Stat3 is required for full neoplastic transformation by the Simian Virus 40 large tumor antigen

KPNA2 promotes angiogenesis by regulating STAT3 phosphorylation

PURPOSE

Angiogenesis is involved in many pathological and physiological processes and is mainly driven by hypoxia. Karyopherin subunit alpha 2 (KPNA2), a member of the nuclear transport protein family, was recently shown to be induced by hypoxia in various types of tumours, so we aimed to investigate the role and mechanism of KPNA2 in angiogenesis under hypoxia.

MATERIALS AND METHODS

After overexpression or knockdown of KPNA2 in human umbilical vein endothelial cells (HUVEC) by adenovirus vector infection, the tube formation, proliferation and migration of HUVEC under hypoxia were detected by tubule formation assay, 5-ethynyl-2'-deoxyuridine (EdU) staining and Transwell assay, respectively. After overexpression or knockdown of KPNA2 in a murine hindlimb ischemia model by local injection of purified adenovirus vector into the gastrocnemius muscle, blood flow changes were examined with a laser Doppler system. Changes in KPNA2-binding proteins under hypoxia were detected by immunoprecipitation-mass spectrometry (IP-MS) and co-immunoprecipitation (Co-IP). The effect of KPNA2 on signal transducer and activator of transcription 3 (STAT3) was detected by Western blotting and quantitative RT‒PCR.

RESULTS

KPNA2 was upregulated in the HUVEC hypoxia model and murine hindlimb ischemia model. Overexpression of KPNA2 increased the proliferation, migration and tube formation of HUVEC under hypoxia, while knockdown of KPNA2 reduced the proliferation, migration and tube formation of HUVEC. Overexpression of KPNA2 promoted the restoration of blood flow in the murine hindlimb ischemia model, while knockout of KPNA2 inhibited the restoration of blood flow in the murine hindlimb ischemia model. Mechanistically, hypoxia promoted the binding of STAT3 to KPNA2. Overexpression of KPNA2 promoted STAT3 phosphorylation and then upregulated vascular endothelial growth factor (VEGF) and angiopoietin 2(ANGPT2), whereas knockdown of KPNA2 inhibited STAT3 phosphorylation and then downregulated VEGF and ANGPT2.

CONCLUSION

Our study demonstrates that hypoxia promotes the binding of STAT3 to KPNA2 and KPNA2 promotes angiogenesis under hypoxia by promoting the binding of STAT3 and JAK1 and regulating STAT3 phosphorylation.

Activated Src requires Cadherin-11, Rac, and gp130 for Stat3 activation and survival of mouse Balb/c3T3 fibroblasts

We previously demonstrated that engagement of cadherins, cell to cell adhesion molecules, triggers a dramatic increase in levels and activity of the Rac/Cdc42 small GTPases, which is followed by secretion of IL6 family cytokines and activation of their common receptor, gp130, in an autocrine manner. This results in phosphorylation of the Signal Transducer and Activator of Transcription-3 (Stat3) on tyrosine-705, which then dimerizes, migrates to the nucleus, and activates transcription of genes involved in cell division and survival. In the present report we demonstrate that, in mouse Balb/c3T3 fibroblasts, mutationally activated Src527F also increases Rac levels, leading to secretion of IL6 family cytokines and gp130 activation, which triggers the Stat3-ptyr705 increase. Interestingly, our results also demonstrate that cadherin-11 is required to preserve gp130 levels for IL6 family signaling. At the same time, however, activated Src527F downregulates cadherin-11, in a quantitative manner. As a result, Src527F expression to intermediate levels allows sufficient cadherin-11, hence gp130 levels for Stat3 activation, as expected. However, expressed to high levels, Src527F eliminates cadherin-11, hence gp130 signaling, thus abolishing Stat3-ptyr705 stimulation. Taken together, these data establish for the first time a loop between Src, cadherin-11, gp130, and Stat3 activation. This fine balance between Src527F and cadherin-11 levels which is required for Stat3 activation and cellular survival could have significant therapeutic implications.

Roads to Stat3 Paved with Cadherins

The engagement of cadherins, cell-to-cell adhesion proteins, triggers a dramatic increase in the levels and activity of the Rac/Cdc42 GTPases, through the inhibition of proteasomal degradation. This leads to an increase in transcription and secretion of IL6 family cytokines, activation of their common receptor, gp130, in an autocrine manner and phosphorylation of the signal transducer and activator of transcription-3 (Stat3) on tyrosine-705 by the Jak kinases. Stat3 subsequently dimerizes, migrates to the nucleus and activates the transcription of genes involved in cell division and survival. The Src oncogene also increases Rac levels, leading to secretion of IL6 family cytokines and gp130 activation, which triggers a Stat3-ptyr705 increase. Interestingly, at the same time, Src downregulates cadherins in a quantitative manner, while cadherins are required to preserve gp130 levels for IL6 family signalling. Therefore, a fine balance between Src527F/Rac/IL6 and Src527F/cadherin/gp130 levels is in existence, which is required for Stat3 activation. This further demonstrates the important role of cadherins in the activation of Stat3, through preservation of gp130 function. Conversely, the absence of cadherin engagement correlates with low Stat3 activity: In sparsely growing cells, both gp130 and Stat3-ptyr705 levels are very low, despite the fact that cSrc is active in the FAK (focal adhesion kinase)/cSrc complex, which further indicates that the engagement of cadherins is important for Stat3 activation, not just their presence. Furthermore, the caveolin-1 protein downregulates Stat3 through binding and sequestration of cadherins to the scaffolding domain of caveolin-1. We hypothesize that the cadherins/Rac/gp130 axis may be a conserved pathway to Stat3 activation in a number of systems. This fact could have significant implications in Stat3 biology, as well as in drug testing and development.

Modulation of Akt vs Stat3 activity by the focal adhesion kinase in non-neoplastic mouse fibroblasts

Adhesion of cells to each other and to the extracellular matrix (ECM) are both required for cellular functions. Cell-to-cell adhesion is mediated by cadherins, and their engagement triggers the activation of Stat3, which offers a potent survival signal. Adhesion to the ECM on the other hand, activates FAK which attracts and activates Src, as well as receptor tyrosine kinases (RTKs), the PI3k/Akt and Ras/Erk pathways. However, the effect of cell density upon FAK and Akt activity has not been examined. We now demonstrate that, interestingly, despite being potent Stat3 activators, Src and RTKs are unable to activate Stat3 in sparsely growing (i.e., without cadherin engagement), non-neoplastic cells attached to the ECM. In contrast, cell aggregation (i.e., cadherin engagement in the absence of adhesion to a solid substratum) was found to activate both Stat3 and Akt. Pharmacologic or genetic reduction of FAK activity abolished Akt activity at low densities, indicating that FAK is an important activator of Akt in this setting. Notably, FAK knockout increased cellular sensitivity to the Stat3 inhibitor CPA7, while FAK reintroduction restored resistance to this drug. These findings suggest a complementary role of integrin/FAK/Akt and cadherin/Stat3-mediated pro-survival pathways, which may be of significance during neoplastic transformation and metastasis.

