Phosphatase Subfamily DSP6
DSP6 is a metazoan cytoplasmic MKP subfamily that selectively dephosphorylates ERK.
DSP6 is found throughout metazoa. It duplicated in vertebrates. The three human members are DUSP6 (MKP3), DUSP7 (MKPX) and DUSP9 (MKP4).
DSP6 has two domains: rhodanese domain and phosphatase domain.
The rhodanese domain inhibit phosphatase domain activity in DUSP6 , which is achieved by the binding of rhodanese domain and phosphatase domain. The binding stabilizes the inactive conformation of the phosphatase catalytic site . The rhodanese domain also mediates interaction with MAP kinases (often ERK) (via kinase interaction motif?). Its binding to MAP kinases induces conformation change in phosphatase domain, which can increase the phosphatase activity .
Rhodanese domain of DSP6 also has two conserved Leu-rich nuclear export signals  (particular Figure 5 and Figure 11).
The rhodanese domain was lost in many nematodes (see technical notes).
DUSP6 preferentially dephosphorylates ERK [5, 6, 7], due to ability of DUSP6 to bind ERK but not p38 or JNK. The interaction is mediated by rhodanese domain (or kinase interaction motif embedded in rhodanese domain?) . Later study has shown DUSP6 is ERK1/2-specific, as it does not inactive ERK5 .
Furthermore, DUSP6, ERK2, and phosphorylated p38alpha can form a stable ternary complex in solution, and the phosphatase activity of DUSP6 toward p38alpha substrate is allosterically regulated by ERK2-DUSP6 interaction. This suggests that DUSP6 may mediate cross-talk between ERK and p38 pathways . (note: this is perhaps the mechanism behind DUSP6 modules DNA damage response .)
- Significant loss of DUSP6 was observed in 100% of 11 esophageal squamous cell carcinoma cell lines and 71% of seven nasopharyngeal carcinoma cell lines .
- DUSP6 plays tumor suppressive role in non-small-cell lung cancers .
- DUSP6 expression was correlated with lower histological grade and lower Ki-67 index in the lung adenocarcinomas .
- DUSP6 expression was down-regulated through hypermethylation at enhancer in some pancreatic cell lines and pancreatic cancer tissues [17, 18].
- Degradation of DUSP6 caused by reactive oxygen species (ROS) leads to aberrant ERK1/2 activation and contributes to tumorigenicity and chemoresistance of human ovarian cancer cells .
- DUSP6 upregulation induced by angiotensin II mediates endothelial cell apoptosis .
But, DUSP6 is up-regulated or not associated with cancers in some cases:
- DUSP6 was up-regulated in endometrial adenocarcinomas , thyroid carcinoma [22, 23], and glioblastomas , MCF-7 breast cancer cells .
- DUSP6 methylation is a rare event in endometrial cancer. Thus, silencing of the DUSP6 phosphatase is unlikely to contribute to constitutive activation of the ERK kinase cascade in endometrial cancer .
- DUSP6 expression was not correlated with Ki-67 index lung squamous cell carcinomas .
(note: DUSP6 upregulation in cancer could be explained by feedback mechanisms?)
DUSP6 (MKP3) inhibits brown adipocyte differentiation perhaps via regulation of Erk phosphorylation .
Like DUSP2, DUSP4 and DUSP4 of DSP1 subfamily, DUSP6 is phosphorylated by ERK . However, the phosphorylation sites are different. DUSP6 is phosphorylated at serines 159 and 197 , which are found in DUSP6 and DUSP7, but not DUSP9 or other DSPs.
DUSP6 expression is regulated by:
- p53. There are two p53 binding sites in DUSP6 promoter . p53 binds to promoter of DUSP2 and DUSP5 of DSP1 subfamily.
- ETS1, a transcription factor that can be activated by Erk2 and Ras at Thr38  (note: a feedback of ERK2, DUSP6, ETS1).
- Wilms tumor protein (WT1), a transcription factor as tumor suppressor, up-regulates DUSP6 expression .
- Nitric oxide down-regulates DUSP6 mRNA levels  (note: which pathway?). Read here for NO's biological function.
DUSP7 is constitutively expressed in a wide variety of human cell lines. DUSP7 is predominantly cytosolic when expressed in COS-1 cells. In common with other members of DSP6 subfamily, DUSP7 shows substrate selectivity ERK > p38 = JNK. DUSP7 binds ERK in vivo. Both ERK and JNK activate DUSP7 phosphatase activity in vitro .
DUSP6 blocks activation of MAP kinases with the selectivity ERK > p38 = JNK. Same as other members in the subfamily, it locates in cytosol [36, 37]. DUSP9 is unique among these cytoplasmic MKPs in containing a conserved PKA consensus phosphorylation site (55)RRXSer-58 immediately adjacent to the kinase interaction motif. DUSP9 is phosphorylated on Ser-58 by PKA in vitro, and phosphorylation abrogates the binding of DUSP9 to both ERK2 and p38alpha MAP kinases .
Decreased expression of DUSP-9 is associated with poor prognosis in clear cell renal cell carcinomas .
Nematodes lost rhodanese domain
We observed the absence of rhodanese domain in C. elegans. We then asked whether the loss is conserved, which can be use to measure the reliability of the loss. We obtained DSP6s from our internal orthology database, which has 203 eukaryotic genomes and 9 nematode genomes. We then searched Pfam domain in the DSP6s using Pfam web server (E-value cutoff 1.0). We found none of the 10 nematode DSPs from 7 nematode genomes has rhodanese domain. The DSP6 subfamily is not found in another 2 nematode genomes: Pristionchus pacificus and Loa loa.
We also BLASTed human DUSP6 against nematode NR protein data set. We found rhodanese-containing DSP6 in most Trichocephalida, which suggest the lost in happened posterior to Trichocephalida diverged from other nematodes.
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