The histone deacetylases HDAC1 and HDAC2 are required for the growth and survival of renal carcinoma cells.

Novel therapies are required for the treatment of metastatic renal cell carcinoma (RCC), which is associated with inoperable disease and patient death. Histone deacetylases (HDACs) are epigenetic modifiers and potential drug targets. Additional information on molecular pathways that are altered by histone deacetylase inhibitors (HDACi) in RCC cells is warranted. It should equally be delineated further which individual members of the 18 mammalian HDACs determine the survival and tumor-associated gene expression programs of such cells. Most importantly, an ongoing dispute whether HDACi promote or suppress metastasis-associated epithelial-to-mesenchymal transition (EMT) has to be resolved before HDACi are considered further as clinically relevant drugs. Here we show how HDACi affect murine and primary human RCC cells. We find that these agents induce morphological alterations resembling the metastasis-associated EMT. However, individual and proteomics-based analyses of epithelial and mesenchymal marker proteins and of EMT-associated transcription factors (EMT-TFs) reveal that HDACi do not trigger EMT. Pathway deconvolution analysis identifies reduced proliferation and apoptosis induction as key effects of HDACi. Furthermore, these drugs lead to a reduction of the cell adhesion molecule E-cadherin and of the platelet-derived growth factor receptor-ß (PDGFRß), which is a key driver of RCC metastasis formation. Accordingly, HDACi reduce the pulmonary spread of syngeneic transplanted renal carcinoma cells in mice. Specific genetic elimination of the histone deacetylases HDAC1/HDAC2 reflects the effects of pharmacological HDAC inhibition regarding growth suppression, apoptosis, and the downregulation of E-cadherin and PDGFRß. Thus, these epigenetic modifiers are non-redundant gatekeepers of cell fate and precise pharmacological targets.

Jak Stat signaling and cancer: Opportunities, benefits and side effects of targeted inhibition.

The effects of Jak Stat signaling and the persistent activation of Stat3 and Stat5 on tumor cell survival, proliferation and invasion have made the Jak Stat pathway a favorite target for drug development and cancer therapy. This notion was strengthened when additional biological functions of Stat signaling in cancer and their roles in the regulation of cytokine dependent inflammation and immunity in the tumor microenvironment were discovered. Stats act not only as transcriptional inducers, but affect gene expression via epigenetic modifications, induce epithelial mesenchymal transition, generate a pro-tumorigenic microenvironment, promote cancer stem cell self-renewal and differentiation, and help to establish the pre-metastatic niche formation. The effects of Jak Stat inhibition on the suppression of pro-inflammatory responses appears most promising and could become a strategy in the prevention of tumor progression. The direct and mediated mechanisms of Jak Stat signaling in and on tumors cells, the interactions with other signaling pathways and transcription factors and the targeting of the functionally crucial secondary modifications of Stat molecules suggest novel approaches to the future development of Jak Stat based cancer therapeutics.

Breast Cancer Patients Have Greatly Benefited from the Progress in Molecular Oncology.

Cancer research has become a global enterprise, and the number of researchers, as well as the cost for their activities, has skyrocketed. The budget for the National Cancer Institute of the United States National Institutes of Health alone was US$5.2 billion in 2015. Since most of the research is funded by public money, it is perfectly legitimate to ask if these large expenses are worth it. In this brief commentary, we recapitulate some of the breakthroughs that mark the history of breast cancer research over the past decades and emphasize the resulting benefits for afflicted women. In 1971, only 40% of women diagnosed with breast cancer would live another 10 years. Today, nearly 80% of women reach that significant milestone in most developed countries. This dramatic change has afforded breast cancer patients many productive years and a better quality of life. Progress resulted largely from advances in the understanding of the molecular details of the disease and their translation into innovative, rationally designed therapies. These developments are founded on the revolution in molecular and cellular biology, an entirely new array of methods and technologies, the enthusiasm, optimism, and diligence of scientists and clinicians, and the considerable funding efforts from public and private sources. We were lucky to be able to spend our productive years in a period of scientific upheaval in which methods and concepts were revolutionized and that allowed us to contribute, within the global scientific community, to the progress in basic science and clinical practice.

