Low dietary inorganic phosphate affects the lung growth of developing mice

The effects and mechanisms of SLC34A2 in tumorigenesis and progression of human non-small cell lung cancer

BACKGROUND

SLC34A2 with highest expressions in lung, small intestine and kidney encoded a type 2b sodium-dependent phosphate transporter (NaPi-IIb). In lung, SLC34A2 only expressed in the apical membrane of type II alveolar epithelium cells (ATII cells) and played a pivotal role during the fetal lung development and embryonic development. ATII cells acting as multifunctional stem cells might transform into NSCLC after undergoing exogenous or endogenous factors. Increasing evidences showed that the genes performing critical roles during embryogenesis were also expressed during the development of cancer. In addition, recent research found the expression of SLC34A2 had a significant difference between the surgical samples of NSCLC and normal tissues, and SLC34A2 was down-regulated in lung adenocarcinoma cell line A549 and up-regulation expression of SLC34A2 could significantly inhibit cell viability and invasion of A549 in vitro. These results suggested SLC34A2 might play an important role in the development of NSCLC. However, the role of SLC34A2 in tumorigenesis and progression of NSCLC remains unknown.

RESULTS

Our study found that SLC34A2 was also significantly down-regulated in 14/15 of examined NSCLC tissues. Moreover, we found that expressions of SLC34A2 were reduced in six NSCLC cell lines for the first time. Our result also revealed a dramatic inhibitory effects of SLC34A2 on cell growth, migration and invasion of several NSCLC cell lines. SLC34A2 also strongly inhibited tumor growth and metastasis ability in A549 subcutaneous tumor model and lung metastasis model, respectively. Further studies found that the suppressive effects of SLC34A2 on tumorigenesis and progression might be associated with the down-regulation of related protein in PI3K/Akt and Ras/Raf/MEK signal pathway.

CONCLUSIONS

For the first time, our data indicated that SLC34A2 could exert significantly suppressive effects on tumorigenesis and progression of NSCLC. SLC34A2 might provide new insights for further understanding the early pathogenesis of human NSCLC.

Elevated expression of SLC34A2 inhibits the viability and invasion of A549 cells

Abnormal expression of solute carrier family 34 (sodium phosphate), member 2 (SLC34A2) in the lung may induce abnormal alveolar type II (AT II) cells to transform into lung adenocarcinoma cells, and may also be important in biological process of lung adenocarcinoma. However, at present, the effects and molecular mechanisms of SLC34A2 in the initiation and progression of lung cancer remain to be elucidated. To the best of our knowledge, the present study revealed for the first time that the expression levels of SLC34A2 were downregulated in the A549 and H1299 lung adenocarcinoma cell lines. Further investigation demonstrated that the elevated expression of SLC34A2 in A549 cells was able to significantly inhibit cell viability and invasion in vitro. In addition, 10 upregulated genes between the A549-P-S cell line stably expressing SLC34A2 and the control cell line A549-P were identified by microarray analysis and quantitative polymerase chain reaction, including seven tumor suppressor genes and three complement genes. Furthermore, the upregulation of complement gene C3 and complement 4B preproprotein (C4b) in A549-P-S cells was confirmed by ELISA analysis and was identified to be correlated with recovering Pi absorption in A549 cells by the phosphomolybdic acid method by enhancing the expression of SLC34A2. Therefore, it was hypothesized that the mechanisms underlying the effect of SLC34A2 on A549 cells might be associated with the activation of the complement alternative pathway (C3 and C4b) and upregulation of the expression of selenium binding protein 1, thioredoxin-interacting protein, PDZK1-interacting protein 1 and dual specificity protein phosphatase 6. Downregulation of SLC34A2 may primarily cause abnormal AT II cells to escape from complement-associated immunosurveillance and abnormally express certain tumor-suppressor genes inducing AT II cells to develop into lung adenocarcinoma. The present study further elucidated the effects and mechanisms of SLC34A2 in the generation and development of lung cancer.