Modulation of Akt vs Stat3 activity by the focal adhesion kinase in non-neoplastic mouse fibroblasts

Adhesion of cells to each other and to the extracellular matrix (ECM) are both required for cellular functions. Cell-to-cell adhesion is mediated by cadherins and their engagement triggers the activation of Stat3, which offers a potent survival signal. Adhesion to the ECM on the other hand, activates FAK which attracts and activates Src, as well as receptor tyrosine kinases (RTKs), the PI3k/Akt and Ras/Erk pathways. However, the effect of cell density upon FAK and Akt activity has not been examined. We now demonstrate that, interestingly, despite being potent Stat3 activators, Src and RTKs are unable to activate Stat3 in sparsely growing (i.e., without cadherin engagement), non-neoplastic cells attached to the ECM. In contrast, cell aggregation (i.e., cadherin engagement in the absence of adhesion to a solid substratum) was found to activate both Stat3 and Akt. Pharmacologic or genetic reduction of FAK activity abolished Akt activity at low densities, indicating that FAK is an important activator of Akt in this setting. Notably, FAK knockout increased cellular sensitivity to the Stat3 inhibitor CPA7, while FAK reintroduction restored resistance to this drug. These findings suggest a complementary role of integrin/FAK/Akt and cadherin/Stat3-mediated pro-survival pathways, which may be of significance during neoplastic transformation and metastasis.

Acetyl-bufalin shows potent efficacy against non-small-cell lung cancer by targeting the CDK9/STAT3 signalling pathway

BACKGROUND

Cyclin-dependent kinase 9 (CDK9) is a promising prognostic marker and therapeutic target in cancers. Bufalin is an effective anti-tumour agent; however, the clinical application of bufalin is limited due to its high toxicity. Acetyl-bufalin, the bufalin prodrug, was designed and synthesised with higher efficiency and lower toxicity.

METHODS

Three non-small-cell lung cancer (NSCLC) cell lines, a xenograft model and a patient-derived xenograft (PDX) model were used to examine the effects of acetyl-bufalin. CDK9/STAT3 involvement was investigated by knockdown with siRNA, proteome microarray assay, western blot analysis and co-immunoprecipitation experiments. Acute toxicity test and pharmacokinetics (PK) study were conducted to assess the safety and PK. The human NSCLC tissues were analysed to verify high CDK9 expression.

RESULTS

We showed that CDK9 induced NSCLC cell proliferation and that this effect was associated with STAT3 activation, specifically an increase in STAT3 phosphorylation and transcription factor activity. Acetyl-bufalin is an effective and safety inhibitor of the CDK9/STAT3 pathway, leading to the impediment of various oncogenic processes in NSCLC. Molecular docking and high-throughput proteomics platform analysis uncovered acetyl-bufalin directly binds to CDK9. Consequently, acetyl-bufalin impaired the complex formation of CDK9 and STAT3, decreased the expressions of P-STAT3, and transcribed target genes such as cyclin B1, CDC2, MCL-1, Survivin, VEGF, BCL2, and it upregulated the expression levels of BAX and caspase-3 activity. Acetyl-bufalin inhibited tumour growth in NSCLC xenograft and PDX models.

CONCLUSIONS

Acetyl-bufalin is a novel blocker of the CDK9/STAT3 pathway thus may have potential in therapy of NSCLC and other cancers.

Effect of caveolin-1 on Stat3-ptyr705 levels in breast and lung carcinoma cells

We recently demonstrated that Cav1 (caveolin-1) is a negative regulator of Stat3 (signal transducer and activator of transcription-3) activity in mouse fibroblasts and human lung carcinoma SHP77 cells. We now examined whether the cellular context may affect their levels as well as the relationship between them, by assessing Cav1 and Stat3-ptyr705 amounts in different cell lines. In MDA-MB-231, A549, and HaCat cells, Cav1 levels were high and Stat3-ptyr705 levels were low, consistent with the notion of a negative effect of endogenous Cav1 on Stat3-ptyr705 levels in these lines. In addition, manipulation of Cav1 levels revealed a negative effect in MCF7 and mouse fibroblast cells, while Cav1 upregulation induced apoptosis in MCF7 cells. In contrast, however, line MRC9 had high Cav1 and high Stat3-ptyr705 levels, indicating that high Cav1 is insufficient to reduce Stat3-ptyr705 levels in this line. MCF7 and LuCi6 cells had very low Cav1 and Stat3-ptyr705 levels, indicating that the low Stat3-ptyr705 can be independent from Cav1 levels altogether. Our results reveal a further level of complexity in the relationship between Cav1 and Stat3-ptyr705 than previously thought. In addition, we demonstrate that in a feedback loop, Stat3 inhibition upregulates Cav1 in HeLa cells but not in other lines tested.

PI3k and Stat3: Oncogenes that are Required for Gap Junctional, Intercellular Communication

Gap junctional, intercellular communication (GJIC) is interrupted in cells transformed by oncogenes such as activated Src. The Src effector, Ras, is required for this effect, so that Ras inhibition restores GJIC in Src-transformed cells. Interestingly, the inhibition of the Src effector phosphatidyl-inositol-3 kinase (PI3k) or Signal Transducer and Activator of Transcription-3 (Stat3) pathways does not restore GJIC. In the contrary, inhibition of PI3k or Stat3 in non-transformed rodent fibroblasts or epithelial cells or certain human lung carcinoma lines with extensive GJIC inhibits communication, while mutational activation of PI3k or Stat3 increases GJIC. Therefore, it appears that oncogenes such as activated Src have a dual role upon GJIC; acting as inhibitors of communication through the Ras pathway, and as activators through activation of PI3k or Stat3. In the presence of high Src activity the inhibitory functions prevail so that the net effect is gap junction closure. PI3k and Stat3 constitute potent survival signals, so that their inhibition in non-transformed cells triggers apoptosis which, in turn, has been independently demonstrated to suppress GJIC. The interruption of gap junctional communication would confine the apoptotic event to single cells and this might be essential for the maintenance of tissue integrity. We hypothesize that the GJIC activation by PI3k or Stat3 may be linked to their survival function.

STAT3 roles in viral infection: antiviral or proviral?