Paul Ehrlich (1854-1915) and His Contributions to the Foundation and Birth of Translational Medicine.

Translational research and precision medicine are based on a profound knowledge of cellular and molecular mechanisms contributing to various physiologic processes and pathologic reactions in diverse organs. Whereas specific molecular interactions and mechanisms have been identified during the past 5 decades, the underlying principles were defined much earlier and originate from to the seminal observations made by outstanding researchers between 1850 and 1915. One of the most outstanding exponents of these scientists is Paul Ehrlich. His work resulted not only in the foundation and birth of modern hematology and immunology, but also led to the development of chemotherapy and specific targeted treatment concepts. In 2015, the Medical University of Vienna organized a memorial meeting, with the aim of honoring Paul Ehrlich's contributions to science, and to commemorate the 100th anniversary of his death. The authors of the current review served as faculty members and dedicate this paper to Paul Ehrlich and his remarkable contributions to medicine.

Improving Drug Penetrability with iRGD Leverages the Therapeutic Response to Sorafenib and Doxorubicin in Hepatocellular Carcinoma.

iRGD is a derivative of the integrin-binding peptide RGD, which selectively increases the penetrability of tumor tissue to various coadministered substances in several preclinical models. In this study, we investigated the ability of iRGD to improve the delivery of sorafenib and doxorubicin therapy in hepatocellular carcinoma (HCC) using established mouse models of the disease. A contrast-enhanced MRI method was developed in parallel to assess the in vivo effects of iRGD in this setting. We found that iRGD improved the delivery of marker substances to the tumors of HCC-bearing mice about three-fold without a parallel increase in normal tissues. Control peptides lacking the critical CendR motif had no effect. Similarly, iRGD also selectively increased the signal intensity from tumors in Gd-DTPA-enhanced MRI. In terms of antitumor efficacy, iRGD coadministration significantly augmented the individual inhibitory effects of sorafenib and doxorubicin without increasing systemic toxicity. Overall, our results offered a preclinical proof of concept for the use of iRGD coadministration as a strategy to widen the therapeutic window for HCC chemotherapy, as monitored by Gd-DTPA-enhanced MRI as a noninvasive, clinically applicable method to identify iRGD-reactive tumors.

5-Lipoxygenase is a direct p53 target gene in humans.

The p53 tumor suppressor plays a critical role in cancer, and more than 50% of human tumors contain mutations or deletions of the TP53 gene. p53 can transactivate or repress target genes in response to diverse stress signals, such as transient growth arrest, DNA repair, cellular differentiation, senescence and apoptosis. Through an unbiased genome-wide ChIP-seq analysis, we have found that 5-lipoxygenase (ALOX5, 5-LO) which is a key enzyme of leukotriene (LT) biosynthesis, is a direct target gene of p53 and its expression is induced by genotoxic stress via actinomycin D (Act.D) or etoposide (Eto) treatment. 5-LO and LTs play a role in immunological diseases as well as in tumorigenesis and tumor growth. p53 binds to a specific binding site consisting of a complete p53 consensus-binding motif in ALOX5 intron G which is located about 64kbp downstream of the transcriptional start site. We confirmed the strong binding of p53 to the 5-LO target site in ChIP-qPCR experiments. Expression analyses by qRT-PCR and immunoblot further revealed that genotoxic stress induces the ALOX5 mRNA and protein expression in a p53-dependent manner. Knockdown of p53 in U2OS cells leads to a downregulation of 5-LO mRNA and protein expression. In addition, immunofluorescence and immunoprecipitation assays indicate the direct binding of 5-LO to p53 protein. Furthermore, we found that 5-LO can inhibit the transcriptional activity of p53 suggesting that 5-LO acts in a negative feedback loop to limit induction of p53 target genes.

