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Dual-specificity phosphatase 9 (DUSP9) belongs to the threonine/tyrosine dual-specific phosphatase family and was first described in 1997, is known to dephosphorylate ERK1/2, p38, JNK and ASK1, and thereby to control various MAPK pathway cascades. As a consequence, DUSP9 plays a major role in human pathologies and more specifically in cardiac dysfunction, liver metabolic syndromes, diabetes, obesity and cancer including drug response and cell stemness.
The mitogen-activated protein kinase (MAPK) signaling pathways are crucial in cell function and homeostasis. MAPKs regulate pathophysiological processes by controlling signal translation and cellular response such as survival, proliferation, differentiation and migration [1][2][3]. They are activated by a double phosphorylation process on tyrosine and threonine residues in a conserved Thr-X-Tyr motif (X being any amino acid) [4][5]. Activation of MAPK pathways triggers multiple intracellular signaling cascades. Each cascade is initiated by a specific signal and leads to the activation of a particular MAPK [6]. Once activated, MAPK can phosphorylate various cytoplasmic and/or nuclear substrates and induce changes in the function of target proteins and gene expression [6]. Spatial localization of MAPKs also determines the target substrates and subsequent cellular effects [7].
Phosphatases reverse phosphorylation and return MAPKs to an inactive state. The dual specificity phosphatase (DUSP) family belongs to the 199 phosphatases encoded in the human genome. This family is composed of 61 phosphatases capable of downregulating MAPKs by dephosphorylating both tyrosine and serine/threonine residues in a single substrate [8][9]. The phosphorylation of proteins is a reversible process. This prevents the abnormal activation of the signal and fine-tunes its activity and downstream effects [3][8]. The balance between phosphorylation and dephosphorylation controls the expression, function, activity and localization of many proteins [8][10]. Dephosphorylation by DUSPs regulates the duration, intensity and spatiotemporal profile of the MAPK signaling cascade [11]. This dephosphorylation takes place thanks to the highly conserved phosphatase site which contains arginine, cysteine and aspartic acid [3][8][12]. In addition to the active site common to all DUSPs, some DUSPs contain a MAP kinase-binding motif (MKB), also called a kinase-interacting motif (KIM), which interacts with the common docking domain of MAPKs to allow the interaction between the enzyme and the substrate [3][8][12][13]. Ten DUSPs containing the KIM domain are classified as typical DUSPs or MAP kinase phosphatases (MKPs) (Table 1), while those which do not have this domain (16 phosphatases in total) are called atypical DUSPs [3][5][14]. However, there are a few exceptions. DUSP2, DUSP5 and DUSP8 are typical DUSPs and contain the KIM domain but they are not called MKPs. On the other hand, DUSP14 and DUSP26, which are atypical DUSPs and do not contain a KIM domain, are called MKP6 and MKP8, respectively (Table 1) [3][14]. Typical DUSPs are the best characterized within the DUSP family [14] and this comprises the typical DUSP named DUSP9 or MKP4, which was first described in 1997 by Muda and collaborators [12]. This 42-kDa protein dephosphorylates several substrates including JNK, p38, the MAPKKK apoptosis signal-regulating kinase 1 (ASK1) and ERK1/2 with a high specificity for ERK kinases [3][12].
Classification | Gene Symbol | Synonyms | Chromosomal Localization | Cell Localization | MAPK Substrates (Others) | Inducible by MAPKs | Main Functions in Physiological and Pathophysiological States |
---|---|---|---|---|---|---|---|
Typical MKPs | DUSP1 | MKP1 | 5 | Nuclear | JNK, p38 > ERK | ERK, p38 | Involved in infectious diseases, pulmonary diseases, inflammatory disorders, atherosclerosis, tumorigenesis and tumor progression [15]. |
DUSP2 | PAC1 | 2 | Nuclear | ERK, JNK, p38 | ERK, JNK | Involved in immune and inflammatory responses, cancer, CLN3 disease and endometriosis [16]. | |
DUSP4 | MKP2 | 8 | Nuclear | ERK, JNK > p38 | ERK | Involved in inflammatory cytokine secretion, susceptibility to sepsis shock, and resistance to Leishmania mexicana infection [17][18]. | |
DUSP5 | hVH3 | 10 | Nuclear | ERK | ERK | Plays an anti-inflammatory role and has tumor suppressive functions in several types of cancer [19]. | |
DUSP6 | MKP3 | 12 | Cytoplasmic | ERK | ERK | Plays a role in carcinogenesis in several cancers as an oncogene or a tumor suppressor [20]. | |
DUSP7 | MKPX | 3 | Cytoplasmic | ERK, JNK, p38 | N/D | Involved in some cancers [21]. | |
DUSP8 | hVH5 | 11 | Dually-located | ERK, JNK, p38 | N/D | Plays a role in the central nervous system, circulatory system, urinary system, immune system, genetic diseases and cancers [22]. | |
DUSP9 | MKP4 | X | Cytoplasmic | ERK >> p38, JNK | N/D | Involved in development of cardiac dystrophy, metabolic diseases and cancers [23][24][25][26][27]. | |
(MAP3K5/ASK1) | |||||||
DUSP10 | MKP5 | 1 | Dually-located | JNK, p38 >> ERK | N/D | Involved in immune response, anti-inflammatory response and some cancers [28]. | |
DUSP16 | MKP7 | 12 | Dually-located | JNK | N/D | Involved in non-alcoholic steatohepatitis and some cancers [29]. | |
Atypical MKPs | DUSP14 | MKP6 | 17 | Dually-located | ERK, JNK, p38 | N/D | Involved in immune response, bone diseases and cancers [30]. |
DUSP26 | MKP8 | 8 | Nuclear | p38 | N/D | Regulates neuronal cell proliferation and acts as an oncogene or a tumor suppressor depending on the cellular context [31]. |