Signal transducer and activator of transcription 3 (STAT3) is a transcription factor which can be activated by cytokines, growth factor receptors, and nonreceptor-like tyrosine kinase. An activated STAT3 translocates into the nucleus and combines with DNA to regulate the expression of target genes involved in cell proliferation, differentiation, apoptosis and metastasis. Recent studies have shown that STAT3 plays important roles in viral infection and pathogenesis. STAT3 exhibits a proviral function in several viral infections, including those of HBV, HCV, HSV-1, varicella zoster virus, human CMV and measles virus. However, in some circumstances, STAT3 has an antiviral function in other viral infections, such as enterovirus 71, severe acute respiratory syndrome coronavirus and human metapneumovirus. This review summarizes the roles of STAT3 in viral infection and pathogenesis, and briefly discusses the molecular mechanisms involved in these processes.

Reciprocal regulation of the Cadherin-11/Stat3 axis by caveolin-1 in mouse fibroblasts and lung carcinoma cells

Caveolin-1 (Cav1) is an integral plasma membrane protein and a complex regulator of signal transduction. The Signal Transducer and Activator of Transcription-3 (Stat3) is activated by a number of receptor and non-receptor tyrosine kinases and is positively implicated in cancer. Despite extensive efforts, the relationship between Cav1 and Stat3 has been a matter of controversy. We previously demonstrated that engagement of E- or N-cadherin or cadherin-11 cell to cell adhesion molecules, as occurs with confluence of cultured cells, triggers a dramatic increase in the levels of tyr705 phosphorylated i.e. activated Stat3, by a mechanism requiring the cRac1 small GTPase. Since confluence was not taken into account in previous studies, we revisited the question of the relationship between Cav1 and Stat3-ptyr705 in non-transformed mouse fibroblasts and in human lung carcinoma cells, by examining their effect at different cell densities. Our results unequivocally demonstrate that Cav1 downregulates cadherin-11, by a mechanism which requires the Cav1 scaffolding domain. This cadherin-11 downregulation, in turn, leads to a reduction in cRac1 and Stat3 activity levels. Furthermore, in a feedback loop possibly through p53 upregulation, Stat3 downregulation increases Cav1 levels. Our data reveal the presence of a potent, negative regulatory loop between Cav1 and cadherin-11/Stat3, leading to Stat3 inhibition and apoptosis.

STAT3 targeting by polyphenols: Novel therapeutic strategy for melanoma

Melanoma or malignant melanocytes appear with the low incidence rate, but very high mortality rate worldwide. Epidemiological studies suggest that polyphenolic compounds contribute for prevention or treatment of several cancers particularly melanoma. Such findings motivate to dig out novel therapeutic strategies against melanoma, including research toward the development of new chemotherapeutic and biologic agents that can target the tumor cells by different mechanisms. Recently, it has been found that signal transducer and activator of transcription 3 (STAT3) is activated in many cancer cases surprisingly. Different evidences supply the aspect that STAT3 activation plays a vital role in the metastasis, including proliferation of cells, survival, invasion, migration, and angiogenesis. This significant feature plays a vital role in various cellular processes, such as cell proliferation and survival. Here, we reviewed the mechanisms of the STAT3 pathway regulation and their role in promoting melanoma. Also, we have evaluated the emerging data on polyphenols (PPs) specifically their contribution in melanoma therapies with an emphasis on their regulatory/inhibitory actions in relation to STAT3 pathway and current progress in the development of phytochemical therapeutic techniques. An understanding of targeting STAT3 by PPs brings an opportunity to melanoma therapy. © 2016 BioFactors, 43(3):347-370, 2017.

A Central Role for STAT3 in Gammaherpesvirus-Life Cycle and -Diseases

Having co-evolved with humans, herpesviruses have adapted to exploit the host molecular machinery to ensure viral persistence. The cellular protein Signal Transducer and Activator of Transcription 3 (STAT3) is a leading example. STAT3 is a prominent transcription factor that functions in a variety of physiologic processes including embryonic development, inflammation, immunity, and wound healing. Generally activated via growth factor and cytokine signaling, STAT3 can transcriptionally drive oncoproteins, pro-survival and pro-proliferative proteins as well as angiogenic factors, thereby contributing to cancer. As in most non-viral cancers, STAT3 is constitutively active in EBV-related B and epithelial cell cancers and in animal models of KSHV-cancers. Again, similar to non-viral cancers, STAT3 contributes to gammaherpesvirus (EBV and KSHV)-mediated cancers by driving cell proliferation, invasion and angiogenesis. Being herpesviruses, EBV and KSHV establish latency in humans with episodic lytic activation. Importantly, both viruses activate STAT3 almost immediately upon infection of primary cells. In the setting of infection of primary B cells by EBV, this rapidly activated STAT3 plays a key role in suppressing the DNA damage response (DDR) to EBV-oncogene triggered replication stress, thereby facilitating B cell proliferation and ultimately establishment of latency. STAT3 also contributes to maintenance of latency by curbing lytic activation of EBV and KSHV in latent cells that express high levels of STAT3. In this way, gammaherpesviruses exploit STAT3 to overcome cellular anti-proliferative and anti-lytic barriers to promote viral persistence. These investigations into gammaherpesviruses and STAT3 have simultaneously revealed a novel function for STAT3 in suppression of the DDR, a process fundamental to physiologic cell proliferation as well as development of cancer.

Cell-cell and cell-matrix adhesion in survival and metastasis: Stat3 versus Akt

Both cell-cell and cell-matrix adhesion are important for epithelial cell differentiation and function. Classical cadherins mediate cell to cell interactions and are potent activators of the signal transducer and activator of transcription (Stat3), thereby offering survival signaling. While the epithelial (E)-cadherin is required for cells to remain tightly associated within differentiated epithelial tissues, cadherin-11 promotes invasion and metastasis, preferentially to the bone. Cell adhesion to the extracellular matrix is mediated through the integrin receptors that bind to the focal adhesion kinase (FAK)/Src complex, thus activating downstream effectors such as Ras/Erk1/2 and PI3k/Akt, but not Stat3. Therefore, at high densities of cultured cells or in epithelial tissues, co-ordinate activation of the complementary cadherin/Stat3 and integrin/FAK pathways can greatly enhance survival and growth of tumor cells. In neoplastically transformed cells on the other hand, a variety of oncogenes including activated Src or receptor tyrosine kinases, activate both pathways. Still, most single-agent therapies directed against these signaling pathways have proven disappointing in the clinic. Combined targeting of the Src/FAK and Stat3 pathways with inhibitory drugs would be expected to have greater efficacy in inhibiting tumor cell survival, and enhancing sensitivity to conventional cytotoxic drugs for treatment of metastatic disease.

