1、1PTEN and TumorAbstract. PTEN was the firstly found cancer suppressor gene with dual-specificity phosphatase (DPS) activities. The gene has connection with the sporadic tumors and hereditary diseases characterized by tumor-like growth in humans. PTEN can specifically dephosphorylate the phosphatidyl
2、inositol-3, 4, 5-triphosphoric acid PIP3, and antagonize PI3K-AKT signaling pathway to be at work, and hence regulates the growth, multiplication, migration and apoptosis of cells. PTEN mutations are mainly reflected in genic mutation and heterozygous losses, which is involved in the occurrance, dev
3、elopment and prognosis of multiple tumors. Key words: cancer suppressor gene, PTEN, tumor, phosphatase 1. Introduction The occurrance, development, invasion and metastasis of tumor are processes of changes in multiple factors, steps and genes, in which the activation of cancer genes and the inactiva
4、tion of cancer suppressor genes are important factors. Li et al 1 (1997) cloned a new gene when researching the primary breast cancer 10q23 homology loss region and named it 2“phosphatase and tensin homology deleted on chromosome 10“ (PTEN). In the same year, Steck et al 2 cloned a tumor suppressor
5、gene in 10q23。3 in the research of glioblastoma multiforme cell line and named it “Mutated in Multiple Advanced Cancers“ (MMAC). Li et al 3 found the epithelial cell regulated by GF-was rich in phosphatase 1 (TEP1). These three genes are identified as a same cancer suppressor gene with dual-specific
6、ity phosphatase activities; the mutation and deletion of this gene has connection with multiple tumors in humans. The existing researches have confirmed the mutation and deletion of this gene in glioblastoma, prostate cancer, breast cancer, endometrial cancer, liver cancer, gastric cancer, carcinoma
7、 of the rectum, esophageal cancer, lung cancer, transitional cell carcinoma of bladder, and even mouth neoplasm, hematological cancer, etc. 1-4 2. Molecular Structure and Biological Characteristics of PTEN PTEN, a total length of 200kb, is situated in the chromosome 10q23.3, and has 9 exons and 8 in
8、trones; and the length of its mRNA is 5.5kb. PTEN, whose main structure functional region is in N-terminal, and cDNA sequence has an open reading frame composed of 1209 nucleotides and encodes 403 3amino acids to constitute a polypeptide chain, and amino acid sequence is highly homogenous with prote
9、in tyrosine phosphatase and serine threonine phosphatase catalytic domain structural motif, was the firstly found cancer suppressor gene with phosphatase activity 1. Also, PTEN is highly homogenous with tensin and auxilin. Tension is a cytoskeletal protein from the interaction of cell adhesion and a
10、ctin filament, and owns lipid phosphatase and protein phosphatases which can dephosphorylate the corresponding locuses of some phospholipids or protein kinases, suppress the periodic duty of cells, mediate apoptosis, and regulate the adhesion, migration and differentiation of cells 5. The active cen
11、ter of PTEN phosphatase has three basic amino acids; if mutation happens, the lipid phosphatase activity will loss, which means most tumor suppressing function disappear, but not impacting the protein phosphatase activity. Therefore, it can be leanred that the tumor suppressing function of PTEN main
12、ly relies on its lipid phosphatase, but its protein phosphatase can still impact the multiplication, adhesion and migration of cells 6. 3. PTEN Tumor Suppressor Mechanism 3.1 Suppressing Cell Growth and Enhancing Apoptosis Under the extracellular signalization of some growth 4factors such as IGF, EG
13、F and PDGF, the intracellular phosphatidylinositol 3-kinases (PI3K) can be activated, hence making the second messenger PIP3 in cell membrane activated; then, the activated PIP3 can activate protein kinase B (PKB) and AKT in cell membrane, hencing making cells into division cycle and suppressing apo
14、ptosis. Besides, AKT can dephosphorylate the original antiapoptotic factor Bad and Caspase-9 to be inactivated, hence preventing the apoptosis further 7. PTEN has lipid phosphatase activity, with PIP3 as its substrate. PTEN dephosphorylates PIP3 through the action of lipid phosphatase and then inact
