglyceride. These findings identify MNK2 as a regulator of adipocyte metabolism, independently of its catalytic activity, and reveal some of the mechanisms by which MNK2 drives AT expansion. The development of an MNK2-targeted therapy may, therefore, be a useful intervention for reducing weight caused by excessive nutrient intake.

Identification of DNA response elements regulating expression of CCAAT/enhancer-binding protein (C/EBP) ß and δ and MAP kinase-interacting kinases during early adipogenesis.

Given the high and increasing prevalence of obesity and associated disorders, such as type-2 diabetes, it is important to understand the mechanisms that regulate lipid storage and the differentiation of fat cells, a process termed adipogenesis. Using the well-established mouse 3T3-L1 in vitro model of adipogenesis, we refine how the induction of two key adipogenic transcription factors, CCAAT/enhancer-binding proteins (C/EBPs) ß and δ are regulated during early adipogenesis. We identify, in the gene promoters of Cebpb and Cebpd, the DNA response elements responsible for binding transcription factors that are activated by cAMP or glucocorticoids. We also show that mitogen-activated protein kinase (MAPK)-interacting kinase 2 (MNK2; Mknk2), which plays a distinct role in diet-induced obesity, is induced during early adipogenesis and identify the functional DNA response elements responsible for regulating its expression. Mknk2 expression is maintained in differentiated 3T3-L1 adipocytes and is expressed at high levels across a range of mouse adipose tissue depots. Together, these new insights help to clarify the transcriptional programme of early adipogenesis and identify Mknk2 as one of potentially many genes up-regulated during adipogenesis.

The MAP kinase-interacting kinases (MNKs) as targets in oncology.

INTRODUCTION: The mitogen-activated protein kinase (MAPK)-interacting kinases (MNKs) are switched on by the oncogenic MAPK (ERK) signalling pathway. They phosphorylate eukaryotic initiation factor (eIF) 4E, a protein which recruits ribosomes to mRNAs and thereby mediates their translation. Importantly, overexpression of eIF4E can transform cells, and its function is controlled by a second oncogenic pathway, mechanistic target of rapamycin complex 1. Areas covered: We have evaluated the literature related to the role of the MNKs in human cancers, including their control by oncogenic signalling pathways; their expression and regulation in cancer cells and preclinical cancer models; and their roles in the proliferation, survival and migration/invasion of cancer cells. We also discuss progress towards generating specific and potent inhibitors of the MNKs and data obtained using such compounds. Expert opinion: The available data indicate that MNKs and/or eIF4E phosphorylation play a role in oncogenic transformation, the progression of at least some tumours and especially in processes related to tumour metastasis. MNKs are clearly druggable targets and, as they are not essential, significant 'side effects' of inhibiting the MNKs are likely to be limited. Further work is required to assess the efficacy of MNK inhibition in tackling tumour development, progression and metastasis.