Screening Active Compounds from Garcinia Species Native to China Reveals Novel Compounds Targeting the STAT/JAK Signaling Pathway

Natural compounds from medicinal plants are important resources for drug development. In a panel of human tumor cells, we screened a library of the natural products from Garcinia species which have anticancer potential to identify new potential therapeutic leads and discovered that caged xanthones were highly effective at suppressing multiple cancer cell lines. Their anticancer activities mainly depended on apoptosis pathways. For compounds in sensitive cancer line, their mechanisms of mode of action were evaluated. 33-Hydroxyepigambogic acid and 35-hydroxyepigambogic acid exhibited about 1 μM IC50 values against JAK2/JAK3 kinases and less than 1 μM IC50 values against NCI-H1650 cell which autocrined IL-6. Thus these two compounds provided a new antitumor molecular scaffold. Our report describes 33-hydroxyepigambogic acid and 35-hydroxyepigambogic acid that inhibited NCI-H1650 cell growth by suppressing constitutive STAT3 activation via direct inhibition of JAK kinase activity.

Targeted Blockade of JAK/STAT3 Signaling Inhibits Ovarian Carcinoma Growth

Ovarian carcinoma is the fifth leading cause of death among women in the United States. Persistent activation of STAT3 is frequently detected in ovarian carcinoma. STAT3 is activated by Janus family kinases (JAK) via cytokine receptors, growth factor receptor, and non-growth factor receptor tyrosine kinases. Activation of STAT3 mediates tumor cell proliferation, survival, motility, invasion, and angiogenesis, and recent work demonstrates that STAT3 activation suppresses antitumor immune responses and supports tumor-promoting inflammation. We hypothesized that therapeutic targeting of the JAK/STAT3 pathway would inhibit tumor growth by direct effects on ovarian carcinoma cells and by inhibition of cells in the tumor microenvironment (TME). To test this, we evaluated the effects of a small-molecule JAK inhibitor, AZD1480, on cell viability, apoptosis, proliferation, migration, and adhesion of ovarian carcinoma cells in vitro. We then evaluated the effects of AZD1480 on in vivo tumor growth and progression, gene expression, tumor-associated matrix metalloproteinase (MMP) activity, and immune cell populations in a transgenic mouse model of ovarian carcinoma. AZD1480 treatment inhibited STAT3 phosphorylation and DNA binding, and migration and adhesion of cultured ovarian carcinoma cells and ovarian tumor growth rate, volume, and ascites production in mice. In addition, drug treatment led to altered gene expression, decreased tumor-associated MMP activity, and fewer suppressor T cells in the peritoneal TME of tumor-bearing mice than control mice. Taken together, our results show pharmacologic inhibition of the JAK2/STAT3 pathway leads to disruption of functions essential for ovarian tumor growth and progression and represents a promising therapeutic strategy.

A functional assay for gap junctional examination; electroporation of adherent cells on indium-tin oxide

In this technique, cells are cultured on a glass slide that is partly coated with indium-tin oxide (ITO), a transparent, electrically conductive material. A variety of molecules, such as peptides or oligonucleotides can be introduced into essentially 100% of the cells in a non-traumatic manner. Here, we describe how it can be used to study intercellular, gap junctional communication. Lucifer yellow penetrates into the cells when an electric pulse, applied to the conductive surface on which they are growing, causes pores to form through the cell membrane. This is electroporation. Cells growing on the nonconductive glass surface immediately adjacent to the electroporated region do not take up Lucifer yellow by electroporation but do acquire the fluorescent dye as it is passed to them via gap junctions that link them to the electroporated cells. The results of the transfer of dye from cell to cell can be observed microscopically under fluorescence illumination. This technique allows for precise quantitation of gap junctional communication. In addition, it can be used for the introduction of peptides or other non-permeant molecules, and the transfer of small electroporated peptides via gap junctions to inhibit the signal in the adjacent, non-electroporated cells is a powerful demonstration of signal inhibition.

Stat3 and gap junctions in normal and lung cancer cells

Gap junctions are channels linking the interiors of neighboring cells. A reduction in gap junctional intercellular communication (GJIC) correlates with high cell proliferation, while oncogene products such as Src suppress GJIC, through the Ras/Raf/Erk and other effector pathways. High Src activity was found to correlate with high levels of the Src effector, Signal Transducer and Activator of Transcription-3 (Stat3) in its tyrosine-705 phosphorylated, i.e., transcriptionally activated form, in the majority of Non-Small Cell Lung Cancer lines examined. However, Stat3 inhibition did not restore GJIC in lines with high Src activity. In the contrary, Stat3 inhibition in normal cells or in lines with low Src activity and high GJIC eliminated gap junctional communication. Therefore, despite the fact that Stat3 is growth promoting and in an activated form acts like an oncogene, it is actually required for junctional permeability.

Engaged for survival: From cadherin ligation to STAT3 activation

In normal tissues or tumors, cells have extensive opportunities for adhesion to their neighbors. This state is mimicked by dense cell cultures. In this review, we integrate some recent findings on a key signal transducer, STAT3 (signal transducer and activator of transcription-3), whose activity is dramatically increased following cadherin-mediated cell to cell adhesion. Cadherin engagement, favored in dense cell cultures, causes a dramatic increase in total Rac/Cdc42 protein levels through inhibition of proteasomal degradation, which is followed by activation of IL-6 and STAT3. The cadherin/Rac/IL-6/STAT3 axis offers a potent survival signal that is a prerequisite for neoplastic transformation, as well as normal tissue function.

Role of STAT3 in cancer metastasis and translational advances

Signal transducer and activator of transcription 3 (STAT3) is a latent cytoplasmic transcription factor, originally discovered as a transducer of signal from cell surface receptors to the nucleus. It is activated by tyrosine phosphorylation at position 705 leading to its dimerization, nuclear translocation, DNA binding, and activation of gene transcription. Under normal physiological conditions, STAT3 activation is tightly regulated. However, compelling evidence suggests that STAT3 is constitutively activated in many cancers and plays a pivotal role in tumor growth and metastasis. It regulates cellular proliferation, invasion, migration, and angiogenesis that are critical for cancer metastasis. In this paper, we first describe the mechanism of STAT3 regulation followed by how STAT3 is involved in cancer metastasis, then we summarize the various small molecule inhibitors that inhibit STAT3 signaling.

Critical analysis of the potential for targeting STAT3 in human malignancy

The signal transducer and activator of transcription (STAT) family of proteins was originally discovered in the context of normal cell biology where they function to transduce intracellular and extracellular signals to the nucleus, ultimately leading to transcription of specific target genes and downstream phenotypic effects. It was quickly appreciated that the STATs, especially STAT3, play a fundamental role in human malignancy. In contrast to normal biology in which transient STAT3 signaling is strictly regulated by a tightly coordinated network of activators and deactivators, STAT3 is constitutively activated in human malignancies. Constitutive STAT3 signaling has been associated with many cancerous phenotypes across nearly all human cancers, including the upregulation of cell growth, proliferation, survival, and motility, among others. Studies involving candidate preclinical STAT3 inhibitors have further demonstrated that the reversal of these phenotypes results from pharmacologic or genetic inhibition of STAT3, suggesting that STAT3 may be a promising target for clinical interventions. Indeed, a Phase 0 clinical trial involving a STAT3 decoy oligonucleotide demonstrated that STAT3 is a drug-gable target in human tumors. Because of the ubiquity of overactive STAT3 in cancer, its role in promoting a wide variety of cancerous phenotypes, and the strong clinical and preclinical studies performed to date, STAT3 represents a promising target for the development of inhibitors for the treatment of human cancers.