15、ivates PIP3, blocking up the signalization pathway of PIP3/AKT/PKB and the cell multiplication, and inducing apoptosis and anoikis, and hence plays a negative regulation role in the cells 8. Meanwhile, PTEN has protein phosphatase acivity, and is able to selectively suppress the activation of extrac
16、ellular signal-regulated kinase (ERK), and hence plays a negative regulation role in the signalization pathway of ERK/mitogen activated protein kinase (MAPK). However, ERK/MAPK pathways exert highly important functions in the cell transformation and cycle regulation, suppressing ERK/MAPK pathways, a
17、nd hence are able to suppress the cell growth, multiplication and differentiation. 53.2 Suppressing the Adhesion and Migration of Tumor Cell Focal adhesion kinase (FAK) is the important molecule of the integrin mediated signal transduction pathways. Integrin combines FAK by forming skeleton complex
18、and make FAK tyrosine dephosphorylated until inactivated, resulting in the improvement of FAK tyrosine dephosphorylation level and increasing the activity of phosphokinase, and hence activates the signal transduction molecules of several kinases related to it, enhancing the invasion migration of cel
19、ls. PIEN can decrease FAK tyrosine dephosphorylation level and suppress its activity, and hence suppresses the integrin mediated cytoadherence, migration and invasion. PIEN counteracts the spreading, migration and invasion of cells by impacting two cytoactive regulation pathways (i.e. FAK/P130cas pa
20、thway and ShcMAPK pathway). PIEN can directly make Shc dephosphorylated and suppress the cell migration mediated by this pathway. The researches of Tamrara et al9，10 show that the expression of the wild-type PIEN can regulate the tyrosine phosphorylation level of FAK and P130 cas (P130-Crk-associate
21、d substrate) down in the U87MG glioblastoma multiforme deleted in PTEN, and suppress the invasion, migration and growth of cells, while the 6PIEN without phosphatase activity has no such a role. PTEN, by inactivating the signal of FAK/P130cas, regulates the tyrosine phosphorylation level of FAK and
22、P130cas down, and hence suppresses the invasion, migration and growth of cells, resulting in the cell anoikis. 3.3 Suppressing the Vascular Formation The growth, invasion and migration of tumor have close connection with the vascular formation, while the vascular formation process is regulated by an
23、giogenesis factor and vascular suppressing factor. The mutation or loss of PTEN can promote the formation of vascular endothelial growth factor and then make the vascular endothelial cell multiplication and tumor capillaries formed, hence increasing the invasion and migration intensity of tumor cell
24、s. The mutation or loss of PTEN may, through the PI3K/AKT pathway, promote the expression of the vascular endothelial growth factor. 4. Mutation and Expression Deletion of PTEN in Tumor The expression levels of all kinds of normal tissues in humans, such as placenta, kidney, liver and brain, are hig
25、h. The abnormal expression of somatic cell PTEN is often seen in multiple sporadic tumors such as glioblastoma multiforme, prostatic cancer, endometrial cancer, thyroid cancer, melanoma, 7lung cancer, gastrointestinal tumor, and mouth and hematologic tumors. The forms of PTEN expression abnormality
26、mainly include gene mutation and deletion. 4.1 Gene Mutation PTEN mutation means that it losses the negative regulation function on cell growth and results in the occurrance of tumor. Based on the mutation sources, PTEN mutation can be divided into germ line mutation and somatic mutation; and the mu
27、tation types include missense mutation, nonsense mutation, insertion and deletion, frameshift mutation and splicing mutation. The nine exons and some intrones in PTEN can be mutated, but the mutation is mainly distributed in exon 5 and exon 8 which are just in the major functional region of PTEN, an
28、d the mutation of exon 5 often causes the loss of phosphatase activity, hencing impacting its function to suppress cell growth 11.There are existing researches12 to prove that PTEN mutation has close connection with the hereditary diseases CS (Cowden Syndrome) and BRRS(Bannayan-Riley-Ruvalcaba syndr
