The tumor-suppressor activity of PTEN is regulated by its carboxyl-terminal region

Abstract

PTEN is a recently identified tumor suppressor inactivated in a variety of cancers such as glioblastoma and endometrial and prostate carcinoma. It contains an amino-terminal phosphatase domain and acts as a phosphatidylinositol 3,4,5-trisphosphate phosphatase antagonizing the activity of the phosphatidylinositol 3-OH kinase. PTEN also contains a carboxyl-terminal domain, and we addressed the role of this region that, analogous to the amino-terminal phosphatase domain, is the target of many mutations identified in tumors. Expression of carboxyl-terminal mutants in PTEN-deficient glioblastoma cells permitted the anchorageindependent growth of the cells that otherwise was suppressed by wild-type PTEN. The stability of these mutants in cells was reduced because of rapid degradation. Although the carboxylterminal region contains regulatory PEST sequences and a PDZ-binding motif, these specific elements were dispensable for the tumor-suppressor function. The study of carboxylterminal point mutations affecting the stability of PTEN revealed that these were located in strongly predicted ␤-strands. Surprisingly, the phosphatase activity of these mutants was affected in correlation with the degree of disruption of these structural elements. We conclude that the carboxyl-terminal region is essential for regulating PTEN stability and enzymatic activity and that mutations in this region are responsible for the reversion of the tumorsuppressor phenotype. We also propose that the molecular conformational changes induced by these mutations constitute the mechanism for PTEN inactivation.PTEN was identified recently as a tumor-suppressor gene located on human chromosome 10q23.3 (1, 2). Deletions or somatic mutations of PTEN occur with high frequency in malignant gliomas (3, 4) and endometrial cancer (5) and with a lower rate in other malignancies such as prostate (6) or small-cell lung cancer (7). Germ-line mutations of PTEN are the cause of Cowden disease, an autosomal-dominant hamartoma syndrome with increased risk for development of tumors in a variety of tissues (8).The PTEN gene contains 9 exons and encodes a 403-aa protein that displays high homology in its N-terminal region to dual-specificity protein phosphatases and also to tensin, a cytoskeleton protein (1,2,9). A recent study demonstrated that PTEN acts as a phospholipid phosphatase dephosphorylating the position D3 of the phosphatidylinositol 3,4,5-trisphosphate (PIP 3 ), which is the direct product of the phosphatidylinositol 3-OH kinase (PI-3 kinase) (10). Subsequent studies have confirmed this finding and have shown that cells lacking wild-type PTEN from PTEN-deficient mice (11), from gliomas (12), or from patients with Cowden disease (13) have elevated levels of PIP 3 . As a result, the activity of protein kinase B (PKB͞Akt) was also elevated in these cells, indicating that PTEN exerts its tumor-suppressor function by negatively regulating the antiapoptotic PI-3 kinase-PKB-signaling pathway.Biological evidence that PTEN acts as a tumor suppressor came from...

The relationship between microsatellite instability and PTEN gene mutations in endometrial cancer

“…In all cases there were 1 or 2 frameshift mutations that caused a premature stop codon before or at aminoacid 343, therefore presumably resulting in a protein with no functional activity. 52,53 Missense mutations were also present in all of these samples and none of them appeared to be polymorphisms as explained before. The presence of more that 2 mutations in the same gene in the same tumor specimen has been reported previously.…”

“…52,53 As for the possible effect of the remaining 2 frameshift mutations, it is known that the last residues of the PTEN protein (PEST domains: residues 350-375 and 379-396, and PDZ domain: residues 400-403) are important for protein stability and for its interaction with other proteins, so they may also be functional. 54 Seventeen different types of variants constituted the 30 missense mutations.…”

The role of PTEN signaling perturbations in cancer and in targeted therapy

“…Aberrant PTEN signaling is associated with metastasis and poor prognosis in breast and other cancers (Saal et al, 2007). Genetic alterations of the PTEN gene range from point mutations (encoding mostly unstable and/or catalytically inactive proteins) to large chromosomal deletions Steck et al, 1997;Georgescu et al, 1999Georgescu et al, , 2000. Among these mutations are structural rearrangements within the PTEN gene (intragenic inversions, insertions, deletions and duplications known as gross PTEN mutations) that occur in BRCA1-associated basal-like breast cancer (Saal et al, 2008).…”

Negative prognostic impact of PTEN mutation in pediatric T-cell acute lymphoblastic leukemia

“…Although several randomized trials have confirmed a better overall and complete response rate for the combination of fludarabine and cyclophosphamide compared with fludarabine alone, no differences in the OS were observed. 7,8 In the British study, the oral formulation of fludarabine was introduced in both fludarabine arms when the drug became available, and differences in response rates between the two routes of administration did not appear significant. Oral fludarabine is an effective treatment for patients with previously untreated B-CLL, with less health resource utilization compared with IV administration.…”