Expression of the miR-302/367 cluster in glioblastoma cells suppresses tumorigenic gene expression patterns and abolishes transformation related phenotypes.

Cellular transformation is initiated by the activation of oncogenes and a closely associated developmental reprogramming of the epigenetic landscape. Transcription factors, regulators of chromatin states and microRNAs influence cell fates in development and stabilize the phenotypes of normal, differentiated cells and of cancer cells. The miR-302/367 cluster, predominantly expressed in human embryonic stem cells (hESs), can promote the cellular reprogramming of human and mouse cells and contribute to the generation of iPSC. We have used the epigenetic reprogramming potential of the miR-302/367 cluster to "de-program" tumor cells, that is, hift their gene expression pattern towards an alternative program associated with more benign cellular phenotypes. Induction of the miR-302/367 cluster in extensively mutated U87MG glioblastoma cells drastically suppressed the expression of transformation related proteins, for example, the reprogramming factors OCT3/4, SOX2, KLF4 and c-MYC, and the transcription factors POU3F2, SALL2 and OLIG2, required for the maintenance of glioblastoma stem-like tumor propagating cells. It also diminished PI3K/AKT and STAT3 signaling, impeded colony formation in soft agar and cell migration and suppressed pro-inflammatory cytokine secretion. At the same time, the miR-302/367 cluster restored the expression of neuronal markers of differentiation. Most notably, miR-302/367 cluster expressing cells lose their ability to form tumors and to establish liver metastasis in nude mice. The induction of the miR-302/367 cluster in U87MG glioblastoma cells suppresses the expression of multiple transformation related genes, abolishes the tumor and metastasis formation potential of these cells and can potentially become a new approach for cancer therapy.

Stat5 Exerts Distinct, Vital Functions in the Cytoplasm and Nucleus of Bcr-Abl+ K562 and Jak2(V617F)+ HEL Leukemia Cells.

Signal transducers and activators of transcription (Stats) play central roles in the conversion of extracellular signals, e.g., cytokines, hormones and growth factors, into tissue and cell type specific gene expression patterns. In normal cells, their signaling potential is strictly limited in extent and duration. The persistent activation of Stat3 or Stat5 is found in many human tumor cells and contributes to their growth and survival. Stat5 activation plays a pivotal role in nearly all hematological malignancies and occurs downstream of oncogenic kinases, e.g., Bcr-Abl in chronic myeloid leukemias (CML) and Jak2(V617F) in other myeloproliferative diseases (MPD). We defined the mechanisms through which Stat5 affects growth and survival of K562 cells, representative of Bcr-Abl positive CML, and HEL cells, representative for Jak2(V617F) positive acute erythroid leukemia. In our experiments we suppressed the protein expression levels of Stat5a and Stat5b through shRNA mediated downregulation and demonstrated the dependence of cell survival on the presence of Stat5. Alternatively, we interfered with the functional capacities of the Stat5 protein through the interaction with a Stat5 specific peptide ligand. This ligand is a Stat5 specific peptide aptamer construct which comprises a 12mer peptide integrated into a modified thioredoxin scaffold, S5-DBD-PA. The peptide sequence specifically recognizes the DNA binding domain (DBD) of Stat5. Complex formation of S5-DBD-PA with Stat5 causes a strong reduction of P-Stat5 in the nuclear fraction of Bcr-Abl-transformed K562 cells and a suppression of Stat5 target genes. Distinct Stat5 mediated survival mechanisms were detected in K562 and Jak2(V617F)-transformed HEL cells. Stat5 is activated in the nuclear and cytosolic compartments of K562 cells and the S5-DBD-PA inhibitor most likely affects the viability of Bcr-Abl+ K562 cells through the inhibition of canonical Stat5 induced target gene transcription. In HEL cells, Stat5 is predominantly present in the cytoplasm and the survival of the Jak2(V617F)+ HEL cells is impeded through the inhibition of the cytoplasmic functions of Stat5.