29、omn) which are characterized by tumor-like growth. The researches of Marsh et al 13 found that about 10%-15% of CS cases are family and 80% has PTEN mutation, but 66.7% mutations occur in the exon 5, 7, 8 and 40% in exon 5. In BRRS, 60% of patients 8have PTEN mutation and their mutation forms are 10
30、q23 deletion, translocation, etc. Endometrial cancer PTEN mutation is characterized by high rates of nonsense mutation and frameshift mutation; the diagnostic rate of endometrial cancer PTEN mutation is 33%-35%. The mutation often occurs in the well-differentiate advanced invasive cancer patients, s
31、howing that PTEN mutation has connection with the starting and ending of endometrial cancer changes. Yao et al14 diagnosed the PTEN/MMACI mutations of 96 primary liver cancer cases, and analyzed the gene complete sequence and showed the results: the exon 5 (K144I) and exon 7 (V255A) of HCC sample B6
32、21 and B62 both encountered missense mutation, and the exon 7 (V255A) mutation lost the phosphatase activity, while exon 5 (K144I) still retained phosphatase activity, and moreover the splicing mutation of the introne 3 of Sample B62 and the silencing mutation (P96P)of the exon 5 of sample B622 were
33、 detected. 4.2 Loss of Heterozygosity (LOH) PTEN express abnormality is mainly caused by genic mutation and deletion. In many primary tumors, LOH is hihly common, and its occurring rate in prostatic cancer, endometrial cancer and advanced glioblastoma multiforme is 60%-80% 15. However, in 9early tum
34、ors, the complete deletion of PTEN occurs only in endometrial cancer and ovarian cancer. Most PTEN complete inactivations occur in advanced tumors such as prostatic cancer and kidney cancer. Fujiward et al 16 found the 10q of 12 cases had a locus at least to have allele deletion in 37 cases of HCC,
35、and 8 cases had part of deletion. Tashiro et al 17 used the microsatellites around 5 PTENs to mark D10s541, D10s215, D10s608, D10s610 and D10s169 to analyze the LOH of endometrial cancer in this region, finding 56% of them at least to carry one of deletions in this region and the deletion occurring
36、to 44% of them near the 5 markers, and simultaneously there were PTEN mutations to accompany in the part of LOH (+) endometrial cancer samples. The researches of Lu et al 18 on 48 ovarian cancer samples found that their PTEN mutation rate was only 4.2%; LOH occurring rate was 39.6%; mRNA expression
37、deletion rate was 18.8%; and the LOH occurring rate of the samples with PTEN protein expression deletion was 69.2%, in which 5.4% had mutation and LOH, namely the biallelic structural abnormality existed. Thus, it was thought that PTEN inactivation in ovarian cancer may have multiple mechanisms, and
38、 protein expression deletion may be important inactivation mechanism. The 10researches of Young et al 19 on the prostatic cancer cell methylation expressed by PTEN deletion able to restore the PTEN expression, indicating its expression deletion sources from promoter methylation and the PTEN complete
39、 inactivation has connection with the malignancy, invasion growth and long-term migration. Therefore, multiple scholars think PTEN can be used as one of the markers for the tumor migration in the occurrance of prostatic cancer, and PTEN promoter high methylation is also the reason for the PTEN expre
40、ssion deletion, having separating thePTEN from a type of pseudogene in human PTEN/MMAI phosphatase, which has transcriptional activity in many cell lines and tissues. The PTEN transcription in some patients has reached to 70%, andPTEN expression has resulted in the complexity of PTEN expression regu
41、lation function. Some researches 20 also discovered that microsatellite instability (MSI) led to PTEN expression deletion. In the researches, the MSI (+) endometrial cancer PTEN mutation rate was found to be 78%; MSI (-) mutation rate was only 35% (P0.05); the occurrance of MSI had connection with DNA mismatch repair (MMR) gene expression inactivation, and also resulted in the complexity of PTEN expression regulation function. PTEN was the firstly found cancer suppressor
