Protein tyrosine phosphatase receptor zeta 1 (PTPRZ1) expression is high in embryonic stem cells, while in the adult its expression is low, except in the nervous system ^[1][12]. In cancer, PTPRZ1 expression is upregulated or downregulated, as discussed later in this resviearchw, but how this is moderated in each case has not been elucidated.

In general, studies related to the regulation of PTPRZ1 expression are few and superficial. It has been shown in astrocytes, for example, that epidermal growth factor (EGF) and transforming growth factor alpha (TGFα) strongly increase the expression of both transmembrane PTPRZ1 isoforms and phosphacan through the EGF receptor, while interferon-gamma and tumor necrosis factor-alpha (TNFα) decrease the expression of phosphacan, but not of the transmembrane PTPRZ1 isoforms. TGFβ1 causes a small upregulation, while fibroblast growth factor 2 (FGF2), IL1, IL6, ciliary neurotrophic factor, leukemia inhibitory factor, and platelet-derived growth factor have no effect ^[2][13]. The small, leucine-rich proteoglycan decorin significantly decreases the protein expression of phosphacan in a mouse model of acute spinal cord injuries ^[3][14], and PTN positively regulates PTPRZ1 expression in glioma and breast cancer cells ^[4][5][8,15]. Chronic oxidative stress induces genomic amplification of the Ptprz1 gene in renal cell carcinoma cells ^[6][16], while doxorubicin has been shown to upregulate PTPRZ1 expression in triple-negative breast cancer cells ^[5][15]. The signaling and/or the transcription factors involved in these effects are not known to date.

PTPRZ1 expression increased after hypoxia-inducible factor (HIF) 2 but not HIF1 vector transfection in HEK293T cells, suggesting that the Ptprz1 gene may be a target of hypoxia ^[7][17]. It was later found that the Ptprz1 promoter contains HIF- and E26 transformation-specific (Ets)-binding motifs, and the preferential activation of Ptprz1 by HIF2, but not HIF1, may derive from the cooperative binding of HIF2 and Ets Like-1 (ELK1) to the nearby corresponding sites on the Ptprz1 promoter ^[8][18]. The tumor-suppressor von Hippel-Lindau (VHL) gene, which plays an important role in regulating the response to hypoxia, suppresses PTPRZ1 expression ^[9][19], potentially by suppressing HIF2 expression. The PPARα agonist clofibrate downregulates the expression of PTPRZ1 in pancreatic cancer cells, by abrogating the binding of nuclear factor-κB (NFκΒ) to the Ptprz1 promoter ^[10][20]. More recently, HOXA5 has been shown to directly bind to the Ptprz1 promoter and upregulate its expression, resulting in increased glioma malignancy ^[11][21].

The first study on the potential involvement of PTPRZ1 in tumor growth referred to the decreased expression of PTPRZ1 in lung adenocarcinomas compared to normal lung tissue, suggesting that PTPRZ1 may act as a tumor-suppressor ^[12][71]. In contrast, PTPRZ1 is highly expressed in small-cell lung carcinoma cells and human neuroendocrine tumor tissues, having an important oncogenic role in a mouse xenograft model of tumor progression ^[13][46]. Such a discrepancy has also been found in a more recent bioinformatic analysis, showing that PTPRZ1 expression is decreased in lung adenocarcinoma but increased in lung squamous cell carcinoma. In both cases, though, PTPRZ1 expression seems inversely correlated to overall or disease-free survival ^[14][3].

