Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi


Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101831


Historical Background

SAP-1 (also known as PTPRH) was originally identified as a receptor-type protein tyrosine phosphatase (RPTP) expressed in a human stomach cancer cell line (Matozaki et al. 1994). This RPTP belongs to the class II subfamily of RPTPs and is further classified as a member of the R3-subtype of RPTPs, such as DEP-1, VE-PTP, and PTPRO, which are characterized by fibronectin type III-like domains in the extracellular region and a single catalytic domain in the cytoplasmic region (Fig. 1a) (Matozaki et al. 2010). The SAP-1 protein contains eight (human) or six (mouse) fibronectin type III-like domains in its extracellular region (Fig. 1b) (Matozaki et al. 1994; Sadakata et al. 2009). In Drosophila melanogaster, PTP10D, PTP4E, and PTP52F are identified as R3-subtype RPTP family members and PTP52F is thought to be a potential ortholog of SAP-1 (Fig. 1b) (Santhanam et al. 2014). The expression of each R3-subtype RPTP in mammals is largely restricted to a single or limited number of cell types (Matozaki et al. 2010). SAP-1 is expressed in epithelial cells in the gastrointestinal tract. VE-PTP and DEP-1 are enriched in endothelial or hematopoietic cells. PTPRO is expressed in neurons and in podocytes of the renal glomerulus. In addition, SAP-1, as well as other members of the R3-subtype RPTP family, is localized specifically at the apical surface of polarized cells. Indeed, mouse SAP-1 is present at the apical surface of epithelial cells in the small intestine and colon (Fig. 2) and localizes to the microvilli of the brush border (Sadakata et al. 2009).
PTPRH, Fig. 1

Structure of SAP-1 and other R3-subtype RPTPs. (a) Structural organization of R3-subtype RPTPs, SAP-1, VE-PTP, DEP-1, and PTPRO. All of these enzymes share a similar structure, with a single catalytic (PTP) domain in the cytoplasmic region and fibronectin type III (FN III)-like domains in the extracellular region. SAP-1, DEP-1, and PTPRO contain eight or fewer type III-like domains in the extracellular region, whereas VE-PTP contains 16 or 17 such domains. PTP, protein tyrosine phosphatase. (b) Schematic representation of the structures of human and mouse SAP-1 and drosophila PTP52F, a potential ortholog of SAP-1. Numbers indicate amino acid residues. The sequence identity between the extracellular or cytoplasmic regions of the human and mouse SAP-1 proteins and drosophila PTP52F is indicated

PTPRH, Fig. 2

Expression of SAP-1 at the apical surface of epithelial cells in the mouse small intestine and colon. Immunostaining of frozen sections of mouse small intestine (left panels) and colon (right panels) with antibodies to SAP-1 (red) and β-catenin (green) as well as staining with DAPI (blue). Bars, 50 μm (upper panels), 20 μm (lower panels). Arrowheads indicate epithelial cells (lower panels). SAP-1 is expressed in epithelial cells of the small intestine and colon and is specifically localized to the apical surface of these polarized cells, whereas β-catenin is present at sites of cell–cell adhesion as well as in the cytoplasm of epithelial cells

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.

Posttranslational modifications of PTPs, such as glycosylation, oxidation, and phosphorylation, play an important role in the modulation of PTP functions (den Hertog et al. 2008). Phosphorylation of several PTPs at either serine or tyrosine residues has been shown to regulate the catalytic activity, intramolecular conformation, and the interaction with other signaling molecules (den Hertog et al. 2008). SAP-1 is a highly glycosylated protein that contains multiple N-glycosylation sites in the extracellular region (Matozaki et al. 1994). In addition, its COOH-terminal region contains the amino acid motif YxNΦ (where Φ represents a hydrophobic amino acid and x any amino acid), which is a conserved motif in mouse and human R3-subtype RPTPs (Fig. 3) (Matozaki et al. 2010; Murata et al. 2010) and is known to be a binding site for SH2 domain-containing proteins such as Src family kinases (SFKs) and Grb2 (Songyang et al. 1993, 1994), whereas PTP52F does not possess the motif in the cytoplasmic region (Fig. 3b). Interestingly, mouse SAP-1 undergoes tyrosine phosphorylation in both intestinal epithelial cells (IECs) and cultured cells exposed to the PTP inhibitor pervanadate, whereas tyrosine phosphorylation of SAP-1 in cultured cells is prevented by the SFK inhibitor PP2 but not PP3, an inactive analog of PP2 (Murata et al. 2010). The tyrosine residue in the YxNΦ motif of mouse SAP-1, when overexpressed in cultured cells, is phosphorylated by coexpression with SFKs Fyn and Src (Murata et al. 2010), suggesting that SFKs participate in tyrosine phosphorylation at the motif. SAP-1 also binds to the Src homology (SH) 2 domain of Fyn via the tyrosine phosphorylated motif. Given that PTPα, an RPTP, interacts with the SH2 domain of c-Src through tyrosine phosphorylation in the COOH-terminal region and consequently activates c-Src by dephosphorylating the COOH-terminus phosphotyrosine reside of c-Src (Pallen 2003), SAP-1 may thus participate in the activation of SFKs in a similar manner to that of PTPα. SAP-1 also interacts with the adaptor molecule Grb2 via the phosphorylated YxNΦ motif (Murata et al. 2010), although the functional role of such a complex formation remains unclear. Grb2 is involved in clathrin-mediated endocytosis of the epidermal growth factor receptor through an interaction of its SH2 domain with the receptor (Sorkin 2004). Therefore, complex formation of SAP-1 with Grb2 may participate in the endocytosis of this RPTP. Interestingly, PTPRO and VE-PTP as well as SAP-1 underwent tyrosine phosphorylation in their COOH-terminal regions, resulting in the promotion of their complex formations with Grb2 or Fyn (Murata et al. 2010). Tyrosine phosphorylation of R3-subtype RPTPs at the YxNΦ motif might thus act as a common mechanism that regulates the cell functions mediated by these enzymes.
PTPRH, Fig. 3