Characterizing dynamic regulatory programs in mouse lung development and their potential association with tumourigenesis via miRNA-TF-mRNA circuits

Background

In dynamic biological processes, genes, transcription factors(TF) and microRNAs(miRNAs) play vital regulation roles. Many researchers have focused on the transcription factors or miRNAs in transcriptional or post transcriptional stage, respectively. However, the transcriptional regulation and post transcriptional regulation is not isolated in the whole dynamic biological processes, there are few reserchers who have tried to consider the network composed by genes, miRNAs and TFs in this dynamic biological processes, especially in the mouse lung development. Moreover, it is widely acknowledged that cancer is a kind of developmental disorders, and some of pathways involved in tissue development might be also implicated in causing cancer. Although it has been found that many genes differentially expressed during mouse lung development are also differentially expressed in lung cancer, very little work has been reported to elucidate the combinational regulatory programs of such kind of associations.

Results

In order to investigate the association of transcriptional and post-transcriptional regulating activities in the mouse lung development, we define the significant triple relations among miRNAs, TFs and mRNAs as circuits. From the lung development time course data GSE21053, we mine 142610 circuit candidates including 96 TFs, 129 miRNAs and 13403 genes. After removing genes with little variation along different time points, we finally find 64760 circuit candidates, containing 8299 genes, 50 TFs, and 118 miRNAs in total. Further analysis on the circuits shows that the circuits vary in different stages of the lung development and play different roles. By investigating the circuits in the context of lung specific genes, we identify out the regulatory combinations for lung specific genes, as well as for those lung non-specific genes. Moreover, we show that the lung non-specific genes involved circuits are functionally related to the lung development. Noticing that some tissue developmental systems may be involved in tumourigenesis, we also check the cancer genes involved circuits, trying to find out their regulatory program, which would be useful for the research of lung cancer.

Conclusions

The relevant transcriptional or post-transcriptional factors and their roles involved in the mouse lung development are both changed greatly in different stages. By investigating the cancer genes involved circuits, we can find miRNAs/TFs playing important roles in tumour progression. Therefore, the miRNA-TF-mRNA circuits can be used in wide translational biomedicine studies, and can provide potential drug targets towards the treatment of lung cancer.

The emergence of phosphate as a specific signaling molecule in bone and other cell types in mammals

Although considerable advances in our understanding of the mechanisms of phosphate homeostasis and skeleton mineralization have recently been made, little is known about the initial events involving the detection of changes in the phosphate serum concentrations and the subsequent downstream regulation cascade. Recent data has strengthened a long-established hypothesis that a phosphate-sensing mechanism may be present in various organs. Such a phosphate sensor would detect changes in serum or local phosphate concentration and would inform the body, the local environment, or the individual cell. This suggests that phosphate in itself could represent a signal regulating multiple factors necessary for diverse biological processes such as bone or vascular calcification. This review summarizes findings supporting the possibility that phosphate represents a signaling molecule, particularly in bone and cartilage, but also in other tissues. The involvement of various signaling pathways (ERK1/2), transcription factors (Fra-1, Runx2) and phosphate transporters (PiT1, PiT2) is discussed.

Low dietary inorganic phosphate stimulates lung tumorigenesis through altering protein translation and cell cycle in K-ras(LA1) mice

Recent surveys indicate that Pi intake has increased steadily as Pi-containing foods have increased. Our previous study demonstrated that high dietary Pi strongly stimulated lung tumorigeneis. In order to answer the issue whether low Pi may be chemopreventive, we examined the effects of low Pi on lung cancer. Eighteen 5-wk-old male K-ras(LA1) lung cancer model mice were randomly allocated to 2 groups. One group was fed a normal diet (0.5% Pi) and other group was fed low Pi (0.1% Pi) diet for 4 wk. Lung cancer development was evaluated by histopathological examination, Western blot, kinase assay, and immunohistochemistry. Low Pi increased the expression of sodium-dependent phosphate co-transporter 2b, and activated Akt signal with decreased PTEN expression in the lungs of K-ras(LA1) mice. Low Pi increased the Akt/mTOR-mediated protein translation through upregulating the phosphorylation of p70S6K and 4E-BP1. In addition, low Pi stimulated cell cycling as evidenced by altered cell cycle regulators such as cyclin D1 and D3. Finally, low Pi increased lung tumorigenesis in K-ras(LA1) mice compared to the normal diet group. Our results clearly demonstrated that low Pi also promoted lung tumorigenesis, thus suggesting that an appropriate intake of dietary Pi may be critical for lung cancer prevention as well as treatment.