Tammy Bowman

While signal transducers and activators of transcription (STATs) were originally discovered as intracellular effectors of normal signaling by cytokines, increasing evidence also points to a role for STAT transcription factors in oncogenesis. Previous studies have demonstrated that one STAT family member, Stat3, possesses constitutively elevated tyrosine phosphorylation and DNA-binding activity in fibroblasts stably transformed by the Src oncoprotein. To determine if this Stat3 activation by Src could induce Stat3-mediated gene expression, luciferase reporter constructs based on synthetic and authentic promoters were transfected into NIH 3T3 cells. Activation of endogenous cellular Stat3 by the Src oncoprotein induced gene expression through a Stat3-specific binding element (TTCCCGAA) of the C-reactive protein gene promoter. A naturally occurring splice variant of human Stat3 protein, Stat3beta, with a deletion in the C-terminal transactivation domain abolished this gene induction in a dominant negative manner. Expression of Stat3beta did not have any effect on a reporter construct based on the c-fos serum response element, which is not dependent on Stat3 signaling, indicating that Stat3beta does not nonspecifically inhibit other signaling pathways or Src function. Transfection of vectors expressing Stat3beta together with Src blocked cell transformation by Src as measured in a quantitative focus formation assay using NIH 3T3 cells. By contrast, Stat3beta had a much less pronounced effect on focus formation induced by the Ras oncoprotein, which does not activate Stat3 signaling. In addition, three independent clones of NIH 3T3 cells stably overexpressing Stat3beta were generated and characterized, demonstrating that Stat3beta overexpression does not have a toxic effect on cell viability. These Stat3beta-overexpressing clones were shown to be deficient in Stat3-mediated signaling and refractory to Src-induced cell transformation. We conclude that Stat3 activation by the Src oncoprotein leads to specific gene regulation and that Stat3 is one of the critical signaling pathways involved in Src oncogenesis. Our findings provide evidence that oncogenesis-associated activation of Stat3 signaling is part of the process of malignant transformation.

PURPOSE: Signal transducer and activator of transcription 3 (Stat3) protein is persistently activated in breast cancer and promotes tumor cell survival. To gain a better understanding of the role of constitutive Stat3 signaling in breast cancer progression, we evaluated the expression profile of potential Stat3-regulated genes that may confer resistance to apoptosis. EXPERIMENTAL DESIGN: Stat3 signaling was blocked with antisense oligonucleotides in human MDA-MB-435s breast cancer cells and Affymetrix GeneChip microarray analysis was done. The candidate Stat3 target gene Survivin was further evaluated in molecular assays using cultured breast cancer cells and immunohistochemistry of breast tumor specimens. RESULTS: Survivin, a member of the inhibitor of apoptosis protein family, was identified as a potential Stat3-regulated gene by microarray analysis. This was confirmed in Survivin gene promoter studies and chromatin immunoprecipitation assays showing that Stat3 directly binds to and regulates the Survivin promoter. Furthermore, direct inhibition of Stat3 signaling blocked the expression of Survivin protein and induced apoptosis in breast cancer cells. Direct inhibition of Survivin expression also induced apoptosis. Increased Survivin protein expression correlates significantly (P = 0.001) with elevated Stat3 activity in primary breast tumor specimens from high-risk patients who were resistant to chemotherapy treatment. CONCLUSIONS: We identify Survivin as a direct downstream target gene of Stat3 in human breast cancer cells that is critical for their survival in culture. Our findings suggest that activated Stat3 signaling contributes to breast cancer progression and resistance to chemotherapy by, at least in part, inducing expression of the antiapoptotic protein, Survivin.

Signal transducers and activators of transcription (STATs) were identified originally as key components of cytokine signaling pathways. More recently, constitutive activation of STAT proteins has been detected in a wide variety of human tumor specimens and tumor cell lines. Here, we examined the activation of one STAT family member, Stat3, in human prostate cancer cell lines and primary prostate tumors. An analysis of 45 adenocarcinomas obtained at radical prostatectomy revealed elevated levels of constitutive Stat3 activation in 37 (82%) of 45 of the tumors compared with matched adjacent nontumor prostate tissues. A highly specific immunohistochemical assay for detection of phospho-Stat3 revealed that elevated Stat3 activity was localized primarily in the tumor cells of prostate carcinoma specimens. Furthermore, higher levels of Stat3 activation in patient specimens were correlated significantly with more malignant tumors exhibiting higher Gleason scores. In addition, all of the three human prostate cancer cell lines examined (DU145, PC3, and LNCaP) displayed constitutive activation of Stat3. Substantially lower levels of Stat3 activation were detected in LNCaP cells; however, stimulation with interleukin 6 induced a significant increase in Stat3 DNA-binding activity in these cells. Moreover, the direct inhibition of constitutive Stat3 signaling in DU145 cells using antisense Stat3 oligonucleotides induced growth inhibition and apoptosis. Our findings demonstrate that constitutive activation of Stat3 occurs frequently in primary prostate adenocarcinomas and is critical for the growth and survival of prostate cancer cells. These studies further suggest that Stat3 signaling represents a potentially novel molecular target for prostate cancer therapy.