ORCID Profile
0000-0003-1905-8010
Current Organisations
University of Cambridge
,
Nanyang Technological University
,
National University of Singapore
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Publisher: Wiley
Date: 08-06-2011
Abstract: Identifying the substrates and biochemical pathway regulated by phosphatases has always been more challenging than finding those regulated by kinases. Here, we report the use of phosphoproteomic methods to analyse the pathways regulated by POPX2 (partner of PIX 2) phosphatase. POPX2 is a serine/threonine phosphatase, found in many cancer types. The levels of the POPX2 have been found to be up-regulated in the more invasive breast cancer cells compared with non-invasive ones. Our observations also suggest that POPX2 level is positively correlated with cell motility. Thus, finding substrates or pathways regulated by POPX2 will help to elucidate the regulatory mechanism of cancer cell motility and invasiveness. We have also developed and validated a protocol using electrostatic repulsion-hydrophilic interaction chromatography (ERLIC) to enrich the phosphopeptides followed by LC-MS/MS to allow comparison between the phosphoproteomes of control and POPX2 overexpressing cells. With this approach, we were able to identify a biochemical pathway through which POPX2 exerts its apparent cellular function: the regulation of activity of glycogen synthase kinase-3, which in turn modulates extracellular signal-regulated kinase and cell motility.
Publisher: The Company of Biologists
Date: 15-02-2008
DOI: 10.1242/JCS.013557
Abstract: Rho GTPases and their downstream effectors regulate changes in the actin cytoskeleton that underlie cell motility and adhesion. They also participate, with RhoA, in the regulation of gene transcription by activating serum response factor (SRF)-mediated transcription from the serum response element (SRE). SRF-mediated transcription is also promoted by several proteins that regulate the polymerization or stability of actin. We have previously identified a family of PP2C phosphatases, POPXs, which can dephosphorylate the CDC42/RAC-activated kinase PAK and downregulate its enzymatic and actin cytoskeletal activity. We now report that POPX2 interacts with the formin protein mDia1 (DIAPH1). This interaction is enhanced when mDia1 is activated by RhoA. The binding of POPX2 to mDia1 or to an mDia-containing complex greatly decreases the ability of mDia1 to activate transcription from the SRE. We propose that the interaction between mDia1 and POPX2 (PPM1F) serves to regulate both the actin cytoskeleton and SRF-mediated transcription, and to link the CDC42/RAC1 pathways with those of RhoA.
Publisher: American Chemical Society (ACS)
Date: 26-04-2013
DOI: 10.1021/PR301113C
Abstract: POPX2 is a serine/threonine phosphatase belonging to the protein phosphatase 2C (PP2C) family that has been found to be elevated in invasive breast cancer cells. Silencing of POPX2 results in lower cell motility and invasiveness. The molecular mechanism of POPX2-regulated cell motility is not well understood. To identify the relevant signaling pathways, we investigated the transcriptome and proteome of POPX2-knockdown MDA-MB-231 breast cancer cells. Our data suggest that POPX2 might be involved in the regulation of focal adhesions and cytoskeleton dynamics through the regulation of MAP kinase (MAPK1/3) and glycogen synthase kinase 3 (GSK3α/β) activities. Silencing POPX2 alters phosphorylation levels of MAPK1/3 and GSK3α/β and results in reduced activity of these kinases. Both MAPK and GSK3 are known to regulate the activities of transcription factors. MAPK1/3 are also implicated in the phosphorylation of stathmin. The level of phospho-stathmin was found to be lower in POPX2 knockdown cells. As phosphorylation of stathmin inhibits its microtubule severing activity, we observed less stable microtubules in POPX2 knockdown cells. Taken together, our data suggest that POPX2 might regulate cell motility through its regulation of the MAPK1/3, leading to changes in the cytoskeleton and cell motility.
Publisher: Elsevier BV
Date: 02-2002
DOI: 10.1016/S0960-9822(02)00652-8
Abstract: The Rho GTPases are involved in many signaling pathways and cellular functions, including the organization of the actin cytoskeleton, regulation of transcription, cell motility, and cell ision. The p21 (Cdc42/Rac)-activated kinase PAK mediates a number of biological effects downstream of these Rho GTPases (reviewed by [1]). The phosphorylation state of mammalian PAK is highly regulated: upon binding of GTPases, PAK is potently activated by autophosphorylation at multiple sites, although the mechanisms of PAK downregulation are not known. We now report two PP2C-like serine/threonine phosphatases (POPX1 and POPX2) that efficiently inactivate PAK. POPX1 was isolated as a binding partner for the PAK interacting guanine nucleotide exchange factor PIX. The dephosphorylating activity of POPX correlates with an ability to block the in vivo effects of active PAK. Consonant with these effects on PAK, POPX can also inhibit actin stress fiber breakdown and morphological changes driven by active Cdc42(V12). The association of the POPX phosphatases with PAK complexes may allow PAK to cycle rapidly between active and inactive states it represents a unique regulatory component of the signaling pathways of the PAK kinase family.
Publisher: American Chemical Society (ACS)
Date: 28-12-2016
DOI: 10.1021/ACS.JPROTEOME.6B00748
Abstract: Cancer metastasis is a complex mechanism involving multiple processes. Previously, our integrative proteome, transcriptome, and phosphoproteome study reported that the levels of serine/threonine phosphatase POPX2 were positively correlated with cancer cell motility through modulating MAPK signaling. Surprisingly, here we found that POPX2 knockdown cells induced more numerous and larger tumor nodules in lungs in longer term animal studies. Interestingly, our analysis of DNA microarray data from cancer patient s les that are available in public databases shows that low POPX2 expression is linked to distant metastasis and poor survival rate. These observations suggest that lower levels of POPX2 may favor tumor progression in later stages of metastasis. We hypothesize that POPX2 may do so by modulation of angiogenesis. Secretome analysis of POPX2-knockdown MDA-MB-231 cells using LC-MS/MS-based SILAC quantitative proteomics and cytokine array show that silencing of POPX2 leads to increased secretion of exosomes, which may, in turn, induce multiple pro-angiogenic cytokines. This study, combined with our previous findings, suggests that a single ubiquitously expressed phosphatase POPX2 influences cancer metastasis via modulating multiple biological processes including MAPK signaling and exosome cytokine secretion.
Publisher: Elsevier BV
Date: 03-2008
DOI: 10.1016/J.BBAMCR.2007.10.024
Abstract: Hepatitis B virus (HBV) is a causative agent for liver diseases including hepatocellular carcinoma. Understanding its interactions with cellular proteins is critical in the elucidation of the mechanisms of disease progression. Using a cell-based HBV replication system, we showed that HBV replication in HepG2 cells resulted in a cellular morphological changes displaying membrane rufflings and lamellipodia like structures reminiscent of cells expressing constitutively activated Rac1. We also showed that activated Rac1 resulted in increased viral replication. HBV replication specifically activated wild type Rac1, but not Cdc42. The Rac1 activation by HBV replication also resulted in the phosphorylation of ERK1/2 and AKT, the downstream targets of Rac1 signaling cascade. The smallest HBV viral protein, HBX, was able to activate the endogenous Rac1 and induce membrane ruffling when transfected into cells. Significantly, HBX was found to directly interact with a Rac1 nucleotide exchange factor (betaPIX) through a SH3 binding motif. Taken together, we have shown the interaction of HBV with the Rho GTPase, affecting cell morphology through the Rac1 activation pathway. HBV may possibly make use of an activated Rac1 signaling pathway for increased replication and resultant metastatic effects.
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Cheng Gee Koh.