Oncolytic Vaccinia Virus Gene Modification and Cytokine Expression Effects on Tumor Infection, Immune Response, and Killing

Oncolytic vaccinia viruses have promising efficacy and safety profiles in cancer therapy. Although antitumor activity can be increased by manipulating viral genes, the relative efficacy of individual modifications has been difficult to assess without side-by-side comparisons. This study sought to compare the initial antitumor activity after intravenous administration of five vaccinia virus variants of the same Western Reserve backbone and thymidine kinase gene deletion in RIP-Tag2 transgenic mice with spontaneous pancreatic neuroendocrine tumors. Tumors had focal regions of infection at 5 days after all viruses. Natural killer (NK) cells were restricted to these sites of infection, but CD8⁺ T cells and tumor cell apoptosis were widespread and varied among the viruses. Antitumor activity of virus VV-A34, bearing amino acid substitution A34^K151E to increase viral spreading, and virus VV-IL2v, expressing a mouse IL2 variant (mIL2v) with attenuated IL2 receptor alpha subunit binding, was similar to control virus VV-GFP. However, antitumor activity was significantly greater after virus VV-A34/IL2v, which expressed mIL2v together with A34^K151E mutation and viral B18R gene deletion, and virus VV-GMCSF that expressed mouse GM-CSF. Both viruses greatly increased expression of CD8 antigens Cd8a/Cd8b1 and cytotoxicity genes granzyme A, granzyme B, Fas ligand, and perforin-1 in tumors. VV-A34/IL2v led to higher serum IL2 and greater tumor expression of death receptor ligand TRAIL, but VV-GMCSF led to higher serum GM-CSF, greater expression of leukocyte chemokines and adhesion molecules, and more neutrophil recruitment. Together, the results show that antitumor activity is similarly increased by viral expression of GM-CSF or IL2v combined with additional genetic modifications.

Abstract A50: Genotype tunes PDAC tension to induce matricellular-fibrosis and tumor aggression

Fibrosis compromises pancreatic ductal carcinoma (PDAC) treatment and contributes to patient mortality. Nevertheless, anti-stromal therapies for pancreatic cancer patients have had mixed results, suggesting there are multifaceted, anti and pro-tumorigenic roles of fibrosis in tumor pathogenesis. We found that human PDACs lacking epithelial TGFβ activity have elevated epithelial Stat3 activity and develop a stiffer, matricellular-enriched fibrosis that associates with high epithelial tension and shorter patient survival. Using several Kras-driven mouse models, we found that both the loss of TGFβ signaling and elevated β1 integrin mechanosignaling engage a positive feedback loop whereby Stat3 signaling modulates pancreatic cancer malignancy by increasing matricellular fibrosis and tissue tension. By contrast, epithelial Stat3 ablation attenuated pancreatic malignancy by reducing the stromal stiffening and epithelial contractility induced by loss of TGFβ signaling. In PDAC patient biopsies, higher matricellular protein and activated Stat3 associated with SMAD4 mutation and shorter survival. The findings implicate epithelial actomyosin tension and matricellular fibrosis in the aggressiveness of SMAD4 mutant pancreatic tumors, and highlight Stat3 as a key driver of the phenotype. These data illustrate how tumor genotype can directly tune tissue mechanics and support the development of genotype-driven, anti-stromal therapies targeting PDAC.

Citation Format: Hanane Laklai, Yekaterina Miroshnikova, Michael Pickup, Eric Collisson, Kim Grace, Alex Barrett, Ryan Hill, Johnathon Lakins, David Schlaepfer, Janna Mouw, Valerie LeBleu, Sergey Novitskiy, Julie Johansen, Valeria Poli, Rahgu Kalluri, Laura Wood, Matthias Hebrok, Kirk Hansen, Harold Moses, Valerie Weaver.{Authors}. Genotype tunes PDAC tension to induce matricellular-fibrosis and tumor aggression. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2016 May 12-15; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(24 Suppl):Abstract nr A50.