COOH-terminal tyrosine phosphorylation sites of SAP-1 and other R3-subtype RPTPs. (a) Tyrosine phosphorylation sites in the COOH-terminal region of mouse SAP-1, VE-PTP, DEP-1, and PTPRO. These RPTPs share the same YxNΦ motif (x, any amino acid; Φ, a hydrophobic amino acid) in the COOH-terminal region (YENV/L/A). Tyrosine phosphorylation of these motifs provides binding sites for Src family kinases (SFKs) or Grb2. (b) The YxNΦ motif in the COOH-terminal region of mouse and human SAP-1 as well as drosophila PTP52F. Mouse and human SAP-1 possess two and one YxNΦ motif(s), respectively, whereas the motif does not exist in the COOH-terminal region of drosophila PTP52F

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).

In Tumorigenesis

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.

Biochemical analyses have revealed that carcinoembryonic antigen-related cell adhesion molecule 20 (CEACAM20), which is a transmembrane protein in the immunoglobulin super family, is a substrate for SAP-1 (Murata et al. 2015). The tyrosine phosphorylation of CEACAM20 is markedly elevated in IECs of SAP-1-deficient mice. CEACAM20, like SAP-1, is highly expressed in IECs and localized to the microvillus. This membrane protein also possesses an immunoreceptor tyrosine-based activation motif (ITAM) in the cytoplasmic region. SFKs likely mediate tyrosine phosphorylation of CEACAM20 at the ITAM in cultured cells. This tyrosine phosphorylation causes the interaction of CEACAM20 with the spleen tyrosine kinase Syk through its SH2 domain, resulting in the activation of Syk and the subsequent production of IL-8, a chemokine, as well as IL-6. In addition, NF-κB and Erk, as downstream effectors of Syk, participate in the CEACAM20-mediated production of IL-8. Consistent with the increased production of IL-8 in cultured cells, the expression levels of mRNAs for KC and MIP-2, orthologs of human IL-8, in epithelial cells isolated from the colon of Sap1−/−Il10−/− mice tend to be higher than those in IL-10-deficient mice. IL-8 is thought to play a major role in the neutrophil infiltration that is frequently associated with colitis lesions in individuals with IBD (Keshavarzian et al. 1999). The exaggeration of colitis in Sap1−/−Il10−/− mice might rely, at least in part, on the enhanced expression of chemokines, such as KC and MIP-2, in IECs of the intestinal mucosa. SAP-1 and CEACAM20 thus likely constitute a regulatory system through which the intestinal epithelium contributes to intestinal immunity (Fig. 4).
PTPRH, Fig. 4

Model for the regulation of intestinal immunity by the SAP-1–CEACAM20 system. SAP-1, together with IL-10, protects against the development of colitis. SAP-1 negatively regulates the function of CEACAM20 by mediating its dephosphorylation. CEACAM20 is phosphorylated by Src family kinases at an immunoreceptor tyrosine-based activation motif (ITAM) in the cytoplasmic region and thereby interacts with the tyrosine kinase Syk. The formation of a complex by tyrosine-phosphorylated CEACAM20 and Syk induces the activation of NF-κB and Erk, leading to an increase in the production of chemokines, such as IL-8, and the promotion of inflammation of the intestinal mucosa. SFK, Src family kinase. IEC, intestinal epithelial cell


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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© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Division of Molecular and Cellular Signaling, Department of Biochemistry and Molecular BiologyKobe University Graduate School of MedicineKobeJapan