Control of adipocyte differentiation and metabolism by mitogen-activated protein kinase (MAPK)- interacting kinases (MNKs)

Abstract

The mitogen-activated protein kinase (MAPK)-interacting kinases (MNKs) are a family of serine/threonine protein kinases that are activated downstream of the ERK1/2 (extracellular regulated kinase) and p38α/β MAPK pathways. The MNKs are encoded on separate genes, Mknk1 and Mknk2, giving rise to two proteins in mice, MNK1 and MNK2, respectively. The MNKs have previously been implicated in metabolic disease and shown to regulate weight gain in mice in response to an energy-rich high-fat diet (HFD). In particular, MNK2-knockout (KO) mice are protected from HFD-induced weight gain and therefore, MNK2 is proposed to regulate adipose tissue expansion. Adipose tissue acts as a stable, long term energy reserve that can be readily mobilised, and plays an important role in regulating whole-body energy homeostasis. It is suggested that MNK activity may be required for the differentiation of new fat cells (adipocytes) and increasing adipocyte lipid storage; however, the function and underlying mechanisms of the MNKs in these processes are yet to be determined. Using the well-established mouse 3T3-L1 in vitro model of adipogenesis, the role of the MNKs in adipocyte differentiation (adipogenesis) and lipid storage was investigated. Inhibition of MNK activity using specific inhibitors failed to impair adipogenesis or lipid accumulation, suggesting MNK activity is not required for adipocyte differentiation and does not regulate lipid storage. Despite this, expression analysis revealed MNK2 to be the predominant isoform in adipose depots. Therefore, to investigate the role of MNK2 specifically in these processes, a 3T3-L1-2KO cell model was generated using CRISPR-Cas9 and small-interfering RNA (siRNA)-mediated knockdown of Mknk2 was also used. Intriguingly, the knockdown of MNK2 was found to decrease lipid accumulation in adipocytes, suggesting non-catalytic functions of MNK2 regulate adipocyte metabolism. It was revealed that MNK2 likely regulates the levels of two major lipogenic transcription factors, ChREBP (carbohydrate response element binding protein) and LPIN1 (Lipin-1), through novel protein-protein interactions. These factors are largely responsible for activating expression of genes in synthetic processes that promote lipid storage in adipocytes, namely de novo lipogenesis (DNL) and triglyceride synthesis. The knockdown of MNK2 also increases the expression of HSL (hormone-sensitive lipase) which increases the breakdown (lipolysis) of stored triglyceride. Herein, a model in which the knockdown of MNK2 in vitro accounts for a significant reduction in adipocyte lipid accumulation is proposed. These findings identify MNK2 as a regulator of adipocyte metabolism and reveal some of the underlying MNK2-driven mechanisms that promote adipose tissue expansion. The development of an MNK2-targeted therapy may therefore be a useful intervention for reducing weight caused by excessive nutrient intake.