Genotype tunes pancreatic ductal adenocarcinoma tissue tension to induce matricellular fibrosis and tumor progression

Fibrosis compromises pancreatic ductal carcinoma (PDAC) treatment and contributes to patient mortality, yet antistromal therapies are controversial. We found that human PDACs with impaired epithelial transforming growth factor-β (TGF-β) signaling have high epithelial STAT3 activity and develop stiff, matricellular-enriched fibrosis associated with high epithelial tension and shorter patient survival. In several KRAS-driven mouse models, both the loss of TGF-β signaling and elevated β1-integrin mechanosignaling engaged a positive feedback loop whereby STAT3 signaling promotes tumor progression by increasing matricellular fibrosis and tissue tension. In contrast, epithelial STAT3 ablation attenuated tumor progression by reducing the stromal stiffening and epithelial contractility induced by loss of TGF-β signaling. In PDAC patient biopsies, higher matricellular protein and activated STAT3 were associated with SMAD4 mutation and shorter survival. The findings implicate epithelial tension and matricellular fibrosis in the aggressiveness of SMAD4 mutant pancreatic tumors and highlight STAT3 and mechanics as key drivers of this phenotype.

Force Matters: Biomechanical Regulation of Cell Invasion and Migration in Disease

Atherosclerosis, cancer, and various chronic fibrotic conditions are characterized by an increase in the migratory behavior of resident cells and the enhanced invasion of assorted exogenous cells across a stiffened extracellular matrix (ECM). This stiffened scaffold aberrantly engages cellular mechanosignaling networks in cells, which promotes the assembly of invadosomes and lamellae for cell invasion and migration. Accordingly, deciphering the conserved molecular mechanisms whereby matrix stiffness fosters invadosome and lamella formation could identify therapeutic targets to treat fibrotic conditions, and reducing ECM stiffness could ameliorate disease progression.

Abstract IA15: Functional diversity of the pancreas tumor stroma

The desmoplastic reaction in pancreatic ductal adenocarcinoma (PDAC) is composed of functionally diverse cellular components embedded in a complex and dynamic fibrotic matrix. In this context, the marked accumulation of stromal myofibroblasts has long been associated with pro-tumorigenic functions, by enhancing tumor cell growth, and limiting immune infiltration and chemotherapeutic drug delivery. Testing of some of these concepts in the clinical care of PDAC patients was however met with paradoxical results. Precise functional auditing of the stromal fibroblasts using genetic targeting in genetically engineered mouse models of PDAC depicted a more complex picture, wherein myofibroblast emerged in this context as tumor-limiting stromal cells. Depletion of the stromal fibroblasts in the early and late stages of PDAC in mice led to more invasive tumors with a negative impact on overall survival, and failed to improve response to Gemcitabine therapy. While our result indicated that suppression of the desmoplastic reaction was associated with decreased tumor angiogenesis, increased intratumoral hypoxia, and epithelial-to-mesenchymal transition of cancer cells with heightened tumorigenicity, our study also informed on the immunomodulatory functions of myofibroblasts in murine PDAC. Depletion of myofibroblasts supported an enhanced response to anti-CTLA4 checkpoint blockade therapy with prolonged survival of mice.

Citation Format: Valerie S. LeBleu, Berna C. Özdemir, Tsvetelina Pentcheva-Hoang, Julienne L. Carstens, Xiaofeng Zheng, Chin-Chia Wu, Tyler Simpson, Hanane Laklai, Hikaru Sugimoto, Christoph Kahlert, Sergey V. Novitskiy, Ana DeJesus Acosta, Padmanee Sharma, Pedram Heidari, Umar Mahmood, Lynda Chin, Harold Moses, Valerie Weaver, Anirban Maitra, James P. Allison and Raghu Kalluri. Functional diversity of the pancreas tumor stroma. [abstract]. In: Proceedings of the Fourth AACR International Conference on Frontiers in Basic Cancer Research; 2015 Oct 23-26; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2016;76(3 Suppl):Abstract nr IA15.

