Abstract

Background: Dual specificity phosphatases (DUSPs) belong to a family of protein tyrosine phosphatases that dephosphorylate serine/threonine and tyrosine residues. In the last decades, DUSPs have been implicated in various physiological and pathological activities. In addition to mitogen-activated protein kinases (MAPKs) as major substrates, other protein, and non-protein substrates can be dephosphorylated by DUSPs. Aberrant regulation of DUSPs has been found in a variety of diseases including cancer, neurological disorders, and renal diseases, suggesting the involvement of DUSPs in disease pathogenesis.

Summary: This article reviews the general characteristics of DUSPs and the progress of research in the field of renal diseases, including diabetic nephropathy, hypertensive nephropathy, chronic kidney disease, acute kidney injury, and lupus nephritis. Since the main biochemical function of DUSPs is to dephosphorylate MAPK activity, reduced DUSPs are found in renal disease models, whereas forced expression of DUSPs reverses disease manifestations, which has been confirmed by transgenic or knockout models.

Keywords: Dual-specificity phosphatases ; Dephosphorylate; Mitogen-activated protein kinases ; Kidney diseases; Cistanche supplements

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Introduction

Protein reversible phosphorylation, an important means of post-translational modification, regulates the biological activity and is involved in various physiopathological processes in eukaryotes. The intracellular homeostasis of protein phosphorylation is maintained by certain protein kinases and protein phosphatases, depending on the intracellular and extracellular environment. Eukaryotes have a variety of protein kinases whose function is to phosphorylate protein substrates at specific amino acid residues. One of the most well-known kinases is the mitogen-activated protein kinase (MAPK) cascade, consisting of a linear array of three kinases. In addition, many protein phosphatases neutralize each other with kinases. Based on structural similarities and their substrates, protein phosphatases can be classified as serine/threonine phosphatases (e.g., PP2A) and protein Tyr phosphatases (e.g., PTP-SL [1]). Among PTPs, dual-specificity phosphatases (DUSPs) have attracted the attention of researchers, and their roles and mechanisms in various disease models have been extensively explored [2].

DUSPs are also known as MAPK-specific phosphatases (MKPs), and the first member, DUSP1/MKP1, was discovered in 1992 [3]. As the name implies, DUSPs possess the biological activity of both serine/threonine phosphatases and Tyr phosphatases. However, their catalytic activity for Tyr phosphatase is much stronger than that of Ser/Thr phosphatase. Therefore, DUSP is included in the superfamily of PTP. In recent years, with the progress of research on DUSPs, the strict regulatory role of cellular reversible phosphorylation has been further recognized, which provides new ideas for the development, progression, and prevention strategies of diseases such as cancer, immune system diseases, and neurological diseases [4,5]. In this review, we will focus on the functions of DUSPs in kidney diseases.

Classification and Functions of DUSPs

Structurally, the catalytic structural domain of DUSPs contains a conserved motif HCxxGxxR [6]. Based on structure, catalytic activity, and substrate, DUSPs can be divided into the following subgroups: (i) MAPKs; (ii) atypical DUSPs; (iii) phosphatases and tensin homologous protein phosphatases; (iv) cell division cycle 14 phosphatases (CDC14s); (v) elastin phosphatases and (vi) phosphatases of the regenerating liver. Among these subgroups, members of the first 2 subgroups have dual specificity for tyrosine and threonine phosphatase functions. Therefore, these members are considered conventional DUSPS (Table 1) and will be discussed in the next section

Mitogen-Activated Protein Kinase

The MAPK pathway regulates many cellular processes, including cell proliferation, differentiation, migration, survival, and apoptosis. Aberrant MAPK signaling is associated with many human diseases [7,8]. the MAPK pathway can be activated by a variety of stimuli, kinases, and other enzymes. MAPK signaling contains a 3-tier cascade including MAP kinase), MAP kinase, and MAPK. there are three main MAPK pathways, extracellular regulated kinases 1 and 2 (ERK1/2), and c- Jun - N-terminal kinases ( The MAPKs pathway is negatively regulated by MKPs, which dephosphorylate active MAPK substrates. To date, there are no clinical trials of MAPK inhibitors or related drugs for the treatment of human disease due to uncertainty of efficacy or the presence of potential adverse effects. Based on the results of current MAPK inhibitor studies, clinical treatments targeting upstream molecules of the MAPK pathway are much better than those targeting downstream molecules. Therefore, MPKs have potential applications in translational medicine.