Functional regulatory role of STAT3 in HPV16-mediated cervical carcinogenesis

Signal transducer and activator of transcription 3 (STAT3) is an oncogenic transcription factor constitutively active and aberrantly expressed in cervical cancer. However, the functional role of STAT3 in regulation of HPV's viral oncogene expression and downstream events associated with cervical carcinogenesis is not known. Our present study performed on HPV16-positive cervical cancer cell lines (SiHa and CaSki) and primary tumor tissues revealed a strong positive correlation of constitutively active STAT3 with expression of HPV16 E6 and E7 oncoproteins and a negative association with levels of p53 and pRB. Pharmacologic targeting of STAT3 expression in cervical cancer cell lines either by STAT3-specific siRNA or blocking its tyrosine phosphorylation by AG490 or curcumin led to dose-dependent accumulation of p53 and pRb in cervical cancer cells. Interestingly, the suppression of STAT3 expression or activation was associated with the gradual loss of HPV16 E6 and E7 expression and was accompanied by loss of cell viability. The viability loss was specifically high in HPV16-positive cells as compared to HPV negative C33a cells. These findings substantiate the regulatory role of STAT3 in HPV16-mediated cervical carcinogenesis. Leads obtained from the present study provide a strong rationale for developing novel STAT3-based approaches for therapeutic interventions against HPV infection to control cervical cancer.

Stat3 is a positive regulator of gap junctional intercellular communication in cultured, human lung carcinoma cells

Background

Neoplastic transformation of cultured cells by a number of oncogenes such as src suppresses gap junctional, intercellular communication (GJIC); however, the role of Src and its effector Signal transducer and activator of transcription-3 (Stat3) upon GJIC in non small cell lung cancer (NSCLC) has not been defined. Immunohistochemical analysis revealed high Src activity in NSCLC biopsy samples compared to normal tissues. Here we explored the potential effect of Src and Stat3 upon GJIC, by assessing the levels of tyr418-phosphorylated Src and tyr705-phosphorylated Stat3, respectively, in a panel of NSCLC cell lines.

Methods

Gap junctional communication was examined by electroporating the fluorescent dye Lucifer yellow into cells grown on a transparent electrode, followed by observation of the migration of the dye to the adjacent, non-electroporated cells under fluorescence illumination.

Results

An inverse relationship between Src activity levels and GJIC was noted; in five lines with high Src activity GJIC was absent, while two lines with extensive GJIC (QU-DB and SK-LuCi6) had low Src levels, similar to a non-transformed, immortalised lung epithelial cell line. Interestingly, examination of the mechanism indicated that Stat3 inhibition in any of the NSCLC lines expressing high endogenous Src activity levels, or in cells where Src was exogenously transduced, did not restore GJIC. On the contrary, Stat3 downregulation in immortalised lung epithelial cells or in the NSCLC lines displaying extensive GJIC actually suppressed junctional permeability.

Conclusions

Our findings demonstrate that although Stat3 is generally growth promoting and in an activated form it can act as an oncogene, it is actually required for gap junctional communication both in nontransformed lung epithelial cells and in certain lung cancer lines that retain extensive GJIC.

STAT3 is activated by JAK2 independent of key oncogenic driver mutations in non-small cell lung carcinoma

Constitutive activation of STAT3 is a common feature in many solid tumors including non-small cell lung carcinoma (NSCLC). While activation of STAT3 is commonly achieved by somatic mutations to JAK2 in hematologic malignancies, similar mutations are not often found in solid tumors. Previous work has instead suggested that STAT3 activation in solid tumors is more commonly induced by hyperactive growth factor receptors or autocrine cytokine signaling. The interplay between STAT3 activation and other well-characterized oncogenic “driver” mutations in NSCLC has not been fully characterized, though constitutive STAT3 activation has been proposed to play an important role in resistance to various small-molecule therapies that target these oncogenes. In this study we demonstrate that STAT3 is constitutively activated in human NSCLC samples and in a variety of NSCLC lines independent of activating KRAS or tyrosine kinase mutations. We further show that genetic or pharmacologic inhibition of the gp130/JAK2 signaling pathway disrupts activation of STAT3. Interestingly, treatment of NSCLC cells with the JAK1/2 inhibitor ruxolitinib has no effect on cell proliferation and viability in two-dimensional culture, but inhibits growth in soft agar and xenograft assays. These data demonstrate that JAK2/STAT3 signaling operates independent of known driver mutations in NSCLC and plays critical roles in tumor cell behavior that may not be effectively inhibited by drugs that selectively target these driver mutations.

The R(h)oads to Stat3: Stat3 activation by the Rho GTPases

The signal transducer and activator of transcription-3 (Stat3) is a member of the STAT family of cytoplasmic transcription factors. Overactivation of Stat3 is detected with high frequency in human cancer and is considered a molecular abnormality that supports the tumor phenotype. Despite concerted investigative efforts, the molecular mechanisms leading to the aberrant Stat3 activation and Stat3-mediated transformation and tumorigenesis are still not clearly defined. Recent evidence reveals a crosstalk close relationship between Stat3 signaling and members of the Rho family of small GTPases, including Rac1, Cdc42 and RhoA. Specifically, Rac1, acting in a complex with the MgcRacGAP (male germ cell RacGAP), promotes tyrosine phosphorylation of Stat3 by the IL6-receptor family/Jak kinase complex, as well as its translocation to the nucleus. Studies have further revealed that the mutational activation of Rac1 and Cdc42 results in Stat3 activation, which occurs in part through the upregulation of IL6 family cytokines that in turn stimulates Stat3 through the Jak kinases. Interestingly, evidence also shows that the engagement of cadherins, cell to cell adhesion molecules, specifically induces a striking increase in Rac1 and Cdc42 protein levels and activity, which in turn results in Stat3 activation. In this review we integrate recent findings clarifying the role of the Rho family GTPases in Stat3 activation in the context of malignant progression.