Adaptive Resistance to Immunotherapy Directed Against p53 Can be Overcome by Global Expression of Tumor-Antigens in Dendritic Cells.

Immunotherapy of cancer utilizes dendritic cells (DCs) for antigen presentation and the induction of tumor-specific immune responses. However, the therapeutic induction of anti-tumor immunity is limited by tumor escape mechanisms. In this study, immortalized dendritic D2SC/1 cells were transduced with a mutated version of the p53 tumor suppressor gene, p53M234I, or p53C132F/E168G, which are overexpressed in MethA fibrosarcoma tumor cells. In addition, D2SC/1 cells were fused with MethA tumor cells to generate a vaccine that potentially expresses a large repertoire of tumor-antigens. Cellular vaccines were transplanted onto Balb/c mice and MethA tumor growth and anti-tumor immune responses were examined in vaccinated animals. D2SC/1-p53M234I and D2SC/1-p53C132F/E168G cells induced strong therapeutic and protective MethA tumor immunity upon transplantation in Balb/c mice. However, in a fraction of immunized mice MethA tumor growth resumed after an extended latency period. Analysis of these tumors indicated loss of p53 expression. Mice, pre-treated with fusion hybrids generated from D2SC/1 and MethA tumor cells, suppressed MethA tumor growth and averted adaptive immune escape. Polyclonal B-cell responses directed against various MethA tumor proteins could be detected in the sera of D2SC/1-MethA inoculated mice. Athymic nude mice and Balb/c mice depleted of CD4(+) or CD8(+) T-cells were not protected against MethA tumor cell growth after immunization with D2SC/1-MethA hybrids. Our results highlight a potential drawback of cancer immunotherapy by demonstrating that the induction of a specific anti-tumor response favors the acquisition of tumor phenotypes promoting immune evasion. In contrast, the application of DC/tumor cell fusion hybrids prevents adaptive immune escape by a T-cell dependent mechanism and provides a simple strategy for personalized anti-cancer treatment without the need of selectively priming the host immune system.

Low PIAS3 expression in malignant mesothelioma is associated with increased STAT3 activation and poor patient survival.

PURPOSE: Deregulation of STAT3 activation is a hallmark of many cancer cells, and the underlying mechanisms are subject to intense investigation. We examined the extent of PIAS3 expression in mesothelioma cells and human tumor samples and determined the functional effects of PIAS3 expression on STAT3 signaling. EXPERIMENTAL DESIGN: We evaluated the expression of PIAS3 in mesothelioma tumors from patients and correlated the expression levels with the course of the disease. We also measured the effects of enhanced PIAS3 activity on STAT3 signaling, cellular growth, and viability in cultured mesothelioma cells. RESULTS: Gene expression databases revealed that mesotheliomas have the lowest levels of PIAS3 transcripts among solid tumors. PIAS3 expression in human mesothelioma tumors is significantly correlated with overall survival intervals (P = 0.058). The high expression of PIAS3 is predictive of a favorable prognosis and decreases the probability of death within one year after diagnosis by 44%. PIAS3 expression is functionally linked to STAT3 activation in mesothelioma cell lines. STAT3 downregulation with siRNA or enhanced expression of PIAS3 both inhibited mesothelioma cell growth and induced apoptosis. Mesothelioma cells are sensitive to curcumin and respond by the induction of PIAS3. Corroborative evidence has been obtained from STAT3 inhibition experiments. Exposure of the cells to a peptide derived from the PIAS3 protein that interferes with STAT3 function resulted in apoptosis induction and the inhibition of cell growth. CONCLUSION: These results suggest that PIAS3 protein expression impacts survival in patients with mesothelioma and that PIAS3 activation could become a therapeutic strategy. Clin Cancer Res; 20(19); 5124-32. ©2014 AACR.

