Regulation of Endogenous Dusp4 by Agonist‐Specific MAPK Activation in Adipocytes
Authors: Guo Wei , Ferguson Bradley S. , Morrison Ron F. 
Published: November 2023

Activation of the ERK, JNK and p38 mitogen activated protein kinases (MAPKs) by different extracellular stimuli leads to a variety of cellular outcomes, such as proliferation, inflammation, differentiation and apoptosis. The signal specificity is shown to be dependent upon the duration, magnitude and localization of MAPK activity. While MAPK activation by upstream signaling is well studied, alteration of biological outcomes by MAPK deactivation is still largely unknown. Growing evidence now supports potential roles for MAPK‐specific, dual‐specificity phosphatases (DUSPs) in the deactivation of MAPK signaling and modulation of obesity‐associated metabolic dysfunction. We initially examined the expression profile of the 10 known DUSPs in insulin‐responsive tissues of genetic‐ and diet‐induced obese mice. We previously observed that dusp4 was significantly more abundant in white adipose tissue relative to liver, skeletal muscle, and heart in lean mice. Dusp4 was induced during early stages of obesity and repressed during later stages correlating with the onset of obesity‐induced inflammation. Since ectopic expression has been shown to artificially shift the balance in protein subcellular localization and possibly affect its function, the objective of this investigation was to elucidate regulatory mechanisms of endogenous dusp4 by ERK, JNK and/or p38 activity in adipocytes. We first investigated induction of dusp4 with different MAPK agonists: TNF‐α, hydrogen peroxide (H2O2), Oncostatin M (OSM), ultraviolet (UV) irradiation, differentiation inducer MDI and phorbol 12‐myristate 13‐acetate (PMA). We demonstrated that MDI lead to immediate, robust phosphorylation of ERK that peaked within 10 mins and decayed to basal levels by 6 hrs. Endogenous dusp4 mRNA and protein levels kinetically increased in a feedback manner where expression was inversely proportional to ERK phosphorylation. PMA stimulation resulted in robust and sustained ERK phosphorylation up to 8 hours, leading to a stronger induction of endogenous dusp4 compared to MDI. Treatment with the ERK inhibitor U0126 suppressed ERK phosphorylation and abolished dusp4 expression under both treatments. However, TNF‐α, H2O2, OSM and UV irradiation were not able to induce dusp4 due to transient ERK activation. In addition, sustained ERK phosphorylation by PMA highly stabilized dusp4 protein, while blocking ERK activity resulted in a much shortened dusp4 half‐life. Interestingly, we further observed the level of PMA‐induced phospho‐ERK did not decrease following induction of dusp4. Collectively, these results demonstrate 1) induction of endogenous dusp4 is tightly regulated by the duration and magnitude of ERK activity; 2) ERK‐induced dusp4 expression by MDI supports a negative feedback model that controls ERK phosphorylation; 3) phospho‐ERK is also involved in dusp4 protein stability; 4) dusp4 may modulate ERK activity in a compartment‐dependent manner that, as a nuclear phosphatase, it is not able to regulate cytosolic ERK activity. Future studies are underway to examine the function of dusp4 in regulating ERK mediated biological outcomes in adipocytes.Support or Funding InformationSupported by NIH‐NIDDK (R15‐DK082799).