Human gliomas are the most highly discussed tumors where PTPRZ1 is overexpressed and may regulate growth and invasion. An earlier study using mRNAs from 23 human glioma tissue samples showed that the transmembrane PTPRZ1 isoforms were expressed in all the lower-grade gliomas, but only in 45% of GBMs. Phosphacan was expressed in all grades ^[15][72]. However, in a subsequent study, PTPRZ1 protein was detected in the tumor cells, was elevated in astrocytic gliomas of different malignancy grades, and was associated with an increasing malignancy grade ^[16][73]. In a more recent study of the mRNA expression profiling of a series of clinical diffuse glioma samples of different grades, PTPRZ1 expression was found to be consistently upregulated in all glioma specimens, but it was significantly lower in GBM compared to lower-grade gliomas ^[17][74]. In GBM cells, PTPRZ1 mediates PTN-induced migration ^[17][18][74,75]. In the same line, PTPRZ1 mRNA was found enriched in GBM samples and positively regulated GBM cell migration ^[19][76]. Overexpression of either the long- or the short-transmembrane PTPRZ1 isoform in human U87MG GBM cells similarly enhanced cell migration and adhesion and activated NFκΒ-dependent signaling, suggesting that both isoforms may be valuable targets for GBM therapy ^[20][66]. Monoclonal antibodies that selectively bind to PTPRZ1 with low nanomolar affinities, coupled to the cytotoxin saporin, were shown to kill human U87MG GBM cells in vitro and delay the corresponding tumors’ growth in a mouse xenograft model in vivo ^[21][77]. Likewise, downregulation of PTPRZ1 expression by siRNA in human GBM U251MG cells injected subcutaneously into nude mice or in an orthotopic intracerebral model has resulted in significantly decreased tumor growth. In these cells, PTN-induced haptotaxis was decreased compared to cells expressing PTPRZ1 ^[22][78]. Using E98 GBM cells, knockdown of the PTPRZ-B isoform resulted in reduced migration and proliferation in vitro and inhibited orthotopic tumor growth in vivo ^[23][79].

In line with the high PTPRZ1 expression in embryonic stem cells, PTPRZ1 is preferentially expressed in GBM stem cells and mediates the stimulatory effects of PTN secreted by tumor-associated macrophages on GBM growth ^[24][48]. PTPRZ1 expression in GBM stem cells was shown to be upregulated by HOXA5 and contribute to cell survival and a worse GBM outcome ^[11][21]. Preferential PTPRZ1 expression in GBM stem cells correlates with that of the glycoprotein M6a (GPM6A) and targeting either of these molecules inhibited the invasive and sphere-forming ability of these cells and enhanced their sensitivity to radiation ^[25][80].

PTPRZ1 has also been included in a group of four genes comprising a gene signature that has been evaluated for the automation of the prediction of 35 brain tumors, distinguishing between GBM and meningioma cases ^[26][81]. This result requires further validation with a larger sample. Being considered a canonical cancer stem cell marker, PTPRZ1 has also been included in an integrative analysis of the heterogeneity present in GBM cancer stem cell populations by using a combination of flow cytometry and bulk and single-cell RNA sequencing. A significant diversity of transcriptional profiles was observed between slow-cycling cells and cells expressing canonical cancer stem cell markers, with very little transcriptional overlapping ^[27][82].

An interesting observation is that the non-coding circular RNA for PTPRZ1 transcribed from the Ptprz1 gene (hsa_circ_0133159) in a microRNA (MiR)-1261-dependent manner is highly expressed in gliomas and regulates the activation of P21-activated kinase 1 (PAK1), thus upregulating glioma cell proliferation and invasion ^[28][83]. A tumor atlas of primary GBMs created by a single-cell RNA-sequencing approach identified the outer radial glia-like cells, a cell-type population that undergoes a characteristic PTPRZ1-mediated mitotic translocation that promotes an invasive behavior ^[29][84]. The enhanced PTPRZ1 expression in GBM tissues is preserved in patient-derived tumorspheres compared to the normal human astrocytes, independently of the p53 mutation status ^[30][85].

Besides gliomas, PTPRZ1 is heterogeneously expressed in individual meningiomas and drives meningioma cell proliferation and tumorigenesis ^[31][86]. It is also the fifth most frequently occurring gene in head and neck cancer, as suggested by analyzing a large cohort of single-cell transcriptomics data ^[32][87], although the functional significance of this observation remains to be studied. In oral cavity squamous cell carcinoma, PTPRZ1 was found by immunohistochemistry to be expressed more frequently in lower-grade tumors and was associated with improved patient survival ^[33][88]. On the other hand, PTPRZ1 overexpression promotes oral submucous fibrosis, a potentially malignant disease of the oral cavity, through the nuclear translocation of β-catenin ^[34][43].

In stomach adenocarcinoma, positive PTPRZ1 immunohistochemical reactivity has been observed and has been associated with gastric cancer progression ^[35][89]. However, PTPRZ1 mRNA levels are significantly lower in gastric adenocarcinoma compared to the corresponding normal tissue ^[14][3].