Endotoxin depletion of recombinant protein preparations through their preferential binding to histidine tags.

The presence of endotoxins in preparations of recombinantly produced therapeutic proteins poses serious problems for patients. Endotoxins can cause fever, respiratory distress syndromes, intravascular coagulation, or endotoxic shock. A number of methods have been devised to remove endotoxins from protein preparations using separation procedures based on molecular mass or charge properties. Most of the methods are limited in their endotoxin removal capacities and lack general applicability. We are describing a biotechnological approach for endotoxin removal. This strategy exploits the observation that endotoxins form micelles that expose negative charges on their surface, leading to preferential binding of endotoxins to cationic surfaces, allowing the separation from their resident protein. Endotoxins exhibit high affinity to stretches of histidines, which are widely used tools to facilitate the purification of recombinant proteins. They bind to nickel ions and are the basis for protein purification from cellular extracts by immobilized metal affinity chromatography. We show that the thrombin-mediated cleavage of two histidine tags from the purified recombinant protein and the adsorption of these histidine tags and their associated endotoxins to a nickel affinity column result in an appreciable depletion of the endotoxins in the purified protein fraction.

STAT3 induction of miR-146b forms a feedback loop to inhibit the NF-κB to IL-6 signaling axis and STAT3-driven cancer phenotypes.

Interleukin-6 (IL-6)-mediated activation of signal transducer and activator of transcription 3 (STAT3) is a mechanism by which chronic inflammation can contribute to cancer and is a common oncogenic event. We discovered a pathway, the loss of which is associated with persistent STAT3 activation in human cancer. We found that the gene encoding the tumor suppressor microRNA miR-146b is a direct STAT3 target gene, and its expression was increased in normal breast epithelial cells but decreased in tumor cells. Methylation of the miR-146b promoter, which inhibited STAT3-mediated induction of expression, was increased in primary breast cancers. Moreover, we found that miR-146b inhibited nuclear factor κB (NF-κB)-dependent production of IL-6, subsequent STAT3 activation, and IL-6/STAT3-driven migration and invasion in breast cancer cells, thereby establishing a negative feedback loop. In addition, higher expression of miR-146b was positively correlated with patient survival in breast cancer subtypes with increased IL6 expression and STAT3 phosphorylation. Our results identify an epigenetic mechanism of crosstalk between STAT3 and NF-κB relevant to constitutive STAT3 activation in malignancy and the role of inflammation in oncogenesis.

Targeting survivin in cancer: novel drug development approaches.

Survivin is a well-established target in experimental cancer therapy. The molecule is over-expressed in most human tumors, but hardly detectable in normal tissues. Multiple functions in different subcellular compartments have been assigned. It participates in the control of cell division, apoptosis, the cellular stress response, and also in the regulation of cell migration and metastasis. Survivin expression has been recognized as a biomarker: high expression indicates an unfavorable prognosis and resistance to chemotherapeutic agents and radiation treatment. Survivin is an unconventional drug target and several indirect approaches have been exploited to affect its function and the phenotype of survivin-expressing cells. Interference with the expression of the survivin gene, the utilization of its messenger RNA, the intracellular localization, the interaction with binding partners, the stability of the survivin protein, and the induction of survivin-specific immune responses have been taken into consideration. A direct strategy to inhibit survivin has been based on the identification of a specifically interacting peptide. This peptide can recognize survivin intracellularly and cause the degradation of the ligand-survivin complex. Technology is being developed that might allow the derivation of small molecular-weight, drug-like compounds that are functionally equivalent to the peptide ligand.

The inhibition of stat5 by a Peptide aptamer ligand specific for the DNA binding domain prevents target gene transactivation and the growth of breast and prostate tumor cells.