Abstract LB-143: Interplay between tissue tension and inflammation in pancreatic tumor progression

Pancreatic ductal adenocarcinoma (PDAC) is amongst the most lethal malignancies due largely to an inability to detect these tumors early and thereafter a lack of effective therapies for late stage cancers (Tempero, 2003). PDAC is characterized by a strong desmoplastic response including significant extracellular matrix (ECM) remodeling that severely compromises treatment and surgical resection (Farrow, 2008). Nevertheless, the molecular mechanisms linking tissue transformation to fibrosis and tumor aggression remain unclear. Tissue fibrosis increases tissue tension and we showed that ECM stiffness and epithelial cell tension drive squamous carcinoma and mammary transformation (Levental, 2009; Samuel, 2011). We therefore asked whether pancreatic fibrosis promotes PDAC progression and aggression by increasing ECM stiffness and tissue tension and how. Inflammation is casually-associated with fibrosis and targeted deletion of the TGFβ receptor 2 (TGFβR2KO) when combined with expression of KRas up regulates CXC chemokine expression to promote PDAC aggression (Ijichi, 2011). Here we report that PDAC progression is accompanied by significant fibrosis and inflammation that correlated strongly with collagen deposition and LOX-dependent cross-linking and ECM stiffening. PDAC progression associated with elevated CXC chemokine levels as well as high epithelial STAT3 activity and elevated mechanosignaling in the epithelium including activated myosin, FAK and YAP and enhanced PDAC contractility. Intriguingly, we showed that PDAC contractility is functionally linked through a reciprocal Cxcr2 receptor - GPCR- JAK-STAT3 - ROCK - FAK signaling circuit that contributes to PDAC aggression and tissue inflammation. Consistently, the pancreas of mice expressing a targeted β1 integrin V737lox/lox clustering mutant, which drives integrin-dependent mechanosignaling in their epithelium showed profound fibrosis, high pSTAT3 and chronic inflammation which progressed to pancreatitis by as early as 3 months of age when combined with the KRas oncogene. Ongoing studies are currently exploring the relevance of Stat3 on pancreatic fibrosis and tumor aggression. Nevertheless, our findings identify a vicious positive feedback mechanism mediated through tissue tension that may account for the strong association between fibrosis, tumor aggression and inflammation in PDAC. (This work was supported by the Tumor Microenvironment Network (TMEN) grant NIH/NCI 1U01 CA151925-01, the Pancreatic Cancer Action Network-AACR Innovative Grant 30-60-25-WEAV, and NIH/NCI R01 CA138818-01A1 to VMW).

Citation Format: Hanane Laklai, Yekaterina Miroshnikova, Jon lakins, Novitskiy Sergey, Agnes Gorska, Lidiya Korets, Raghu Kalluri, Harold Moses, Valerie Weaver. Interplay between tissue tension and inflammation in pancreatic tumor progression. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-143. doi:10.1158/1538-7445.AM2014-LB-143

Depletion of carcinoma-associated fibroblasts and fibrosis induces immunosuppression and accelerates pancreas cancer with reduced survival

Pancreatic ductal adenocarcinoma (PDAC) is associated with marked fibrosis and stromal myofibroblasts, but their functional contribution remains unknown. Transgenic mice with the ability to delete αSMA(+) myofibroblasts in pancreatic cancer were generated. Depletion starting at either noninvasive precursor (pancreatic intraepithelial neoplasia) or the PDAC stage led to invasive, undifferentiated tumors with enhanced hypoxia, epithelial-to-mesenchymal transition, and cancer stem cells, with diminished animal survival. In PDAC patients, fewer myofibroblasts in their tumors also correlated with reduced survival. Suppressed immune surveillance with increased CD4(+)Foxp3(+) Tregs was observed in myofibroblast-depleted mouse tumors. Although myofibroblast-depleted tumors did not respond to gemcitabine, anti-CTLA4 immunotherapy reversed disease acceleration and prolonged animal survival. This study underscores the need for caution in targeting carcinoma-associated fibroblasts in PDAC.

