The protein tyrosine phosphatase (PTP) PTPL1/PTPN13 is a candidate tumor suppressor gene. Indeed, PTPL1 activity has been reported recently to be decreased through somatic mutations, allelic loss, or promoter methylation in some tumors. We showed previously that its expression was necessary for inhibition of Akt activation and induction of apoptosis by antiestrogens in breast cancer cells. Implications of the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway in cancer progression are now well established, and our study was therefore designed to define whether PTPL1 is sufficient to inhibit this pathway and, if so, to identify a direct substrate of this PTP, which may trigger a proapoptotic effect. We first show by complementary approaches that PTPL1 specifically dephosphorylates insulin receptor substrate-1 (IRS-1) in vitro and in cellulo. Next, our experiments using a dominant-negative mutant and RNA interference confirm the crucial role of PTPL1 in IRS-1 dephosphorylation. Finally, we report that PTPL1 expression is sufficient to block the IRS-

The protein tyrosine phosphatase PTPL1/PTPN13, the activity of which is decreased through allelic loss, promoter methylation, or somatic mutations in some tumors, has been proposed as a tumor suppressor gene. Moreover, our recent clinical study identified PTPL1 expression level as an independent prognostic indicator of a favorable outcome for patients with breast cancer. However, how PTPL1 can affect tumor aggressiveness has not been characterized. Here, we first show that PTPL1 expression, assessed by immunohistochemistry, is decreased in breast cancer and metastasis specimens compared with nonmalignant tissues. Second, to evaluate whether PTPL1 plays a critical role in breast cancer progression, RNA interference experiments were performed in poorly tumorigenic MCF-7 breast cancer cells. PTPL1 inhibition drastically increased tumor growth in athymic mice and also enhanced several parameters associated with tumor progression, including cell proliferation on extracellular matrix components and cell invasion. Furthermore, the inhibition of Src kinase expression drastically blocked the effects of PTPL1 silencing on cell growth. In PTPL1 knockdown cells, the phosphorylation of Src on tyrosine 419 is increased, leading to the activation of its downstream substrates Fak and p130cas. Finally, substrate-trapping experiments revealed that Src tyrosine 419 is a direct target of the phosphatase. Thus, by identification of PTPL1 as the first phosphatase able to inhibit Src through direct dephosphorylation in intact cells, we presently describe a new mechanism by which PTPL1 inhibits breast tumor aggressiveness. Cancer Res; 70(12); 5116-26. ©2010 AACR.

Purpose:The main goal of sentinel lymph node (SLN) detection in head and neck squamous cell carcinomas is to limit neck dissections to pN+ cases only. However, intraoperative + diagnosis cannot be routinely done using the current gold standard, serial step sectioning with immunohistochemistry. Real-time quantitative reverse transcription-PCR (RT-PCR) is potentially compatible with intraoperative use, proving highly sensitive in detecting molecular markers. This study postoperatively assessed the accuracy of quantitative RT-PCR in staging patients from their SLN. Experimental Design: A combined analysis on the same SLN by serial step sectioning with immunohistochemistry and quantitative RT-PCR targeting cytokeratins 5, 14, and 17 was done in 18 consecutive patients with oral or oropharyngeal squamous cell carcinoma and 10 control subjects. Results: From 71lymph nodes examined, mRNA levels (KRT) were linked to metastasis size for the three cytokeratins studied (Pearson correlation coefficient, r = 0.89, 0.73, and 0.77 for KRT 5, 14, and 17 respectively; P < 0.05). Histopathology-positive SLNs (macro-and micrometastases) showed higher mRNA values than negative SLNs for KRT 17 (P < 10 À4) and KRT 14 (P < 10 À2 ). KRT 5 showed nonsignificant results. KRT17 seemed to be the most accurate marker for the diagnosis of micrometastases of a size >450 Am. Smaller micrometastases and isolated tumor cells did not provide results above the background level. Receiver operating characteristic curve analysis for KRT17 identified a cutoff value where patient staging reached 100% specificity and sensitivity for macro-and micrometastases. Conclusion: Quantitative RT-PCR for SLN staging in cN 0 patients with oral and oropharyngeal squamous cell carcinoma seems to be a promising approach.The extent of lymph node involvement in clinically and computed tomography scan N 0 (cN 0 ) oral and oropharyngeal squamous cell carcinomas is f30% to 40% and is one of the main prognostic factors (1). Until recently, a neck dissection was advocated routinely both to assess nodal involvement and to remove occult minimal residual cancer (2). A more recent approach consists of limiting lymph node surgery to a staging procedure by taking only the sentinel lymph nodes (SLN) which are representative of the whole neck node system (3). Such a strategy aims to permit more thorough analysis of only a few lymph nodes to enhance the sensitivity and the specificity of the diagnosis of lymph node invasion and thus select only pN+ patients on whom to perform a neck dissection. Intraoperative diagnosis must therefore have a predictive negative value close to 100%. At present, many methods of SLN analysis are used. The reference method for pathologic analysis is histopathologic examination by serial step sectioning with immunohistochemistry, staining with anticytokeratin antibodies in cases of squamous cell carcinoma (4, 5). Such analysis is able to diagnose three levels of nodal invasion (6): isolated tumor cells <0.2 mm, micrometastases V2 mm, and macromet...