The MPK family has 10 members. Each member consists of an n-terminal MAPK-binding (MKB) structural domain and a c-terminal conserved DUSP catalytic domain. the kinase interaction motif of the MKB structural domain determines enzyme specificity through docking interactions with MAPK [9]. Notably, both phosphorylated and unphosphorylated MAPKs can bind to MPKs [10]. Upon binding to MAPK, the conformation of MPKs protein is altered, which increases the catalytic activity of MKPs.

According to the cellular distribution of MPKs members, they can be divided into three subgroups: (1) nuclear MPKs, including DUSP1/MKP1, DUSP2, DUSP4/MKP2 and DUSP5; (2) cytoplasmic MPKs, including DUSP6/ MKP3, DUSP7, DUSP9/MKP4; (3) nuclear and cytoplasmic MPKs, including DUSP8, DUSP10/MKP5 and DUSP16/MKP7. although these MPKs have been reported to dephosphorylate MAPKs, the substrates of MPKs have not been precisely defined. the substrate preference of MPKs is related to the nature of the binding site and cell type as well as the scaffolding proteins of MAPKs [11].

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Atypical DUSPs

Atypical DUSP is structurally similar to MPK. These proteins are usually smaller than MPK. Atypical DUSPs contain the DSP catalytic domain but lack the n-terminal MKB structural domain. In addition, some members contain CH2 structural domains, carbohydrate-binding structural domains, and arginine-rich or proline-rich regions. In mammalian tissues, there are approximately 20 members of DUSP3, DUSP11, DUSP12, DUSP13, DUSP14, DUSP15, DUSP18, DUSP19, DUSP21, DUSP22, DUSP23, DUSP26, DUSP27, DUSP28, EPM2A dextran phosphatase, protein tyrosine phosphatase mitochondrial 1, RNA guanylyltransferase and 5 ' -phosphatase, serine/threonine/tyrosine interacting proteins. However, according to phylogenetic analysis [12], these atypical DUSPs are derived from a common ancestor.

As atypical DUSPs lack the specific MKB structural domain of MKPs, their exact substrates remain uncertain. Among these atypical DUSP members, DUSP14/MKP6 and DUSP26/MKP8 have been reported to bind to MAPKs and regulate signaling pathways. In addition to MAKPs, atypical DUSPs can also act on other phosphorylated protein substrates. In addition, they can dephosphorylate non-protein substrates, such as RNA or lipids. For example, DUSP11 has intrinsic phosphatase activity, and its mRNA triphosphatase activity is greater than that of protein phosphatases [13]. In addition, atypical DUSPs can act as scaffolding proteins and facilitate the interaction of signaling proteins [14]. Thus, atypical DUSPs have a wide range of substrate specificity and multiple physiological functions.

Expression and Regulation of DUSPs in Kidney

The major cellular components of the kidney include tubular epithelial cells, thylakoid cells, podocytes, and interstitial cells, which are most frequently studied in models of renal disease. In the glomerulus, the major component is the thylakoid cells, which account for about 30 - 40% of the total cells [15].DUSPs are expressed at different levels in different types of renal cells. Renal tubular epithelial cells are the most abundant cell type in the kidney and these genes, including DUSP1/MKP1, DUSP4/MKP2, and DUSP7, have been reported to be expressed in tubular cells in vivo under various renal disease models or in immortalized cell lines such as HK-2. In addition, DUSP1/ MKP1 and DUSP10/MKP5 are expressed in renal thylakoid cells, and DUSP4/MKP2 and DUSP6/MKP3 are expressed in podocytes [16,17]. In addition, vascular cells in the kidney, such as vascular smooth muscle cells (VSMCs), also express DUSPs.

DUSP1/MKP1 and DUSP5 have been reported to be expressed in VSMCs and may play a role in hypertension-related kidney disease. Notably, resident immune cells in the kidney such as t cells, b cells, and monocytes can express DUSPs [18,19]. Although the expression of some members of DUSPs has been revealed and compared under these disease models, the gene expression of most members is unknown. Furthermore, the exact expression levels of DUSPs are not known. Recently, single-cell sequencing has offered the possibility to compare the expression levels of individual genes. Based on the Human Protein Atlas data, the expression levels of DUSPs can be arbitrarily classified into four groups. dUSP23 belongs to the high expression group, DUSP1/MKP1, DUSP6/MKP3, DUSP3, DUSP11, DUSP15, DUSP24/serine/threonine/tyrosine interacting protein l1, protein tyrosine phosphatase mitochondrial 1 belong to the moderate expression group. DUSP13, DUSP21, and DUSP27 are not expressed in the kidney, and the members on the left belong to the low-expression group. Therefore, the gene expression of DUSPs needs to be further investigated, especially in disease models.

