RESUMO
To improve cancer patient outcome significantly, we must understand the mechanisms regulating stem-like cancer cells, which have been implicated as a cause of metastasis and treatment resistance. The transcription factor C/EBPδ can exhibit pro- and anti-tumorigenic activities, but the mechanisms underlying the complexity of its functions are poorly understood. Here we identify a role for breast cancer cell intrinsic C/EBPδ in promoting phenotypes that have been associated with cancer stem cells (CSCs). While C/EBPδ expression is not abundant in most metastatic breast cancers, our data support a pro-tumorigenic role of C/EBPδ when expressed in subsets of tumor cells and/or through transient activation by the tumor microenvironment or loss of substrate adhesion. Using genetic mouse models and human breast cancer cell lines, we show that deletion or depletion of C/EBPδ reduced expression of stem cell factors and stemnness markers, sphere formation and self-renewal, along with growth of tumors and established experimental metastases in vivo. C/EBPδ is also known as a mediator of the innate immune response, which is enhanced by hypoxia and interleukin-6 (IL-6) signaling, two conditions that also play important roles in cancer progression. Our mechanistic data reveal C/EBPδ as a link that engages two positive feedback loops, in part by directly targeting the IL-6 receptor (IL6RA) gene, and, thus, amplifying IL-6 and HIF-1 signaling. This study provides a molecular mechanism for the synergism of tumor microenvironmental conditions in cancer progression with potential implications for the targeting of CSCs.
Assuntos
Neoplasias da Mama/patologia , Proteína delta de Ligação ao Facilitador CCAAT/metabolismo , Subunidade alfa do Fator 1 Induzível por Hipóxia/metabolismo , Interleucina-6/metabolismo , Células-Tronco Neoplásicas/patologia , Animais , Neoplasias da Mama/metabolismo , Proteína delta de Ligação ao Facilitador CCAAT/genética , Linhagem Celular Tumoral , Proteína 7 com Repetições F-Box-WD/genética , Proteína 7 com Repetições F-Box-WD/metabolismo , Feminino , Regulação Neoplásica da Expressão Gênica , Humanos , Subunidade alfa do Fator 1 Induzível por Hipóxia/genética , Neoplasias Pulmonares/genética , Neoplasias Pulmonares/patologia , Neoplasias Pulmonares/secundário , Camundongos Knockout , Células-Tronco Neoplásicas/metabolismo , Receptores de Interleucina-6/genética , Transdução de Sinais/genética , Microambiente Tumoral , Ensaios Antitumorais Modelo de XenoenxertoRESUMO
Toll-like receptor 4 (Tlr4) has a pivotal role in innate immune responses, and the transcription factor CCAAT/enhancer binding protein delta (C/EBPδ, Cebpd) is a Tlr4-induced gene. Here we identify a positive feedback loop in which C/EBPδ activates Tlr4 gene expression in macrophages and tumour cells. In addition, we discovered a negative feedback loop whereby the tumour suppressor FBXW7α (FBW7, Cdc4), whose gene expression is inhibited by C/EBPδ, targets C/EBPδ for degradation when C/EBPδ is phosphorylated by GSK-3ß. Consequently, FBXW7α suppresses Tlr4 expression and responses to the ligand lipopolysaccharide. FBXW7α depletion alone is sufficient to augment pro-inflammatory signalling in vivo. Moreover, as inflammatory pathways are known to modulate tumour biology, Cebpd null mammary tumours, which have reduced metastatic potential, show altered expression of inflammation-associated genes. Together, these findings reveal a role for C/EBPδ upstream of Tlr4 signalling and uncover a function for FBXW7α as an attenuator of inflammatory signalling.