PTPRZ1 mRNA levels are decreased in colorectal cancers compared to those in adjacent normal mucosae ^[36][90]. In a promoter methylation analysis of 131 surgical specimens obtained from patients with sporadic colorectal cancers, the Ptprz1 promoter was found hypermethylated in tumor cells compared to the corresponding adjacent normal tissue ^[37][91], supporting a decreased PTPRZ1 expression in colorectal cancer. In a following study that involved 102 colorectal cancer tissues, amplification in the region containing the Ptprz1 gene was observed in 20% of cases ^[38][92]. By analyzing the mRNA levels of PTPRZ1 in 16 tissues obtained from patients with colorectal carcinoma, no significant difference in the Ptprz1 gene expression was found compared to the corresponding normal tissues ^[39][93]; similarly, the protein PTPRZ1 levels were not found to be different between 25 colorectal carcinoma and 5 normal tissues, studied by immunohistochemistry and Western blot analysis ^[40][94]. In a recent bioinformatic analysis, PTPRZ1 mRNA levels were significantly decreased in both colon and rectum adenocarcinomas, and this decrease may be associated with overall and/or disease-free survival ^[14][3].

PTPRZ1 was found by immunohistochemistry to be overexpressed in human primary and metastatic melanomas, but not in the melanocytes of healthy skin ^[41][95]. However, mRNA levels of PTPRZ1 were not found significantly different between normal and tumor tissues in skin cutaneous melanoma ^[14][3]. In uveal melanoma cells, PTPRZ1 is overexpressed and positively affects proliferation and invasion in vitro ^[42][39].

PTPRZ1 is expressed in different types of human breast cancers, both in the breast cancer cells themselves and in carcinoma-associated fibroblasts. PTPRZ1 protein has been detected by immunohistochemistry in the cell membrane, the cytoplasm, and the nucleus in different breast cancer cells ^[43][96], in line with another study showing cytoplasmic and nuclear PTPRZ1 localization in endothelial cells ^[44][33]. In triple-negative breast cancer, PTPRZ1 is overexpressed, as assessed by immunohistochemistry in 325 cases of breast cancer, and it may be an independent risk indicator for recurrence and metastasis ^[45][97]. Using microarray data from the GEPIA database, PTPRZ1 was found significantly downregulated in breast cancer samples derived from patients that received no chemotherapy compared to samples from the normal group. PTPRZ1 expression was found to be significantly upregulated following chemotherapy, based on data from the GEO database. Doxorubicin enhanced both PTPRZ1 and PTN expression in the triple-negative breast cancer MDA-MB-231 and MDA-MB-453 cells, promoting cell proliferation and inhibiting apoptosis through PTPRZ1-dependent activation of NFκΒ ^[5][15]. High PTPRZ1 expression seems to correlate with a negative effect on overall patient survival ^[14][3]. In a study that analyzed the differential RNA expression patterns between breast cancer with and without bone metastasis, using 1091 primary breast cancer samples included in The Cancer Genome Atlas database, a significant correlation was found between PTPRZ1 expression and the survival rate in breast cancer patients with bone metastasis ^[46][98].

In cervical carcinoma, PTPRZ1 expression was found significantly higher compared to the normal cervical epithelium, significantly higher in patients with smaller (≤2 cm) compared to those with larger (>2 cm) tumor sizes, and higher in squamous cell carcinoma than in adenocarcinoma ^[47][99]. In a bioinformatic analysis, PTPRZ1 was also found to be upregulated in cervical squamous cell carcinoma and endocervical adenocarcinoma and might be associated with better overall and disease-free survival ^[14][3].

PTPRZ1 is expressed in epithelial ovarian cancer cells and enhances cell viability through the inhibition of apoptosis. It was also found upregulated in serous ovarian tumor tissue relative to normal ovarian surface epithelial tissue ^[48][100]. However, PTPRZ1 mRNA levels were found to be significantly downregulated in ovarian serous cystadenocarcinoma ^[14][3], in line with a recent study using transcriptomic data, showing an abnormally low PTPRZ1 expression in ovarian cancer tissues and in cisplatin-resistant ovarian cancer cells. Overexpression of PTPRZ1 enhanced the sensitivity of ovarian cancer cells to cisplatin and enhanced cell apoptosis in vitro, and inhibited tumor growth and resistance to cisplatin in vivo ^[49][63].

In prostate cancer, PTPRZ1 mRNA levels are significantly decreased compared to normal prostate tissue ^[14][3]. This is in line with data showing that downregulation of PTPRZ1 expression in human prostate DU145 and PC3 cells initiated EMT and enhanced prostate cancer cell migration and invasion in vitro, and metastasis in vivo ^[50][101].