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Gene expression and phosphatase activity of DUSPs can be regulated by gene transcription, translational regulation, protein modification, or protein stability. Aberrant expression of DUSPs has been found in different models in which the regulation of DUSPs may occur at the transcriptional level. For example, both DUSP1/MKP1 protein and mRNA levels were downregulated in diabetic kidney tissue [20]. Similarly, diabetes and high glucose exposure reduced DUSP4/MKP2 expression at the transcriptional level in cultured foot cells and glomeruli, which resulted in enhanced p38 and JNK activity.

As expected, the transcription of DUSPs can be regulated by epigenetic means such as DNA methylation, in addition to other important transcription factors. Currently, DNA methylation usually represents an epigenetic marker that represses gene expression. hypermethylation of the promoter region of DUSPs leads to reduced expression, which has been demonstrated in many cell types. Significantly reduced expression of DUSP1/MKP1 in breast cancer cell lines and invasive breast tumors was found to be associated with DNA methylation by methylation-specific PCR analysis, and DUSP1/MKP1 promoter methylation may be a potential breast cancer biomarker for breast malignancies [21]. Similarly, Tögel et al [22] found that DUSP5 was methylated in colorectal cancers with high CMP, although this epigenetic change by itself could not explain the reduced DUSP5 expression in cancer cells. In addition to DNA methylation, histone modifications are also involved in the regulation of DUSPs. Modifications include histone acetylation, methylation, and phosphorylation, and the effect on gene expression depends on the nature of the histone modifications. Recently, Hofmann et al. [23] demonstrated that CREMα can regulate DUSP4 expression in effector T cells through p300-induced histone acetylation at the DUSP4 promoter. In a renal disease model, Coit et al. [19] analyzed genome-wide DNA methylation changes in naive CD4+ T cells from lupus patients with or without renal involvement and healthy controls and found DUSP5 hypermethylation.DUSP5 demethylation may lead to defective ERK signaling pathways in lupus T cells. Recent studies on the epigenetic regulation of DUSPs in kidney disease are relatively underdeveloped. More attention should be given to this area of research.

In addition, the expression of DUSPs can be regulated at the post-transcriptional or translational level. Non-coding RNAs play an important role in the regulation of DUSPs. These RNAs, such as miRNAs and long-stranded non-coding RNAs (lncRNAs), can act directly or indirectly. Usually, miRNAs can act by directly targeting DUSP mRNA, for example, miR-107 was shown to target the 3 ' UTR of DUSP7 in endothelial cells [24], and DUSP4 is a downstream target of miR-122-5p [25]. These miRNAs result in reduced translation of DUSPs proteins. Unlike miRNAs, lncRNA can act indirectly by binding to miRNAs or recruiting other proteins. Recently, lncRNA AZIN1 was found to downregulate miR- 513b-5p by sponging miR- 513b-5p, which in turn targets DUSP11 in tumor tissues. more interestingly, another study showed that lncRNA CASC9 can recruit histone methyltransferase EZH2 to epigenetically regulate DUSP1/ MKP1 expression. These studies revealed the mechanisms of translational regulation of DUSPs.

Finally, the regulation of DUSPs can also occur at the protein level.DUSP proteins can be acetylated, phosphorylated, methylated, or ubiquitinated. These modifications can affect their biological activity or protein stability. detailed regulation of DUSPs at the protein level can be found in other reviews [28].

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DUSPs in Kidney Disease

The role of DUSPs has been studied in renal disease models such as diabetic nephropathy (DN), hypertensive nephropathy, chronic kidney disease (CKD), acute kidney injury (AKI), and lupus nephritis (LN) with a focus on DUSPs. Since the main biochemical function of DUSPs is to dephosphorylate MAPKs activity, reduced DUSPs were found in renal disease models, whereas forced DUSPs expression reversed the disease manifestations. At the molecular level, most studies have focused on the regulation of MAPKs by DUSPs. Recent advances have revealed a clinical translation of such renal diseases based on DUSPs.

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From Haiyang Li; Jiachuan Xiong; Yu Du; Yinghui Huang; Jinghong Zhao.

Department of Nephrology, The Key Laboratory for the Prevention and Treatment of Chronic Kidney Disease of Chongqing, Chongqing Clinical Research Center of Kidney and Urology Diseases, Xinqiao Hospital, Army Medical University (Third Military Medical University), Chongqing, PR China