Dual roles of hemidesmosomal proteins in the pancreatic epithelium: the phosphoinositide 3-kinase decides

Given the failure of chemo- and biotherapies to fight advanced pancreatic cancer, one major challenge is to identify critical events that initiate invasion. One priming step in epithelia carcinogenesis is the disruption of epithelial cell anchorage to the basement membrane which can be provided by hemidesmosomes (HDs). However, the existence of HDs in pancreatic ductal epithelium and their role in carcinogenesis remain unexplored. HDs have been explored in normal and cancer pancreatic cells, and patient samples. Unique cancer cell models where HD assembly can be pharmacologically manipulated by somatostatin/sst2 signaling have been then used to investigate the role and molecular mechanisms of dynamic HD during pancreatic carcinogenesis. We surprisingly report the presence of mature type-1 HDs comprising the integrin α6β4 and bullous pemphigoid antigen BP180 in the human pancreatic ductal epithelium. Importantly, HDs are shown to disassemble during pancreatic carcinogenesis. HD breakdown requires phosphoinositide 3-kinase (PI3K)-dependent induction of the matrix-metalloprotease MMP-9, which cleaves BP180. Consequently, integrin α6β4 delocalizes to the cell-leading edges where it paradoxically promotes cell migration and invasion through S100A4 activation. As S100A4 in turn stimulates MMP-9 expression, a vicious cycle maintains BP180 cleavage. Inactivation of this PI3K-MMP-9-S100A4 signaling loop conversely blocks BP180 cleavage, induces HD reassembly and inhibits cell invasion. We conclude that mature type-1 HDs are critical anchoring structures for the pancreatic ductal epithelium whose disruption, upon PI3K activation during carcinogenesis, provokes pancreatic cancer cell migration and invasion.

Stromally derived lysyl oxidase promotes metastasis of transforming growth factor-β-deficient mouse mammary carcinomas

The tumor stromal environment can dictate many aspects of tumor progression. A complete understanding of factors driving stromal activation and their role in tumor metastasis is critical to furthering research with the goal of therapeutic intervention. Polyoma middle T-induced mammary carcinomas lacking the type II TGF-β receptor (PyMT(mgko)) are highly metastatic compared with control PyMT-induced carcinomas (PyMT(fl/fl)). We hypothesized that the PyMT(mgko)-activated stroma interacts with carcinoma cells to promote invasion and metastasis. We show that the extracellular matrix associated with PyMT(mgko) tumors is stiffer and has more fibrillar collagen and increased expression of the collagen crosslinking enzyme lysyl oxidase (LOX) compared with PyMT(fl/fl) mammary carcinomas. Inhibition of LOX activity in PyMT(mgko) mice had no effect on tumor latency and size, but significantly decreased tumor metastasis through inhibition of tumor cell intravasation. This phenotype was associated with a decrease in keratin 14-positive myoepithelial cells in PyMT(mgko) tumors following LOX inhibition as well as a decrease in focal adhesion formation. Interestingly, the primary source of LOX was found to be activated fibroblasts. LOX expression in these fibroblasts can be driven by myeloid cell-derived TGF-β, which is significantly linked to human breast cancer. Overall, stromal expansion in PyMT(mgko) tumors is likely caused through the modulation of immune cell infiltrates to promote fibroblast activation. This feeds back to the epithelium to promote metastasis by modulating phenotypic characteristics of basal cells. Our data indicate that epithelial induction of microenvironmental changes can play a significant role in tumorigenesis and attenuating these changes can inhibit metastasis. Cancer Res; 73(17); 5336-46. ©2013 AACR.

βArrestin-1 and Mcl-1 modulate self-renewal growth of cancer stem-like side-population cells in non-small cell lung cancer

Side population (SP) cells have been reported to have properties of cancer stem-like cells (CSCs) in non-small cell lung carcinoma (NSCLC), yet their molecular features have not been fully elucidated. Here we show that, NSCLC-SP cells were enriched in G₀/G­₁ phase of cell cycle, had higher aldehyde dehydrogenase activity as well as higher clonogenic and self-renewing ability compared to main population (MP) cells. Interestingly, SP cells were also able to trans-differentiate into angiogenic tubules in vitro and were highly tumorigenic as compared to MP cells. SP-derived tumors demonstrated the intratumoral heterogeneity comprising of both SP and MP cells, suggesting the self-renewal and differentiation ability of SP cells are manifested in vivo as well. βArrestin-1 (βArr1) is involved in the progression of various cancers including NSCLCs and we find that depletion of βArr1 significantly blocked the SP phenotype; whereas depletion of βArr2 had relatively minor effects. Ectopic expression of βArr1 resulted in increased SP frequency and ABCG2 expression while abrogation of βArr1 expression suppressed the self-renewal growth and expansion of A549 cells. Anti-apoptotic protein Mcl-1 is known to be one of the key regulators of self-renewal of tissue stem cells and is thought to contribute to survival of NSCLC cells. Our experiments show that higher levels of Mcl-1 were expressed in SP cells compared to MP cells at both transcriptional and translational levels. In addition, Obatoclax, a pharmacological inhibitor of Mcl-1, could effectively prevent the self-renewal of both EGFR-inhibitor sensitive and resistant NSCLC cells. In conclusion, our findings suggest that βArr1 and Mcl-1 are involved in the self-renewal and expansion of NSCLC-CSCs and are potential targets for anti-cancer therapy.

