Regulation of the Catalytic Activity of SAP-1 and its Posttranscriptional Modification
Protein tyrosine phosphatases (PTPs) are enzymes that can catalyze the dephosphorylation of their tyrosine-phosphorylated substrates. Dimerization of RPTPs by extracellular ligands and changes in oxidation state are known to contribute to the modulation of their catalytic activity (den Hertog et al. 2008). Although extracellular ligands for SAP-1 have not been identified yet, SAP-1 catalytic activity has been shown to be modulated by reversible dimerization, which could be controlled by the redox state of the extracellular environment (Walchli et al. 2005). Indeed, SAP-1 overexpressed in cultured cells is present as a homodimer and this dimerization is mediated by the extracellular domain. In contrast, the homodimer state of SAP-1 is reduced by exposure of SAP-1-overexpressing cells to a reducing agent. Moreover, the deletion of the extracellular domain of SAP-1 results in an increase in PTP activity. Therefore, the homodimer formation of SAP-1 is likely to attenuate the catalytic activity. However, how SAP-1 dimerization is regulated under physiological conditions remains unknown.
Physiological and Pathological Roles of SAP-1
Overexpression of SAP-1 has been reported to inhibit the proliferation of cultured cells through attenuation of growth factor-induced activation of MAPK or through induction of caspase-dependent apoptosis (Noguchi et al. 2001; Takada et al. 2002). SAP-1 is also involved in the regulation of the reorganization of the actin-based cytoskeleton in cultured cells through dephosphorylation of several focal adhesion-associated proteins, such as focal adhesion kinase, p130cas, and paxillin (Noguchi et al. 2001). Moreover, PTP52F, which is enriched in the midgut tissue of prepupal flies, has been demonstrated to mediate the dephosphorylation of transitional endoplasmic reticulum ATPase (TER94), a regulator of the ubiquitin proteasome system, thereby regulating the destruction of the larval midgut during metamorphosis via enhancement of autophagic and apoptotic cell death (Santhanam et al. 2014). Given the predominant expression of SAP-1 in gastrointestinal epithelial cells and its localization to the microvilli of these cells (Sadakata et al. 2009), these findings suggest that SAP-1 plays a role in the growth and survival of gastrointestinal epithelial cells as well as in the maintenance of microvillus architecture. However, ablation of SAP-1 in mice did not result in marked changes in the proliferation and survival of IECs or the morphology of IECs, including that of microvilli or of tight or adherens junctions between these cells (Sadakata et al. 2009). The expression level of SAP-1 in the mouse intestine is also minimal during embryonic development and increases markedly after birth (Sadakata et al. 2009). Thus, these results suggest that mouse SAP-1 is dispensable for the determination of cell growth, survival, and architecture of IECs, although such functions of SAP-1 might be complemented by other RPTPs, including DEP-1 expressed in gastrointestinal epithelial cells as well as in hematopoietic cells and endothelial cells. By contrast, with use of SAP-1-deficient mice, this RPTP has been shown to be involved in the modulation of tumor formation in the intestine (Sadakata et al. 2009). Recently, SAP-1 was also demonstrated to play an important role in the regulation of intestinal immunity (Murata et al. 2015).
The expression of the SAP-1 protein or its mRNA has been shown to be increased in human colorectal adenocarcinomas (Seo et al. 1997) or nonsmall cell lung carcinomas (NSCLCs) (Sato et al. 2015). Overexpression of SAP-1 was observed in 2 of 17 (11.8%) adenomas with moderate or severe dysplasia and in 19 of 48 (40%) adenocarcinomas (Seo et al. 1997). The frequency of SAP-1 expression in well-differentiated adenocarcinomas was higher than that in moderately and poorly differentiated adenocarcinomas combined. It is thus possible that the expression level of SAP-1 is frequently increased in human colorectal cancers and that such an increase in the SAP-1 expression level occurs relatively late in the adenoma-carcinoma sequence. The expression level of SAP-1 mRNA in the tumor tissue of patients with nonsmall cell lung carcinoma is also markedly elevated compared with that in adjacent normal tissue and is correlated with smoking status but not with other clinicopathological parameters such as age, gender, histological type, and pathological TNM stage (Sato et al. 2015). Such an increase in the expression of SAP-1 mRNA in tumor tissue is also correlated with the reduction in DNA methylation at a single CpG site located in SAP-1 intron 1. Moreover, the treatment of lung cancer cell lines with low SAP-1 expression with 5-aza-2′-deoxycytidine, a DNA methylation inhibitor, results in the upregulation of the gene expression. Thus, increased expression of SAP-1 mRNA in NSCLC might be attributable to SAP-1 DNA hypomethylation. However, whether overexpression of SAP-1 has relevance to the development and progression of human colorectal adenocarcinoma or NSCLC remains to be determined.
