Frequent Loss of PTEN Expression Is Linked to Elevated Phosphorylated Akt Levels, but Not Associated with p27 and Cyclin D1 Expression, in Primary Epithelial Ovarian Carcinomas

Abstract

PTEN (MMAC1/TEP1), a tumor suppressor gene on chromosome subband 10q23.3, is variably mutated and/or deleted in a variety of human cancers. Germline mutations in PTEN, which encode a dual-specificity phosphatase, have been implicated in at least two hamartoma tumor syndromes that exhibit some clinical overlap, Cowden syndrome and BannayanRiley-Ruvalcaba syndrome. Among several series of ovarian cancers, the frequency of loss of heterozygosity (LOH) of markers flanking and within PTEN, is ϳ30 to 50%, and the somatic intragenic PTEN mutation frequency is <10%. In this study, we screened primary adenocarcinomas of the ovary for LOH of polymorphic markers within and flanking the PTEN gene and for intragenic mutations of the PTEN gene and compared them to PTEN expression using immunohistochemistry. Furthermore, we sought to detect the expression of the presumed downstream targets of PTEN, such as P-Akt, p27, and cyclin D1 by immunohistochemistry. LOH at 10q23 was observed in 29 of 64 (45%) cases. Of the 117 samples, 6 somatic intragenic PTEN mutations, 1 germline mutation, and 1 novel polymorphism were found in 7 (6%) patients. Immunostaining of 49 ovarian cancer samples revealed that 13 (27%) were PTEN immunostain-negative, 25 (51%) had reduced staining, and the rest (22%) were PTEN expression-positive. Among the 44 informative tumors assessed for 10q23 LOH and PTEN immunostaining, there was an association between 10q23 LOH and decreased or absent staining (P ‫؍‬ 0.0317). Of note, there were five (11%) tumors with neither mutation nor deletion that exhibited no PTEN expression and 10 (25%) others without mutation or deletion but had decreased PTEN expression. Among the 49 tumors available for immunohistochemistry, 28 (57%) showed P-Akt-positive staining, 24 (49%) had decreased p27 staining, and cyclin D1 was overexpressed in 35 (79%) cases. In general, P-Akt expression was inversely correlated with PTEN expression (P ‫؍‬ 0.0083). These data suggest that disruption of PTEN by several mechanisms, allelic loss, intragenic mutation, or epigenetic silencing, all contribute to epithelial ovarian carcinogenesis, and that epigenetic silencing is a significant mechanism. The Akt pathway is prominently involved, but clearly not in all cases. Surprisingly, despite in vitro demonstration that p27 and cyclin D1 lies downstream of PTEN and Akt, there was no correlation between p27 and cyclin D1 expression and PTEN or P-Akt status. Thus, in vivo, although PTEN and Akt play a prominent role in ovarian carcinogenesis, p27 and cyclin D1 might not be the primary downstream targets. Alternatively, these observations could also suggest that pathways involving other than Akt, p27 and cyclin D1 that lie downstream of PTEN play roles in ovarian carcinogenesis.

Searching for potential biomarkers of cisplatin resistance in human ovarian cancer using a label‐free LC/MS‐based protein quantification method

“…This is consistent with the previous report that reduced expression of PTEN contributes to cisplatin resistance [73]. Other evidence suggests that the PTEN also helps control cell migration, focal adhesion of cells to surrounding tissues, and angiogenesis through dephosphorylation of focal adhesion kinases [74]. Through the inactivation of these kinases whose activities are phosphorylation dependant; PTEN could have lasting affects on the cell's behavior to certain stimuli.…”

“…In our study, the resistant cell lines showed an increase in this 26S proteasome which could be a major factor in the resistance to cisplatin that these cell lines display. Phosphorylation and dephosphorylation represents another class of PTMs that has been shown to have major effects on the activation of certain downstream signaling events that are involved in cell proliferation [73][74][75]. The enzymes responsible for these PTMs include kinases and phophatases.…”

“…These functions prevent uncontrolled cell growth that can lead to the formation of tumors [76]. In ovarian cancer and other cancers, PTEN is often found to be mutated and/or deleted, resulting in functional inactivation of the gene and thus elevation of many cancers [73,74]. Serine/threonine kinase Akt maintains cell survival through suppression of apoptosis when phosphorylated [76].…”

Dysregulated PTEN‐PKB and negative receptor status in human breast cancer

“…40,45 Immunohistochemical analysis of PTEN expression is a powerful tool by which one can evaluate its role in human malignancies. This has been conducted for several human cancers such as prostate, 46 thyroid, 47 endometrial, 48 melanoma, 49 ovarian 50 and gastric 51 carcinomas, to name a few. Thus far, 3 groups have reported PTEN immunostaining in breast cancer and observed its absent or reduced expression in 33-50% of such cases.…”

The Major Target of the Endogenously Generated Reactive Oxygen Species in Response to Insulin Stimulation Is Phosphatase and Tensin Homolog and Not Phosphoinositide-3 Kinase (PI-3 Kinase) in the PI-3 Kinase/Akt Pathway

“…Because PTEN has a critical role in antagonizing PI-3 kinase signaling pathways, it might be expected that PTEN would be the target of complex control mechanisms. Only limited studies have addressed this issue, but evidence is now emerging for functional regulation of PTEN at the level of protein stability and localization, as well as transcription of the PTEN gene (Adey et al, 2000;Vazquez et al, 2000Vazquez et al, , 2001Kurose et al, 2001;Bastola et al, 2002).It has been demonstrated that the activity of PI-3 kinase, the level of 3-phosphorylated PIs, and the activity of Akt are increased in cells exposed to H 2 O 2 (Konishi et al, 1997;Shaw et al, 1998;Sonoda et al, 1999;Tanaka et al, 1999;Qin et al, 2000). This H 2 O 2 -mediated activation of PI-3 kinase/Akt pathway is considered to be the result of reversible oxidation and inactivation of protein tyrosine phosphatase (PTP) family protein such as PTEN that have a critical cysteine residue in the active site (Denu and Tanner, 1998;Lee et al, 1998;Meng et al, 2002).…”