Abstract 4381: EGFR-Src signaling regulates self-renewal of cancer stem like cells from NSCLC through Sox2

NSCLC is the most common form of lung cancer with 5-year survival rate of approximately 10%. Cancer stem cell model has been proposed for tumor-hetrogeniety, carcinogenesis and relapse after therapy. According to this model, cancer stem like cells (CSCs) are defined as self-renewing tumor cells, able to initiate and maintain the tumor. Here, we isolated CSCs based on “side populaton” (SP) phenotype, a functional property of adult stem cells to efflux Hoechst-33352 dye through ABCG2 transporter. Irrespective of genotype of the cells, we observed SP-cells in H358, H292, H1650, H1975, A549 and H460 cell lines. SP phenotype was also displayed by clinical-human xenografts, which could be specifically blocked by ABCG2 inhibitor. Relative tumorigenic potential of SP and MP (main population) cells was determined by subcutaneous or orthotopic xenograft-implantation in SCID mice. As low as 1×103 SP-cells isolated from H1650 cells could form subcutaneous tumor. Similarly, 5×104 SP-cells from A549 could establish orthotopic tumors in lung. Flow-cytometry analysis of subcutaneous tumors demonstrated the asymmetric division of SP-cells generating MP-cells within the tumors. In vitro analysis showed higher expression of ABCG2 and anti-apoptotic protein MCL1 in SP-cells as compared to MP cells. Further, SP-cells displayed higher expression of mesenchymal marker vimentin and lower levels of epithelial marker E-cadherin, suggesting the EMT-like features in SP-cells. SP-cells were found to express embryonic self-renewal factors Oct4, Sox2 and Nanog and demonstrated self-renewal capability by growing as suspended spheres in serum free, stem cell-selective medium. To successfully eliminate these CSCs from tumors, it is necessary to understand the mechanisms facilitating its self-renewal. Based on the favorable clinical outcome of EGFR- and Src-targeted therapy against certain NSCLCs, we hypothesized that the activated EGFR and Src signaling might play important roles in CSCs of NSCLC. This hypothesis was tested using biochemical as well as genetic inhibitor tools against EGFR and Src. SiRNA against EGFR significantly blocked the SP phenotype by downregulating ABCG2 protein levels in all the tested cell lines. Importantly, inhibitor of EGFR (Gefitinib, Erlotinib, BIBW2992) and Src (Dasatinib, PP2) completely blocked the self-renewal capability of SP-cells, as assessed by sphere formation assays. Further, blocking of EGFR and Src signaling by SiRNA or inhibitors resulted in downregulation of the protein levels of Sox2 in SP cells. Importantly, SiRNA against Sox2 significantly blocked the SP phenotype as well as self-renewal capacity of SP-cells. Our findings reveal an important role of EGFR-Src signaling axis in self-renewal and expansion of NSCLC-CSCs by regulating the Sox2 activity in the cells.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4381. doi:10.1158/1538-7445.AM2011-4381

Abstract 5181: Forced hemidesmosome assembly blocks pancreatic cancer cell invasiveness

Hemidesmosomes (HDs) play a critical role in epithelia integrity by anchoring epithelial cells to the basement membrane (BM). These structures, present at the basal cell surface, link the intracellular intermediate filament network with the extracellular laminins of the BM. Interestingly, HDs are frequently lost in cancer cells, thereby facilitating epithelial tumor cell detachment from the BM.