Of note is that ablation of SAP-1 attenuates tumorigenesis in Apcmin/+ mice, which harbor a heterozygous mutation of the adenomatous polyposis coli (APC) gene and develop adenoma in the colon (Sadakata et al. 2009). The number of large adenomas in SAP-1-deficient mice is markedly decreased, whereas the number of small adenomas is similar to that in Apcmin/+ mice, suggesting that SAP-1 is involved in tumor expansion but not in the initial transformation of normal epithelial cells into dysplastic cells. APC has been shown to be a negative regulator of Wnt signaling, which is implicated in tumorigenesis in the intestine (Clarke 2006). Functional impairment of the APC protein in Apcmin/+ mice causes stabilization and marked accumulation of β-catenin, which initiates transformation of normal epithelial cells and promotes tumorigenesis as a result of the constitutive activation of the β-catenin/transcription factor 4 (TCF4) pathway (Clarke 2006). However, SAP-1-deficient Apcmin/+ mice exhibit no difference from Apcmin/+ mice in the cytoplasmic and nuclear accumulation of β-catenin in adenomas (Sadakata et al. 2009). Therefore, SAP-1 is unlikely to participate in the regulation of the β-catenin/TCF4 pathway itself. Instead, given that VE-PTP and DEP-1 are thought to activate SFKs (Chabot et al. 2009; Mori et al. 2010), SAP-1 might contribute to the promotion of intestinal cell proliferation through SFK activation.
In contrast, the expression level of SAP-1 has been shown to be decreased in advanced or moderately differentiated human hepatocellular carcinoma (HCC), whereas the expression level in well-differentiated HCC is similar to the surrounding noncancerous tissue (Nagano et al. 2003). Overexpression of SAP-1 in HCC cell lines, which were derived from human poorly differentiated HCC, results in a change in morphology and a marked reduction in both migratory activity and growth rate. Given that tissue invasion and metastasis are characteristic features of advanced HCC and are critically regulated by cell motility, it is thus possible that SAP-1 contributes, at least in part, to the modulation of advanced HCC to metastasize and invade tissue. However, how SAP-1 expression is regulated in human HCC during its de-differentiation remains unclear. SAP-1 also might have different roles in tumor development and progression in the different types and stages of cancers.
In Intestinal Immunity
IECs provide a physical barrier that protects the body from the external environment, which includes the vast array of microbes present in the intestinal lumen (Peterson and Artis 2014). These cells also produce a variety of mucus and antimicrobial peptides, which prevent the growth of pathogenic microbes, as well as cytokines and chemokines (Peterson and Artis 2014). These epithelial functions are thought to play an important role in the regulation of immune responses in the intestinal mucosa (Peterson and Artis 2014). Ablation of SAP-1 in IL-10-deficient mice (Sap1−/−Il10−/− mice), an inflammatory bowel disease (IBD) model, exaggerates the severity of spontaneous colitis compared with that observed in IL-10-deficient mice, with increased expression of mRNAs for various inflammatory cytokines and chemokines, such as tumor necrosis factor α (TNF-α), interferon γ (IFN-γ), interleukin (IL)-6, keratinocyte-derived chemokine (KC), and macrophage inflammatory protein 2 (MIP-2) in the colon (Murata et al. 2015). Therefore, SAP-1, together with IL-10, protects against the development of colitis through regulation of intestinal immune responses.
SAP-1 is a member of the R3-subtype RPTP family characterized by the fibronectin type III-like domain in the extracellular region and a single catalytic domain in the cytoplasmic region. Mouse SAP-1 is specifically expressed in epithelial cells in the gastrointestinal tract and localized at the apical side of these cells. The catalytic activity of SAP-1 and its interaction with other signaling molecules are likely to be modulated by the redox state of the extracellular environment and posttranslational modifications, such as protein tyrosine phosphorylation. Studies using cultured cells suggest that SAP-1 is involved in the regulation of cellular functions, including cell proliferation, cell survival, changes in cell morphology, and cell adhesion, through dephosphorylation of its substrates. The expression level of the SAP-1 protein or its mRNA has also been demonstrated to be increased or decreased in different types of tumors, suggesting that SAP-1 possesses protumorigenic and antitumorigenic activity. Consistent with the protumorigenic roles of SAP-1, a study using SAP-1 knockout mice revealed that SAP-1 was involved in the promotion of tumor expansion in the intestine of Apcmin/+ mice. Moreover, SAP-1 has also been shown to act as a modulator of intestinal immunity, at least in part, thought the regulation of chemokine production by IECs and to contribute to the protection against colitis together with IL-10. Therefore, SAP-1 might be a potential therapeutic target of gastrointestinal cancers or IBD.
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