Human TMEM174 that is highly expressed in kidney tissue activates AP-1 and promotes cell proliferation

The roles and functions of TMEM protein family members in cancers, cardiovascular and kidney diseases (Review)

Transmembrane protein (TMEM) is a type of membrane proteins, encoded by TMEM gene, also known as integral membrane protein. TMEM gene family contains various members and its encoded proteins have various functions and expressed in numerous organs. It has been proved to be widely involved in the formation of a lot of organelle membranes, enzymes, receptors and channels, mediating numerous normal physiological functions and regulating various disease processes. At present, accumulating evidences at home and abroad have shown that TMEM is involved in regulating the occurrence and development of different tumors, cardiovascular and kidney diseases. The improved understanding of molecular mechanisms of TMEM genes and proteins may provide new directions and ideas for the prevention, diagnosis and treatment of diseases. In the present review, the roles of TMEM and biological functions in various cancers, cardiovascular and kidney diseases were discussed.

Identification of a risk model for prognostic and therapeutic prediction in renal cell carcinoma based on infiltrating M0 cells

The tumor microenvironment (TME) comprises immune-infiltrating cells that are closely linked to tumor development. By screening and analyzing genes associated with tumor-infiltrating M0 cells, we developed a risk model to provide therapeutic and prognostic guidance in clear cell renal cell carcinoma (ccRCC). First, the infiltration abundance of each immune cell type and its correlation with patient prognosis were analyzed. After assessing the potential link between the depth of immune cell infiltration and prognosis, we screened the infiltrating M0 cells to establish a risk model centered on three key genes (TMEN174, LRRC19, and SAA1). The correlation analysis indicated a positive correlation between the risk score and various stages of the tumor immune cycle, including B-cell recruitment. Furthermore, the risk score was positively correlated with CD8 expression and several popular immune checkpoints (ICs) (TIGIT, CTLA4, CD274, LAG3, and PDCD1). Additionally, the high-risk group (HRG) had higher scores for tumor immune dysfunction and exclusion (TIDE) and exclusion than the low-risk group (LRG). Importantly, the risk score was negatively correlated with the immunotherapy-related pathway enrichment scores, and the LRG showed a greater therapeutic benefit than the HRG. Differences in sensitivity to targeted drugs between the HRG and LRG were analyzed. For commonly used targeted drugs in RCC, including axitinib, pazopanib, temsirolimus, and sunitinib, LRG had lower IC50 values, indicating increased sensitivity. Finally, immunohistochemistry results of 66 paraffin-embedded specimens indicated that SAA1 was strongly expressed in the tumor samples and was associated with tumor metastasis, stage, and grade. SAA1 was found to have a significant pro-tumorigenic effect by experimental validation. In summary, these data confirmed that tumor-infiltrating M0 cells play a key role in the prognosis and treatment of patients with ccRCC. This discovery offers new insights and directions for the prognostic prediction and treatment of ccRCC.

Targeted Disruption of a Proximal Tubule-Specific TMEM174 Gene in Mice Causes Hyperphosphatemia and Vascular Calcification

BACKGROUND

The proximal tubules play a critical role in phosphate (Pi) homeostasis by reabsorbing Pi via sodium-dependent Pi cotransporters. NPT2A is a major proximal-specific Pi cotransporter, whose expression is regulated by circulating hormones, such as parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23). In this study, we aimed to find a novel regulator in Pi homeostasis.

METHODS

Using RNA-seq and RT-qPCR analysis, we identified proximal tubule cell-enriched genes. We next used RNAi screening of the identified proximal tubular cell-enriched genes to identify a novel proximal tubule-specific gene that contributes to FGF23- and PTH-mediated inhibition of Pi uptake and NPT2 reduction. We created mice lacking this novel regulator of Pi homeostasis to examine whether the novel regulator contributes to Pi homeostasis in vivo.

RESULTS

We identified 54 kidney-enriched genes, 19 of which are expressed in renal primary proximal tubule cells. One of the proximal tubule-specific genes, TMEM174, interacted with NPT2A, and its knockdown blocked the reduction of NPT2A protein by FGF23 and PTH treatments in human and opossum proximal tubule cells. TMEM174 KO mice had significantly increased levels of serum Pi, FGF23, and PTH, resulting in vascular calcification.

CONCLUSIONS

TMEM174 is a novel regulator of Pi homeostasis that interacts with NPT2A.