Gambogic acid inhibits STAT3 phosphorylation through activation of protein tyrosine phosphatase SHP-1: potential role in proliferation and apoptosis

The transcription factor, STAT3, is associated with proliferation, survival, and metastasis of cancer cells. We investigated whether gambogic acid (GA), a xanthone derived from the resin of traditional Chinese medicine, Garcinia hanburyi (mangosteen), can regulate the STAT3 pathway, leading to suppression of growth and sensitization of cancer cells. We found that GA induced apoptosis in human multiple myeloma cells that correlated with the inhibition of both constitutive and inducible STAT3 activation. STAT3 phosphorylation at both tyrosine residue 705 and serine residue 727 was inhibited by GA. STAT3 suppression was mediated through the inhibition of activation of the protein tyrosine kinases Janus-activated kinase 1 (JAK1) and JAK2. Treatment with the protein tyrosine phosphatase (PTP) inhibitor pervanadate reversed the GA-induced downregulation of STAT3, suggesting the involvement of a PTP. We also found that GA induced the expression of the PTP SHP-1. Deletion of the SHP-1 gene by siRNA suppressed the ability of GA to inhibit STAT3 activation and to induce apoptosis, suggesting the critical role of SHP-1 in its action. Moreover, GA downregulated the expression of STAT3-regulated antiapoptotic (Bcl-2, Bcl-xL, and Mcl-1), proliferative (cyclin D1), and angiogenic (VEGF) proteins, and this correlated with suppression of proliferation and induction of apoptosis. Overall, these results suggest that GA blocks STAT3 activation, leading to suppression of tumor cell proliferation and induction of apoptosis.

Aberrant expression and constitutive activation of STAT3 in cervical carcinogenesis: implications in high-risk human papillomavirus infection

Background

Recent observations indicate potential role of transcription factor STAT3 in cervical cancer development but its role specifically with respect to HPV infection is not known. Present study has been designed to investigate expression and activation of STAT3 in cervical precancer and cancer in relation to HPV infection during cervical carcinogenesis. Established cervical cancer cell lines and prospectively-collected cervical precancer and cancer tissues were analyzed for the HPV positivity and evaluated for STAT3 expression and its phosphorylation by immunoblotting and immunohistochemistry whereas STAT3-specific DNA binding activity was examined by gel-shift assays.

Results

Analysis of 120 tissues from cervical precancer and cancer lesions or from normal cervix revealed differentially high levels of constitutively active STAT3 in cervical precancer and cancer lesions, whereas it was absent in normal controls. Similarly, a high level of constitutively active STAT3 expression was observed in HPV-positive cervical cancer cell lines when compared to that of HPV-negative cells. Expression and activity of STAT3 were found to change as a function of severity of cervical lesions from precancer to cancer. Expression of active pSTAT3 was specifically high in cervical precancer and cancer lesions found positive for HPV16. Interestingly, site-specific accumulation of STAT3 was observed in basal and suprabasal layers of HPV16-positive early precancer lesions which is indicative of possible involvement of STAT3 in establishment of HPV infection. In HPV16-positive cases, STAT3 expression and activity were distinctively higher in poorly-differentiated lesions with advanced histopathological grades.

Conclusion

We demonstrate that in the presence of HPV16, STAT3 is aberrantly-expressed and constitutively-activated in cervical cancer which increases as the lesion progresses thus indicating its potential role in progression of HPV16-mediated cervical carcinogenesis.

Housekeeping genes; expression levels may change with density of cultured cells

Western blotting is a powerful technique to characterize a multitude of cellular proteins. As an internal control, the blots are commonly probed for "housekeeping" gene products. In this communication, we show that cell confluence significantly affects the levels of two such widely used proteins, alpha-tubulin and Glyceraldehyde-3-Phosphate Dehydrogenase. On the other hand the levels of heat-shock protein-90 and beta-actin remained unchanged at a wide range of cell densities, making these proteins into more reliable loading controls.

Deregulated NFATc1 activity transforms murine fibroblasts via an autocrine growth factor-mediated Stat3-dependent pathway

The nuclear factor of activated T cells (NFAT) family of transcription factors has recently been implicated with a role in tumorigenesis. Forced expression of a constitutively active NFATc1 mutant (caNFATc1) has been shown to transform immortalized murine fibroblasts in vitro, while constitutive activation of the NFAT-signaling pathway has been found in a number of human cancers, where it has been shown to contribute towards various aspects of the tumor phenotype. Here we have investigated the molecular mechanisms underlying the oncogenic potential of deregulated NFAT activity. We now show that ectopic expression of caNFATc1 in murine 3T3-L1 fibroblasts induces the secretion of an autocrine factor(s) that is sufficient to promote the transformed phenotype. We further demonstrate that this NFATc1-induced autocrine factor(s) specifically induces the tyrosine phosphorylation of the Stat3 transcription factor via a JAK kinase-dependent pathway. Interestingly, this effect of sustained NFAT signaling on the autocrine growth factor-mediated activation of Stat3 is not restricted to murine fibroblasts, but is also observed in the PANC-1 and MCF10A human cell lines. Most importantly, we find that the shRNA-mediated depletion of endogenous Stat3 significantly attenuates the ability of caNFATc1 to transform 3T3-L1 fibroblasts. Taken together, our results afford significant new insights into the molecular mechanisms underlying the oncogenic potential of deregulated NFATc1 activity by demonstrating that constitutive NFATc1 activity transforms cells via an autocrine factor-mediated pathway that is critically dependent upon the activity of the Stat3 transcription factor.

Beyond structure, to survival: activation of Stat3 by cadherin engagement

Cells in normal tissues or in tumors have extensive opportunities for adhesion to their neighbors and the importance of cell to cell contact in the study of fundamental cellular processes is beginning to emerge. In this review, we discuss recent evidence of dramatic changes in the activity of an important signal transducer found to be profoundly affected by cell to cell adhesion, the signal transducer and activator of transcription-3 (Stat3). Direct cadherin engagement, growth of cells to postconfluence, or formation of multicellular aggregates were found to induce a striking increase in the levels of Stat3 activity, Rac1/Cdc42, and members of the IL6 receptor family in different settings. This activation was specific to Stat3, in that the levels of the extracellular signal regulated kinase (Erk1/2), a signal transducer often coordinately activated with Stat3 by a number of growth factors or oncogenes, remained unaffected by cell density. Density-dependent Stat3 activation may play a key role in survival, and could contribute to the establishment of cell polarity. It is clear that at any given time the total Stat3 activity levels in a cell are the sum of the effects of cell to cell adhesion plus the conventional Stat3 activating factors present.