The signal transducer and activator of transcription Stat5 is transiently activated by growth factor and cytokine signals in normal cells, but its persistent activation has been observed in a wide range of human tumors. Aberrant Stat5 activity was initially observed in leukemias, but subsequently also found in carcinomas. We investigated the importance of Stat5 in human tumor cell lines. shRNA mediated downregulation of Stat5 revealed the dependence of prostate and breast cancer cells on the expression of this transcription factor. We extended these inhibition studies and derived a peptide aptamer (PA) ligand, which directly interacts with the DNA-binding domain of Stat5 in a yeast-two-hybrid screen. The Stat5 specific PA sequence is embedded in a thioredoxin (hTRX) scaffold protein. The resulting recombinant protein S5-DBD-PA was expressed in bacteria, purified and introduced into tumor cells by protein transduction. Alternatively, S5-DBD-PA was expressed in the tumor cells after infection with a S5-DBD-PA encoding gene transfer vector. Both strategies impaired the DNA-binding ability of Stat5, suppressed Stat5 dependent transactivation and caused its intracellular degradation. Our experiments describe a peptide based inhibitor of Stat5 protein activity which can serve as a lead for the development of a clinically useful compound for cancer treatment.

STAT3 silencing inhibits glioma single cell infiltration and tumor growth.

BACKGROUND: Diffuse infiltration remains the fulcrum of glioblastoma's incurability, leading inevitably to recurrence. Therefore, uncovering the pathological mechanism is imperative. Because signal transducer and activator of transcription 3 (STAT3) correlates with glioma malignancy and predicts poor clinical outcome, we determined its role in glioma single cell infiltration and tumor growth. METHODS: STAT3 was silenced in Tu-2449 glioma cells via lentiviral gene transfer. Target gene expression was measured by real-time reverse transcription PCR, Western blotting, and immunohistochemistry. Microvilli were visualized by staining with wheat germ agglutinin. Migration and invasion were measured by Scratch and Matrigel chamber assays. Diffuse infiltration was studied in 350-µm-thick organotypic tissue cultures over 14 days using cells tagged with enhanced green fluorescent protein and live confocal laser scanning microscopy. Survival of tumor-bearing syngeneic, immunocompetent B6C3F1 mice was analyzed by Kaplan-Meier plots. RESULTS: STAT3 silencing reduced cell migration and invasion in vitro and stopped single cell infiltration ex vivo, while STAT3-expressing cells disseminated through the neuropil at ∼100 µm/day. STAT3 silencing reduced transcription of several tumor progression genes. Mice with intracranial STAT3 knockdown tumors had a significant (P< .0007) survival advantage over controls, yielding 27% long-term survival. STAT3 knockdown reduced podoplanin expression 50-fold and inhibited concurrent microvilli formation. STAT3 knockdown tumors exhibited a weaker podoplanin immunoreactivity compared with controls. Podoplanin staining was diffuse, preferentially at tumor margins, and absent in normal brain. CONCLUSIONS: Our results show compelling evidence that STAT3 is a key driver of diffuse infiltration and glioma growth and might therefore represent a promising target for an anti-invasive therapy.

Resistance of Cancer Cells to Targeted Therapies Through the Activation of Compensating Signaling Loops.