In renal cell carcinoma cell lines, downregulation of PTPRZ1 expression by siRNA has been shown to decrease the amounts of nuclear β-catenin, decrease the expression of target genes, and suppress cell proliferation ^[6][16]. In contrast, PTPRZ1 is overexpressed in renal cell carcinomas following the loss of VHL activation, activates β-catenin, and enhances cell proliferation ^[9][19]. However, PTPRZ1 mRNA levels are decreased in renal cell carcinomas ^[14][3].

In a sample of 30 osteosarcoma patients, the Ptprz1 gene was overexpressed in 73% and under-expressed in 27% of cases. There was no correlation between the Ptprz1 gene expression profile, clinicopathological parameters, and survival ^[51][102]. However, a later study using Ptprz1−/− 129SV/Ev Trp53-heterozygous mice^−/− 129SV/Ev Trp53-heterozygous mice, it was found that Ptprz1 gene deletion enhanced osteosarcoma development, characterized by enhanced tyrosine phosphorylation and cell proliferation, suggesting that PTPRZ1 acts as a tumor-suppressor for osteosarcoma ^[52][103].

PTPRZ1 expression is also increased in lymphoma tissues from patients with diffuse large B lymphoma, especially of high risk, correlates with the proportion of tumor-associated macrophages that promote lymphoma growth, and coincides with an increased proportion of cancer stem cells ^[53][104].

A summary of the data related to PTPRZ1 expression and its role in different cancer types is presented in Table 1.

To identify driver fusion proteins in GBM, RNA-sequencing of 272 gliomas identified PTPRZ1-MET (ZM) fusion transcripts only in grade III astrocytomas or secondary GBMs. The fusion transcripts result in a protein that has a molecular mass of 145 kDa, which is difficult to distinguish from wildtype MET ^[54][55][56][105,106,107]. The ZM fusion protein forms homodimers or heterodimers with wildtype MET, leading to enhanced MET tyrosine phosphorylation levels in the absence of its ligand, hepatocyte growth factor, although the latter can further activate ZM autophosphorylation ^[57][108]. The ZM fusion protein has been described to contain sequences encoding the carbonic anhydrase and the fibronectin type III domain of PTPRZ1 fused to the dimerization domain, immunoglobulin-like domains, transmembrane domain, and the tyrosine kinase domain of MET ^[57][108], or to result from the highly active promoter of the Ptprz1 gene fused to exons 2–21 of Met, leading to overexpressed MET and activated downstream signaling ^[58][59][109,110]. In all cases, GBMs harboring a ZM fusion exhibit a more aggressive phenotype and are associated with a poor patient prognosis ^[57][108]. PTPRZ1-MET fusion proteins are also found in pediatric GBMs with enhanced MET expression and activity, sensitive to MET inhibitors ^[58][59][109,110]. Exosomes from GBM cells harboring the ZM fusion have higher MET expression and activity compared to those from non-ZM fusion GBM cells, and they induce EMT when they are transferred to non-ZM fusion GBM cells or normal human astrocytes ^[60][111]. ZM fusion has also been detected in a small number of brain metastases of lung cancer ^[61][112].

In spitzoid neoplasms, the PTPRZ1-NFAM1 fusion gene has been identified in two patients, associated with a copy gain in the kinase fusion gene. NFAT-activated protein 1, which is the protein encoded by Nfam1, is a transmembrane receptor that regulates cytokine production and is mostly expressed in immune cells ^[62][113].

Another fusion transcript, PTPRZ1-ETV1, has been more recently identified in 6% of the tested gliomas, including GBMs, one anaplastic oligodendroglioma, and one pilocytic astrocytoma. This fusion consists of the Ptprz1 promoter in frame with the highly conserved DNA-binding domain of the ETV1 transcription factor. The latter is a member of the ETS family of transcription factors, known as oncogenic drivers in different types of tumors ^[63][114]. To date, this fusion protein’s prognostic or therapeutic value is unknown.

More recently, PTPRZ1 has been identified as a BRAF fusion partner in juvenile pilocytic astrocytomas, but information on the functional significance of this fusion protein is missing ^[64][115].