Activated Rac1 requires gp130 for Stat3 activation, cell proliferation and migration

Rac1 (Rac) is a member of the Rho family of small GTPases which controls cell migration by regulating the organization of actin filaments. Previous results suggested that mutationally activated forms of the Rho GTPases can activate the Signal Transducer and Activator of Transcription-3 (Stat3), but the exact mechanism is a matter of controversy. We recently demonstrated that Stat3 activity of cultured cells increases dramatically following E-cadherin engagement. To better understand this pathway, we now compared Stat3 activity levels in mouse HC11 cells before and after expression of the mutationally activated Rac1 (Rac(V12)), at different cell densities. The results revealed for the first time a dramatic increase in protein levels and activity of both the endogenous Rac and Rac(V12) with cell density, which was due to inhibition of proteasomal degradation. In addition, Rac(V12)-expressing cells had higher Stat3, tyrosine-705 phosphorylation and activity levels at all densities, indicating that Rac(V12) is able to activate Stat3. Further examination of the mechanism of Stat3 activation showed that Rac(V12) expression caused a surge in mRNA of Interleukin-6 (IL6) family cytokines, known potent Stat3 activators. Knockdown of gp130, the common subunit of this family reduced Stat3 activity, indicating that these cytokines may be responsible for the Stat3 activation by Rac(V12). The upregulation of IL6 family cytokines was required for cell migration and proliferation induced by Rac(V12), as shown by gp130 knockdown experiments, thus demonstrating that the gp130/Stat3 axis represents an essential effector of activated Rac for the regulation of key cellular functions.

Stat3 activity is required for gap junctional permeability in normal rat liver epithelial cells

Neoplastic transformation by oncogenes such as activated Src is known to suppress gap junctional, intercellular communication (GJIC). One of the Src effector pathways leading to GJIC suppression and transformation is the Ras/Raf/Mek/Erk, so that inhibition of this pathway in vSrc-transformed cells restores GJIC. A distinct Src downstream effector required for neoplasia is the signal transducer and activator of transcription-3 (Stat3). To examine the role of Stat3 upon the Src-mediated, GJIC suppression, Stat3 was downregulated in rat liver epithelial cells expressing activated Src through treatment with the CPA7, Stat3 inhibitor, or through infection with a retroviral vector expressing a Stat3-specific shRNA. GJIC was examined by electroporating the fluorescent dye, Lucifer yellow, into cells grown on two coplanar electrodes of electrically conductive, optically transparent, indium-tin oxide, followed by observation of the migration of the dye to the adjacent, nonelectroporated cells under fluorescence illumination. The results demonstrate that, contrary to inhibition of the Ras pathway, Stat3 inhibition in cells expressing activated Src does not restore GJIC. On the contrary, Stat3 inhibition in normal cells with high GJIC levels eliminated junctional permeability. Therefore, Stat3's function is actually required for the maintenance of junctional permeability, although Stat3 generally promotes growth and in an activated form can act as an oncogene.

SKI-606 (bosutinib), a novel Src kinase inhibitor, suppresses migration and invasion of human breast cancer cells

Src family kinase activity is elevated in many human tumors, including breast cancer, and is often associated with aggressive disease. We examined the effects of SKI-606 (bosutinib), a selective Src family kinase inhibitor, on human cancer cells derived from breast cancer patients to assess its potential for breast cancer treatment. Our results show that SKI-606 caused a decrease in cell motility and invasion of breast cancer cell lines with an IC50 of approximately 250 nmol/L, which was also the IC50 for inhibition of cellular Src kinase activity in intact tumor cells. These changes were accompanied by an increase in cell-to-cell adhesion and membrane localization of beta-catenin. By contrast, cell proliferation and survival were unaffected by SKI-606 at concentrations sufficient to block cell migration and invasion. Analysis of downstream effectors of Src revealed that SKI-606 inhibits the phosphorylation of focal adhesion kinase (FAK), proline-rich tyrosine kinase 2 (Pyk2), and Crk-associated substrate (p130Cas), with an IC50 similar to inhibition of cellular Src kinase. Our findings indicate that SKI-606 inhibits signaling pathways involved in controlling tumor cell motility and invasion, suggesting that SKI-606 is a promising therapeutic for breast cancer.

Synthesis, characterization and Stat3 inhibitory properties of the prototypical platinum(IV) anticancer drug, [PtCl3(NO2)(NH3)2] (CPA-7)

This paper describes a reinvestigation of the literature concerning the synthesis and structural characterization of the platinum(IV)-based anticancer drug known as CPA-7 and believed to be the compound fac-[PtCl3(NO2)(NH 3)2]. CPA-7 has previously been extensively investigated for its ability to control tumor cell growth by inhibition of Stat3 signaling, but very little information is available concerning its synthesis or spectroscopic properties. A reproducible synthetic route is shown to produce an active material which is characterized by IR and (1)H, (14)N, (15)N, and (195)Pt NMR spectroscopy, and single crystal X-ray crystallography. The freshly prepared drug is obtained as a single isomer which may in fact be fac- or mer-[PtCl3(NO2)(NH3)2], but recrystallization resulted in a disordered crystal containing approximately equal amounts of the two geometric isomers.

Crif1 is a novel transcriptional coactivator of STAT3

Signal transducer and activator of transcription 3 (STAT3) is a transcriptional factor that performs a broad spectrum of biological functions in response to various stimuli. However, no specific coactivator that regulates the transcriptional activity of STAT3 has been identified. Here we report that CR6-interacting factor 1 (Crif1) is a specific transcriptional coactivator of STAT3, but not of STAT1 or STAT5a. Crif1 interacts with STAT3 and positively regulates its transcriptional activity. Crif1-/- embryos were lethal around embryonic day 6.5, and manifested developmental arrest accompanied with defective proliferation and massive apoptosis. The expression of STAT3 target genes was markedly reduced in a Crif1-/- blastocyst culture and in Oncostatin M-stimulated Crif1-deficient MEFs. Importantly, the key activities of constitutively active STAT3-C, such as transcription, DNA binding, and cellular transformation, were abolished in the Crif1-null MEFs, suggesting the essential role of Crif1 in the transcriptional activity of STAT3. Our results reveal that Crif1 is a novel and essential transcriptional coactivator of STAT3 that modulates its DNA binding ability, and shed light on the regulation of oncogenic STAT3.

Differential effects of c-Ras upon transformation, adipocytic differentiation, and apoptosis mediated by the simian virus 40 large tumor antigen

To investigate the functional relationship between the ability of the simian virus 40 large tumor antigen (TAg) to transform and its ability to block adipocytic differentiation and induce apoptosis, we expressed TAg in C3H10T1/2 (10T1/2)-derived preadipocytes. The results demonstrated that differentiation could be suppressed at lower TAg levels than at the levels required for full neoplastic conversion. Progressively higher TAg levels were accompanied by apoptosis induction in this system. To further examine the role of the cellular Ras protooncogene product (Ras) in TAg function, TAg was expressed in 10T1/2-derived preadipocytes rendered deficient in Ras activity by transfection with inducible or constitutive antisense ras gene constructs. The results indicated that Ras is required for TAg-mediated transformation and for suppression of adipocytic differentiation, while TAg-mediated apoptosis following serum starvation was independent from Ras action. Unexpectedly, our results further demonstrated a dramatic reduction in the levels of the TAg protein itself as differentiation progressed in Ras-knockdown cells, with a concomitant reduction in TAg's ability to induce apoptosis as a result. These findings suggest that Ras, although cytoplasmic, is an integral component of the pathway whereby TAg, an oncoprotein believed to have primarily nuclear targets, suppresses differentiation or induces neoplastic conversion of murine preadipocytes.

