RESUMO
Chronic Myeloid Leukemia (CML) is characterized by translocations between chromosomes 9 and 22, resulting in expression of Bcr-abl oncogenes. Although the clinical course of CML was revolutionized by development of Bcr-abl-directed tyrosine kinase inhibitors (TKIs), CML is not cured by these agents. Specifically, the majority of subjects relapsed in clinical trials attempting TKI discontinuation, suggesting persistence of leukemia stem cells (LSCs) even in molecular remission. Identifying mechanisms of CML-LSC persistence may suggest rationale therapeutic targets to augment TKI efficacy and lead to cure. Apoptosis resistance is one proposed mechanism. In prior studies, we identified increased expression of Growth Arrest Specific 2 (Gas2; a Calpain inhibitor) in Bcr-abl+ bone marrow progenitor cells. A number of previously described Calpain substrates might influence apoptosis in CML, including ßcatenin and the X-linked Inhibitor of Apoptosis Protein 1 (Xiap1). We previously found Gas2/Calpain dependent stabilization of ßcatenin in CML, and increased expression of ßcatenin target genes, including Survivin (also an IAP). In the current work, we investigate contributions of Survivin and Xiap1 to Fas-resistance in Bcr-abl+ bone marrow cells. Inhibitors of these proteins are currently in clinical trials for other malignancies, but a role for either IAP in CML-LSC persistence is unknown.
RESUMO
Icsbp/Irf8 is an interferon regulatory transcription factor that functions as a suppressor of myeloid leukemias. Consistent with this activity, Icsbp represses a set of genes encoding proteins that promote cell proliferation/survival. One such gene encodes Gas2, a calpain inhibitor. We previously found that increased Gas2-expression in Bcr-abl+ cells stabilized ßcatenin; a Calpain substrate. This was of interest, because ßcatenin contributes to disease progression in chronic myeloid leukemia (CML). Calpain has additional substrates implicated in leukemogenesis, including Stat5. In the current study, we hypothesized that Stat5 activity in CML is regulated by Gas2/Calpain. We found that Bcr-abl-induced, Shp2-dependent dephosphorylation of Icsbp impaired repression of GAS2 by this transcription factor. The consequent decrease in Calpain activity stabilized Stat5 protein; increasing the absolute abundance of both phospho and total Stat5. This enhanced repression of the IRF8 promoter by Stat5 in a manner dependent on Icsbp, Gas2 and Calpain, but not Stat5 tyrosine phosphorylation. During normal myelopoiesis, increased expression and phosphorylation of Icsbp inhibits Calpain. In contrast, constitutive activation of Shp2 in Bcr-abl+ cells impairs regulation of Gas2/Calpain by Icsbp, aberrantly stabilizing Stat5 and enhancing IRF8 repression. This novel feedback mechanism enhances leukemogenesis by increasing Stat5 and decreasing Icsbp. Bcr-abl targeted tyrosine kinase inhibitors (TKIs) provide long term disease control, but CML is not cured by these agents. Our studies suggest targeting Calpain might be a rational therapeutic approach to decrease persistent leukemia stem cells (LSCs) during TKI-treatment.
Assuntos
Calpaína/metabolismo , Proteínas de Fusão bcr-abl/metabolismo , Fatores Reguladores de Interferon/genética , Leucemia Mielogênica Crônica BCR-ABL Positiva/metabolismo , Proteínas dos Microfilamentos/metabolismo , Fator de Transcrição STAT5/metabolismo , Animais , Linhagem Celular Tumoral , Humanos , Leucemia Mielogênica Crônica BCR-ABL Positiva/genética , Camundongos , Fosforilação , Regiões Promotoras Genéticas , Estabilidade Proteica , Proteína Tirosina Fosfatase não Receptora Tipo 11 , Fator de Transcrição STAT5/químicaRESUMO
Emergency granulopoiesis occurs in response to infectious or inflammatory challenge and is a component of the innate immune response. Some molecular events involved in initiating emergency granulopoiesis are known, but termination of this process is less well defined. In this study, we found that the interferon consensus sequence binding protein (Icsbp/Irf8) was required to terminate emergency granulopoiesis. Icsbp is an interferon regulatory transcription factor with leukemia suppressor activity. Expression of Icsbp is decreased in chronic myeloid leukemia, and Icsbp(-/-) mice exhibit progressive granulocytosis with evolution to blast crisis, similar to the course of human chronic myeloid leukemia. In this study, we found aberrantly sustained granulocyte production in Icsbp(-/-) mice after stimulation of an emergency granulopoiesis response. Icsbp represses transcription of the genes encoding Fas-associated phosphatase 1 (Fap1) and growth arrest-specific 2 (Gas2) and activates genes encoding Fanconi C and F. After stimulation of emergency granulopoiesis, we found increased and sustained expression of Fap1 and Gas2 in bone marrow myeloid progenitor cells from Icsbp(-/-) mice in comparison with the wild type. This was associated with resistance to Fas-induced apoptosis and increased ß-catenin activity in these cells. We also found that repeated episodes of emergency granulopoiesis accelerated progression to acute myeloid leukemia in Icsbp(-/-) mice. This was associated with impaired Fanconi C and F expression and increased sensitivity to DNA damage in bone marrow myeloid progenitors. Our results suggest that impaired Icsbp expression enhances leukemogenesis by deregulating processes that normally limit granulocyte expansion during the innate immune response.
Assuntos
Granulócitos/metabolismo , Imunidade Inata , Fatores Reguladores de Interferon/metabolismo , Leucopoese/fisiologia , Animais , Apoptose/genética , Granulócitos/citologia , Humanos , Fatores Reguladores de Interferon/genética , Camundongos , Camundongos Knockout , Proteínas dos Microfilamentos/genética , Proteínas dos Microfilamentos/metabolismo , Proteína Tirosina Fosfatase não Receptora Tipo 13/genética , Proteína Tirosina Fosfatase não Receptora Tipo 13/metabolismo , beta Catenina/genética , beta Catenina/metabolismo , Receptor fas/genética , Receptor fas/metabolismoRESUMO
The effects of prolactin (PRL) during the pathogenesis of breast cancer are mediated in part though Stat5 activity enhanced by its interaction with its transcriptional inducer, the prolyl isomerase cyclophilin B (CypB). We have demonstrated that knockdown of CypB decreases cell growth, proliferation, and migration, and CypB expression is associated with malignant progression of breast cancer. In this study, we examined the effect of CypB knockdown on PRL signaling in breast cancer cells. CypB knockdown with two independent siRNAs was shown to impair PRL-induced reporter expression in breast cancer cell line. cDNA microarray analysis was performed on these cells to assess the effect of CypB reduction, and revealed a significant decrease in PRL-induced endogenous gene expression in two breast cancer cell lines. Parallel functional assays revealed corresponding alterations of both anchorage-independent cell growth and cell motility of breast cancer cells. Our results demonstrate that CypB expression levels significantly modulate PRL-induced function in breast cancer cells ultimately resulting in enhanced levels of PRL-responsive gene expression, cell growth, and migration. Given the increasingly appreciated role of PRL in the pathogenesis of breast cancer, the actions of CypB detailed here are of biological significance.