4. Pharmacological Targeting of Protein Tyrosine Phosphatase Receptor Zeta PRZ1 in Cancer

TP inhibitors are not extensively exploited, and no such drugs exist in the clinic to date. The first small molecule that selectively and potently inhibits the catalytic activity of PTPRZ1 was found by in vitro screening of a chemical library and was named SCB4380. This molecule inhibited rat C6 glioma cell proliferation and migration in vitro and inhibited tumor growth in vivo, favoring the notion that selective inhibition of PTPRZ1 may be a promising therapeutic approach in GBM ^[65][119]. In the same line, another cell-permeable small-molecule inhibitor of PTPRZ1, NAZ2329, was found to decrease the expression of the stem cell transcription factor SOX2 in rat C6 and human U251 GBM cells, thus inhibiting their growth in vitro and in vivo. NAZ2329 and the alkylating agent temozolomide have a synergistic effect in decreasing GBM cell growth ^[66][120]. A few more blood–brain barrier-permeable small molecules that selectively interact with the intracellular D1 domain of PTPRZ1 and inhibit its TP activity were designed and found to increase the tyrosine phosphorylation of known PTPRZ1 substrates ^[67][121]. The effect of these latter compounds on angiogenesis and cancer growth is being tested. Besides small molecules that target the TP activity of PTPRZ1, another approach is to develop molecules that would inhibit ligand binding to the extracellular PRPRZ1 domains. Early studies in neurons have shown that polyclonal antibodies against the extracellular domain of PTPRZ1 suppress PTN-induced neuronal migration ^[68][122]. Antibodies targeting the extracellular domain of the short PTPRZ1 isoform have also been shown to modestly delay GBM growth in mice in vivo. When such antibodies were coupled to a cytotoxin, they killed human U87MG GBM cells in vitro and significantly delayed GBM growth in a mouse xenograft model ^[21][77]. More recently, it was shown that an antibody against the extracellular PTPRZ1 domain in GBM stem cells inhibits PTN binding and thus suppresses GBM growth in mice, leading to prolonged survival ^[24][48]. PTPRZ1 has also been identified as one of the GBM-associated antigens that could be considered a target for immunotherapy in both HLA-A 02-positive ^[69][123] and negative ^[70][124] GBM. The PTPRZ1 peptides, together with the rest of the HLA-A2-restricted tumor-associated antigens identified by peptidomics, have been used to formulate a multi-peptide vaccine (IMA950) that has entered phase I/II clinical trials for gliomas and showed spontaneous antigen-specific T-cell responses that were better in grade II and III compared to GBM patients ^[71][125]. Using an in-silico approach, another PTPRZ1 domain was found to induce the host’s B- and T-cell immune response against GBM and was fused with domains from other proteins to construct and characterize a multi-domain recombinant vaccine that will be validated by further in vitro and in vivo experimental studies ^[72][126]. A more recent approach developed for the treatment of highly aggressive uveal melanoma was the construction of indocyanine green-labeled manganese metal-organic framework nanoparticles that carry siRNA for the lncRNA OUM1 and PTPRZ1, together with cisplatin. Nanoparticles were linked with the RGD peptide for targeting and were shown to kill uveal melanoma in vitro and delay tumor growth in vivo through selective siRNA knockdown and enhanced cisplatin cytotoxicity ^[42][39]. Besides developing drugs that target PTPRZ1 to inhibit its signaling, PTPRZ1 has also been considered as a cell membrane molecule exploited to target GBM stem cells with cytotoxic chemotherapeutics. In a study using patient-derived GBM tissues cultured in a microchannel network chip, resistance to temozolomide and radiation is observed, similar to what is observed in GBM patients, and can be overcome by nanovesicles displaying an anti-PTPRZ1 peptide and loaded with temozolomide ^[73][127]. In a similar approach, a self-assembled hybrid micelle that can cross the blood–brain barrier, and simultaneously target M2-like tumor-associated macrophages by a specific peptide and GBM stem cells by an anti-PTPRZ1 antibody, was developed as a nanocarrier to deliver the chemotherapeutic agent doxorubicin to the GBM tissue. This nanocarrier was shown to be effective in reshaping the immune microenvironment and decreasing the growth and the invasive potential of GBM stem cells ^[74][128]. One drawback of this latter study is that the anti-PTPRZ1 antibody used targets the intracellular TP domain of PTPRZ1, and it is not clear or shown how this antibody targets GBM stem cells. Finally, a recent study that used single-cell RNA-sequencing datasets from 37 GBM patients to look for GBM stem-like marker candidates identified PTPRZ1 as one of the most highly expressed surface markers to be used for GBM stem cell isolation and identification ^[75][129].