Leukemia inhibitor factor (LIF) inhibits HIV-1 replication via restriction of stat 3 activation

Leukemia inhibitor factor (LIF) has been shown to potently inhibit HIV-1 replication in vitro and in human organ explant cultures. Furthermore, LIF activates the Jak/Stat signaling pathway with which many viruses, including HIV-1, interfere. We used CXCR4 and the LIF signaling receptor (gp130)-expressing cMAGI cells transfected with CD4, CCR5, and HIV-LTR-beta-galactosidase as a model system to investigate the potential involvement of Stat proteins in the anti-HIV-1 effect of LIF. Pretreatment with recombinant human (rh)LIF resulted in a significantly reduced uptake of HIV-1(BaL) , HIV-1(LAI), and SIVmac251 viral particles without affecting uptake of murine leukemia retroviral particles. HIV-1(BaL), HIV-1(LAI), as well as rhLIF selectively induced phosphorylation of Stat 3 but not Stat 1 or Stat 5. However, treatment of cMAGI cells with rhLIF prior to HIV-1 infection downregulated the HIV-1-mediated Stat 3 phosphorylation. In addition, peripheral blood mononuclear cells (PBMCs) transfected with Stat 3 siRNA prior to HIV-1(LAI) or HIV-1(BaL) infection produced significantly less HIV-1 p24 antigen as compared to nontransfected HIV-1(LAI) and HIV-1(BaL)-infected PBMCs. Thus, the Jak/Stat signaling pathway is important for the HIV-1 replication life cycle and rhLIF excerts its anti-HIV-1 activity by disrupting this signaling cascade.

Bub1 up-regulation and hyperphosphorylation promote malignant transformation in SV40 tag-induced transgenic mouse models

Rodents do not naturally develop prostate cancer. Currently, most widely used genetically engineered mouse prostate cancer models use SV40 T/tag oncogene. To understand the mechanism underlying prostate cancer development in transgenic and knock-in SV40 Tag mouse models, we did cDNA microarray analyses, comparing gene expression profiles of prostate cancer tissues from early-, late-, and advance-stage androgen-independent prostate cancers. Of the 67 genes that were up-regulated by > or = 10-fold, 40 are known to be required for chromosome stability. In particular, the spindle checkpoint component Bub1 was persistently up-regulated from early to advanced androgen-independent prostate cancer lesions. Significantly, Bub1, which is required for accurate chromosome segregation during mitosis, has recently been reported to bind SV40 Tag. Consistent with a spindle checkpoint defect, flow cytometry experiments indicate that advanced androgen-independent prostate cancer tumors exhibit aneuploidy, along with up-regulation of levels of both Bub1 mRNA and Bub1 protein or hyperphosphorylation. Importantly, up-regulation and hyperphosphorylation of Bub1 were also observed in established human prostate cancer cell lines and in clinical studies. Furthermore, analysis of human prostate cancer lines showed impaired spindle checkpoint function and endoreduplication following exposure to spindle toxins. Small interfering RNA-mediated repression of Bub1 in the human prostate cancer line PC-3 restrained cell proliferation, an effect mimicked by inhibition of mitogen-activated protein kinase, an upstream activator of Bub1. Thus, by perturbing Bub1 function, our observations suggest a new mechanism whereby the SV40 Tag oncoprotein promotes chromosomal instability and aneuploidy in transgenic mouse prostate cancer models. Whereas the exact details of this mechanism remain unclear, our novel findings raise the possibility of exploiting Bub1 as a new therapeutic target in the treatment of prostate cancer, the most common cancer in adult men in North America.

The STAT3 oncogene as a predictive marker of drug resistance

Constitutive activation of STAT3 (signal transducer and activator of transcription) has been reported in several primary cancers and tumor cell lines where it induces cell transformation through a combined inhibition of apoptosis and cell-cycle activation. Several studies have suggested that STAT3 prevents cell-cycle arrest and cell death through upregulation of survival proteins and downregulation of tumor suppressors. As a consequence of anti-apoptotic and proliferative lesions, we propose that this oncogenic pathway is also involved in intrinsic drug resistance and that STAT3-expressing tumors are resistant to chemotherapeutic agents. If this hypothesis is correct, the detection of the activated form of this protein should help to define subsets of tumors that fail to respond to chemotherapy. Furthermore, interfering with the STAT3 oncogenic pathway might restore the sensitivity to anticancer drugs.

Adenovirus E1A requires c-Ras for full neoplastic transformation or suppression of differentiation of murine preadipocytes

We recently demonstrated that the Adenovirus-5 E1A gene products (E1A), known E2F activators, can block the differentiation of murine preadipocytes and that differentiation suppression occurs at lower levels than required for full neoplastic transformation. Progressively higher levels were accompanied by apoptosis induction. To examine the role of the cellular Ras protooncogene product (Ras) in E1A function, E1A was expressed in C3H10T(1/2) (10T(1/2))-derived preadipocytes rendered deficient in Ras activity by transfection with inducible or constitutive antisense ras gene constructs (Ras-knockdowns). The results showed that, although even low amounts of E1A could block the differentiation of 10T(1/2) preadipocytes with normal Ras levels, even the highest E1A levels were unable to block the differentiation or induce transformation of Ras-knockdown preadipocytes. Ras downregulation did not affect E2F activation by E1A. Interestingly, our results further demonstrated a dramatic reduction in the levels of the E1A protein itself as differentiation progressed, with a concomitant reduction in E1A's ability to induce apoptosis as a result. These findings suggest for the first time that Ras, although cytoplasmic, is an integral component of the pathway whereby E1A, an oncoprotein believed to have primarily nuclear targets, suppresses differentiation or induces neoplastic conversion of murine preadipocytes.

The role of SV40 in malignant mesothelioma and other human malignancies

SV40 is a DNA tumor virus thrust upon human populations primarily as a contaminant in various vaccine preparations. Some estimates suggest that millions of people are currently infected with the virus. The virus causes primary brain tumors, bone tumors, lymphomas, and mesotheliomas when injected into some rodent models. It has also been detected in a similar spectrum of human tumors. However, epidemiological studies have failed to conclusively demonstrate a higher incidence of disease in affected populations. To date, over 60 reports from 49 different laboratories have shown SV40 sequences in tissues from human cancer patients. Six studies, however, have failed to detect evidence of virus in similar tissues. Some have suggested that SV40 may act as a cocarcinogen with asbestos to cause mesothelioma formation, or that it may be responsible for the 10-20% of mesotheliomas with no reported history of asbestos exposure. This report briefly covers the historical evidence for SV40 carcinogenesis and then covers experiments now underway to better understand the role of SV40 in human mesotheliomas.