Surprisingly, we here provide evidence that a strong immunoreactivity for the integrin α6β4 and BP180, two specific hemidesmosomal proteins, is observed in human pancreatic ductal adenocarcinoma cells, where their function served paradoxically to stimulate migration and invasion. Using human pancreatic cancer cells in culture, we described molecular events explaining why these proteins are hijacked from their primary function and, importantly, how to force back the reassembly of HDs, thereby decreasing cancer cell migration and invasion.

This is achieved by introducing in these cells the somatostatin receptor sst2, whose expression is lost in 90% of pancreatic cancers, and which we have shown to have oncosuppressive functions including inhibition of tumor progression and metastasis. Cell treatment with somatostatin forced HDs reassembly by stimulating two separate critical events, as demonstrated in pancreatic cancer cells and in migrating keratinocytes: 1- the dephosphorylation of the α6β4 integrin which drives its relocalization from lamellipodia, where it is delocalised upon its phosphorylation by growth factors in migrating cells, to HDs; 2- the inhibition of the cleavage of BP180 into its pro-migratory BP120 form, which is here shown to account for the strong BP180 immunoreactivity observed in human pancreatic tumor samples as compared to normal pancreas where the full-length BP180 form is mostly expressed. BP180 cleavage into BP120 is here shown to rely at least on the activity of the MMP-9 metalloprotease whose activity is inhibited by sst2. Strikingly, knocking-down BP180 expression (siRNA) impairs somatostatin-induced HDs assembly in sst2-expressing cells. Interestingly, BP180 siRNA partially revert sst2 inhibitory role on in vitro and in vivo cell migration and invasion, as demonstrated using the chick chorioallatoic membrane model whereby tumor progression of pancreatic cancer cell xenografts is monitored. We have identified novel and original mechanisms for somatostatin-activated sst2 to revert cancer cell pro-migratory phenotype by facilitating HD assembly and cancer cell anchorage to the BM.

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5181.

Netrin-1 mediates early events in pancreatic adenocarcinoma progression, acting on tumor and endothelial cells

BACKGROUND & AIMS

Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal cancers. It is characterized by substantial tumor cell invasion and early-stage metastasis. We developed an in vivo model to analyze interactions between cancer and stromal cells during early stages of PDAC.

METHODS

Human pancreatic adenocarcinoma cells were grafted onto the chick chorioallantoic membrane (CAM). Human and chicken GeneChips were used simultaneously to study gene regulation during PDAC cell invasion. Bioinformatic analysis was used to identify human orthologs and cell specificity of gene expression. The effects of netrin-1 encoded by NTN1 were investigated in adhesion, invasion, and apoptosis assays. The effects of NTN1 silencing with small interfering RNAs were investigated in PDAC cells in vivo. NTN1 expression was measured in human PDAC samples.

RESULTS

PDAC cells rapidly invade the CAM stroma and remodel the CAM vasculature. Around 800 stromal genes were up-regulated by >2-fold; the angiogenesis regulators vascular endothelial growth factor D, thrombospondin 1, and CD151 were among the most highly regulated genes. Silencing of tumor cell NTN1, which is up-regulated 4-fold in the PDAC model, inhibited tumor cell invasion in vivo. Netrin-1 conferred apoptosis resistance to tumor and endothelial cells in vitro, induced their invasion, and provided an adhesive substrate for tumor cells. NTN1 and its gene product are strongly overexpressed in human PDAC samples.

CONCLUSIONS

We developed a useful tool to study the invasive mechanisms of early-stage PDAC. Netrin-1 might be an important regulator of pancreatic tumor growth that functions in tumor and endothelial cells.

Antitumor effects of somatostatin

Since its discovery three decades ago as an inhibitor of GH release from the pituitary gland, somatostatin has attracted much attention because of its functional role in the regulation of a wide variety of physiological functions in the brain, pituitary, pancreas, gastrointestinal tract, adrenals, thyroid, kidney and immune system. In addition to its negative role in the control of endocrine and exocrine secretions, somatostatin and analogs also exert inhibitory effects on the proliferation and survival of normal and tumor cells. Over the past 15 years, studies have begun to reveal some of the molecular mechanisms underlying the antitumor activity of somatostatin. This review covers the present knowledge in the antitumor effect of somatostatin and analogs and discusses the perspectives of novel clinical strategies based on somatostatin receptor sst2 gene transfer therapy.