Tmem174, a regulator of phosphate transporter prevents hyperphosphatemia

Renal type II sodium-dependent inorganic phosphate (Pi) transporters NaPi2a and NaPi2c cooperate with other organs to strictly regulate the plasma Pi concentration. A high Pi load induces expression and secretion of the phosphaturic hormones parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) that enhance urinary Pi excretion and prevent the onset of hyperphosphatemia. How FGF23 secretion from bone is increased by a high Pi load and the setpoint of the plasma Pi concentration, however, are unclear. Here, we investigated the role of Transmembrane protein 174 (Tmem174) and observed evidence for gene co-expression networks in NaPi2a and NaPi2c function. Tmem174 is localized in the renal proximal tubules and interacts with NaPi2a, but not NaPi2c. In Tmem174-knockout (KO) mice, the serum FGF23 concentration was markedly increased but increased Pi excretion and hypophosphatemia were not observed. In addition, Tmem174-KO mice exhibit reduced NaPi2a responsiveness to FGF23 and PTH administration. Furthermore, a dietary Pi load causes marked hyperphosphatemia and abnormal NaPi2a regulation in Tmem174-KO mice. Thus, Tmem174 is thought to be associated with FGF23 induction in bones and the regulation of NaPi2a to prevent an increase in the plasma Pi concentration due to a high Pi load and kidney injury.

Tmem63c is a potential pro-survival factor in angiotensin II-treated human podocytes

AIMS

Human podocytes (hPC) play an important role in the pathogenesis of renal diseases. In this context, angiotensin II (Ang II) and nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) play a crucial role in podocyte injury. Recently, transmembrane protein (Tmem) 63c, a member of the Tmem-family was found to be expressed in kidney and associated with podocyte function. In this study, we analysed the expression regulation and functional impact of Tmem63c on cell viability and apoptosis in hPC in the context of Ang II activation.

MATERIALS AND METHODS

Expression of Tmem63c in response to Ang II and the NFκB inhibitor Bay 11-7082 was analysed by Real-Time PCR and Western blotting. Cellular functions were determined by functional assays.

KEY FINDINGS

We found Ang II to induce Tmem63c expression in hPC in a concentration-dependent manner. Inhibition of NFκB by Bay 11-7082 reduced basal as well as Ang II-induced Tmem63c expression. SiRNA-mediated down-regulation of Tmem63c diminished cell viability and protein kinase B (Akt) signaling and increased cell apoptosis of resting as well as Ang II-activated hPC.

SIGNIFICANCE

These data show that Ang II induced the expression of Tmem63c in hPC, possibly via NFκB-dependent mechanisms. Moreover, down-regulation of Tmem63c was associated with reduced cell viability, indicating Tmem63c to be a potential pro-survival factor in hPC.

Identification and functional analysis of a novel splice variant of AC3-33 in breast cancer

Alternative RNA splicing plays a key role in regulating gene function and influencing protein expression diversity. In the present study, an AC-33 transcript variant (NCBI Reference Sequence: NM_001308229.1), splice variant (sv)AC3-33, was successfully cloned from the MCF-7 breast cancer cell line by reverse transcription PCR using primers based on expressed sequence tags. The aim of the present study was to investigate the structure and function of svAC3-33. svAC3-33 has an open reading frame of 1,825 base pairs, lacks AC3-33 exon 2 and is encoded by 294 amino acids. svAC3-33 is localized within the cytoplasm. The Cell Counting Kit-8 and EdU detection of cell proliferation assays showed that svAC3-33 inhibited MCF-7 cell proliferation. Similarly, svAC3-33 knockdown by RNA interference was shown to have the opposite effect by repressing the cell cycle progression of breast cancer cells. Furthermore, the data indicated that svAC3-33 may upregulate the expression of p21. The present study provides evidence that the increased expression of svAC3-33 may inhibit the activity of the transcription factor AP-1. The luciferase reporter gene assay detected a downregulation of the expression of c-Jun, but not c-Fos, which in turn affected cell proliferation. In conclusion, these results indicated a function for svAC3-33 in inhibiting the cell proliferation of MCF-7 cells by regulating the AP-1 signaling pathway.

SZRD1 is a Novel Protein that Functions as a Potential Tumor Suppressor in Cervical Cancer

SZRD1 is a novel gene screened out by high-throughput platform, and so far there exists no systematic function reports. The purpose of our study is to discover the function and mechanism of this novel human gene. Bioinformatics analysis indicates that SZRD1 is a highly conserved intracellular protein. After overexpression of SZRD1, we found that SZRD1 could arrest the cell cycle in G2 phase and play a role in inhibiting cell proliferation and inducing apoptosis. In contrast, after knockdown of endogenous SZRD1, we concluded that it could promote cell proliferation. The mechanism investigations showed that overexpression of SZRD1 could downregulate the phosphorylation of ERK1/2, AKT, STAT3 and downstream signaling molecules, and then arrest the cells in G2 phase by upregulating P21. Tissue microarray analysis showed that the expression of SZRD1 was downregulated in cervical squamous cell carcinomas compared with normal squamous epithelium, and the ratio of downregulation correlated with the stage of the cancer. Overall, we clarified the function of this novel protein SZRD1, which indicated it may be a potential novel tumor suppressor in cervical cancer.