The emergence of low molecular weight kinase inhibitors as "targeted" drugs has led to remarkable advances in the treatment of cancer patients. The clinical benefits of these tumor therapies, however, vary widely in patient populations and with duration of treatment. Intrinsic and acquired resistance against such drugs limits their efficacy. In addition to the well studied mechanisms of resistance based upon drug transport and metabolism, genetic alterations in drug target structures and the activation of compensatory cell signaling have received recent attention. Adaptive responses can be triggered which counteract the initial dependence of tumor cells upon a particular signaling molecule and allow only a transient inhibition of tumor cell growth. These compensating signaling mechanisms are often based upon the relief of repression of regulatory feedback loops. They might involve cell autonomous, intracellular events or they can be mediated via the secretion of growth factor receptor ligands into the tumor microenvironment and signal induction in an auto- or paracrine fashion. The transcription factors Stat3 and Stat5 mediate the biological functions of cytokines, interleukins and growth factors and can be considered as endpoints of multiple signaling pathways. In normal cells this activation is transient and the Stat molecules return to their non-phosphorylated state within a short time period. In tumor cells the balance between activating and de-activating signals is disturbed resulting in the persistent activation of Stat3 or Stat5. The constant activation of Stat3 induces the expression of target genes, which cause the proliferation and survival of cancer cells, as well as their migration and invasive behavior. Activating components of the Jak-Stat pathway have been recognized as potentially valuable drug targets and important principles of compensatory signaling circuit induction during targeted drug treatment have been discovered in the context of kinase inhibition studies in HNSCC cells [1]. The treatment of HNSCC with a specific inhibitor of c-Src, initially resulted in reduced Stat3 and Stat5 activation and subsequently an arrest of cell proliferation and increased apoptosis. However, the inhibition of c-Src only caused a persistent inhibition of Stat5, whereas the inhibition of Stat3 was only transient. The activation of Stat3 was restored within a short time period in the presence of the c-Src inhibitor. This process is mediated through the suppression of P-Stat5 activity and the decrease in the expression of the Stat5 dependent target gene SOCS2, a negative regulator of Jak2. Jak2 activity is enhanced upon SOCS2 downregulation and causes the reactivation of Stat3. A similar observation has been made upon inhibition of Bmx, bone marrow kinase x-linked, activated in the murine glioma cell lines Tu-2449 and Tu-9648. Its inhibition resulted in a transient decrease of P-Stat3 and the induction of a compensatory Stat3 activation mechanism, possibly through the relief of negative feedback inhibition and Jak2 activation. These observations indicate that the inhibition of a single tyrosine kinase might not be sufficient to induce lasting therapeutic effects in cancer patients. Compensatory kinases and pathways might become activated and maintain the growth and survival of tumor cells. The definition of these escape pathways and their preemptive inhibition will suggest effective new combination therapies for cancer.

Increasing the range of drug targets: interacting peptides provide leads for the development of oncoprotein inhibitors.

Two limiting aspects are mainly responsible for the sluggish development of new cancer drugs. They concern the chemical properties of potential drug molecules and the structural prerequisites for drug targets. The chemical properties which are being considered desirable for potential drugs are rather restrictive and mainly dictated by the rules of oral availability. Drug target structures are mostly defined as molecules which comprise binding pockets for low molecular weight compounds. These low molecular weight compounds then serve as leads for the derivation of analogs which recognize the same site and which can function as competitive or irreversibly binding inhibitors. The extension of the range of drug targets and the design of suitable lead compounds will be one of the most challenging tasks for drug developers in the future. Such auxiliary drug target structures can be found in the complex networks of interacting proteins which constitute the intracellular signal transduction cascades. The transient assembly of high molecular weight complexes, based on the specific interactions of particular domains, and usually regulated by secondary modifications, propagates extracellular signals through the cytoplasm and into the nucleus. Aberrations in the formation of protein complexes, or in the regulation of their disassembly, often trigger pathological conditions. The interference with interactions of proteins or the interactions of proteins with DNA offer new opportunities for drug discovery and development. Protein complexes which are indispensable for the growth and survival of cancer cells, proteins to which these cells are "addicted," appear most suited for such an approach. Stat3 and Survivin have been used as model proteins. Specific peptide ligands able to recognize and suppress the functions of crucial interaction surfaces of these proteins have been derived and shown to be able to induce cancer cell death. However, further technology development is required to turn such ligands into useful drugs. The technology comprises three steps: (1) the identification of a peptide ligand which specifically interacts with a crucial functional domain of a target protein, (2) the induction of a desired cellular phenotype upon intracellular interaction of the peptide ligand with its target structure and (3) the replacement of the peptide ligand with a functionally equivalent low molecular weight, drug like compound and its optimization through medicinal chemistry.