Analysis of TMEM174 gene expression in various renal cancer types by RNA in situ hybridization

Transmembrane protein 174 (TMEM174) mRNA is easily detectable in human kidney tissues and activates AP-1 and promotes 293T cell proliferation. In the present study, RNA in situ hybridization was used to detect TMEM174 gene expression in various malignant renal cancer and normal renal tissues. The results showed that TMEM174 exhibits differential expression in renal tissues, with a high positive rate of expression in squamous cell carcinoma with necrosis, papillary renal cell carcinoma and transitional cell carcinoma, and a low positive rate of expression in clear cell carcinoma, interstitial nephritis, undifferentiated carcinoma, retroperitoneal metastatic clear cell carcinoma, adrenal gland metastatic clear cell carcinoma, pelvic cavity metastatic chromophobe carcinoma, severe atypical hyperplasia of transitional epithelium and hyperplasia. Extremely weak expression was exhibited in collecting duct carcinoma, Wilms’ tumor, chronic pyelonephritis, acute pyelonephritis, cancer adjacent normal renal tissue and normal renal tissue. In conclusion, the TMEM174 gene exhibited high expression levels in certain renal carcinomas, which may indicate that TMEM174 may have a significant role in the development and progression of these renal carcinomas.

Analysis of promoters and CREB/AP-1 binding sites of the human TMEM174 gene

Transmembrane protein 174 (TMEM174) is a type III transmembrane protein with no clear signal peptide. The N and C terminals are located inside the cell. Our previous study demonstrated high expression of TMEM174 in the kidney and its potential involvement in renal cancer based on its capacity to stimulate cell proliferation. However, the mechanism by which TMEM174 promotes proliferation at the transcriptional level remains to be elucidated. In the present study, the TMEM174 promoter region was amplified from whole blood DNA. Six different regions of the regulatory sequences of the TMEM174 promoter region including ~2.5 kb of the upstream region were cloned into the dual luciferase expression vector pGL3-basic. Comparison of the activity of these fragments in dual luciferase reporter assays revealed higher levels of activity for the fragments spanning −186 to +674, −700 to +674, −1,000 to +674 and −2,500 to +1 bp. Lower levels of activity were detected for the fragments spanning −466 to +674 and −890 to +674 bp. The highest activity was detected for the fragment spanning −186 to +674 bp. Electrophoretic mobility shift assay (EMSA) was performed to determine effective transcription factor binding sites. Specific binding of the cyclic-AMP response element binding (CREB) within the TMEM174 gene promoter region was demonstrated, although binding of the activator protein-1 (AP-1) was also detected in this region. In conclusion, these results suggest that the core promoter region of the human TMEM174 gene is located within the region spanning −186 to +674 bp and that the transcription factors CREB and AP-1 are involved in the transcriptional regulation of this gene.

Exome sequencing identifies a new candidate mutation for susceptibility to diabetes in a family with highly aggregated type 2 diabetes

The aim of this study was to investigate the genetic background of familial clustering of diabetes using genome-wide linkage analysis combined with exome sequencing. We recruited a Japanese family with a 3-generation history of diabetes. The family comprised 16 members, 13 having been diagnosed with diabetes. Nine members had been diagnosed before the age of 40. Linkage analysis was performed assuming an autosomal dominant model. Linkage regions were observed on chromosomes 4q34, 5q11-q13, and 12p11-q22 and the logarithm of odds (LOD) scores were 1.80. To identify the susceptibility variants, we performed exome sequencing of an affected family member. We predicted that the familial clustering of diabetes is caused by a rare non-synonymous variant, and focused our analysis on non-synonymous variants absent in dbSNP131. Exome sequencing identified 10 such variants in the linkage regions, 7 of which were concordant with the affection status in the family. One hundred five normal subjects and 67 lean diabetes subjects were genotyped for the 7 variants; the only variant found to be significantly more frequent in the diabetes subjects than in the normal subjects was the N1072K variant of the early endosome antigen 1 (EEA1) gene (0 in normal subjects and 4 in diabetes subjects, p=0.022). We therefore propose that the N1072K variant of the EEA1 gene is a candidate mutation for susceptibility to diabetes in the Japanese population.