The MicroRNA let-7a Modulates Interleukin-6-dependent STAT-3 Survival Signaling in Malignant Human Cholangiocytes

Abstract

The inflammation-associated cytokine interleukin-6 (IL-6) can contribute to tumor growth and resistance to therapy by the activation of survival mechanisms. In several human cancers, IL-6-activated survival signaling involves the signal transducers and activators of transcription (Stat) factors or protein kinase cascades. microRNAs (miRNAs) are endogenous regulators of gene expression that are altered in expression in many cancers. However, the effect of inflammatory cytokines on miRNA expression and the role of miRNA in modulating IL-6-mediated cell survival are unknown. We investigated the involvement of miRNA in malignant cholangiocytes stably transfected to overexpress IL-6, which enhances tumor growth in vivo by inhibition of apoptosis. We provide evidence that (i) miRNA expression both in vitro and in vivo is altered by overexpression of IL-6; (ii) selective miRNAs including let-7a are up-regulated and contribute to the survival effects of enforced IL-6 activity; and (iii) let-7a contributes to the constitutively increased phosphorylation of Stat-3 by a mechanism involving the neurofibromatosis 2 (NF2) gene. These findings reveal a novel mechanism by which IL-6 mediates tumor cell survival that may be therapeutically targeted and emphasize the presence of complex interrelationships between deregulated expression of miRNA and transcription factors in human cancers.Increased expression of the inflammation-associated cytokine interleukin-6 (IL-6) 2 occurs in chronic inflammatory conditions and in several human cancers such as multiple myeloma, prostate cancer, and cholangiocarcinoma. IL-6 has been implicated in tumor growth in many of these tumors, and elevated IL-6 expression has been associated with poor outcomes and resistance to chemotherapy (1). Experimentally, growth of prostate cancer and cholangiocarcinoma xenografts in athymic mice has been shown to be increased by enforced expression of IL-6 by activation of cell survival signaling (2, 3). The mechanisms by which IL-6 promotes cell survival in cancers are of considerable interest because they may be therapeutically targeted.IL-6-activated survival signaling has been shown to involve the signal transducers and activators of transcription (Stat) factors or various protein kinase cascades (4, 5). Although constitutive activation of Stat has been described in many cancers, the precise mechanisms involved are incompletely understood. Cholangiocarcinomas are highly resistant to chemotherapy. However, inhibition of IL-6-dependent pathways such as the Jak-Stat pathway, phosphatidylinositol 3-kinase, or the p38 MAPK pathways can enhance chemotherapy-induced cell death. Thus, aberrant IL-6-dependent survival signaling may contribute to the refractoriness of cholangiocarcinoma to most chemotherapeutic agents.We sought to understand the role of microRNAs (miRNAs) in IL-6-mediated tumor cell survival. miRNAs are endogenous regulators of gene expression that are altered in expression in many cancers (6-8). The expression of several miRNAs in cholangiocarcinoma xe...

Viral hepatitis-associated intrahepatic cholangiocarcinoma shares common disease processes with hepatocellular carcinoma

“…Sustained cell proliferation in a micro-environment rich in inflammatory cells, cytokines/chemokines, growth factors and DNA-damaging agents (such as reactive oxygen and nitrogen species because of long-term inflammation) will lead to permanent genetic alterations and subsequent neoplastic transformation of the proliferating cells (Komori et al, 2008). For example, interleukin-6, an inflammatory cytokine, promotes human cholangiocarcinoma cells grown in vivo by inhibiting apoptosis through the activation of miRNAs including miR let-7a and miR370, thereby modulating the activation of STAT-3 pathways (Meng et al, 2007(Meng et al, , 2008Smirnova et al, 2007). It has also been shown that the sustained upregulation of the transcription factor nuclear factor kappaB (NF-kB) in the liver cells, through the paracrine action of tumour necrosis factor-a secreted from the neighbouring endothelia and inflammatory cells, may lead to the tumour development (Maeda et al, 2005;Choi et al, 2006;Sakurai et al, 2006;Luedde et al, 2007), given the activation of mitogenic and anti-apoptotic genes through NF-kB pathways.…”

Epigenetic Down-Regulation of the Tumor Suppressor Gene PRDM1/Blimp-1 in Diffuse Large B Cell Lymphomas

“…There are also reports that suggest an oncogenic function for let-7. 40,41 Thus, it is conceivable that let-7 may have pleiotropic cell-type specific biological effects depending on the milieu of expressed genes inside the cells. In DLBCL, abnormal down-regulation of the tumor suppressor PRDM1 due to an overexpressed let-7 may represent one of the mechanisms by which the latter promotes lymphomagenesis.…”

Role of the AKT pathway in microRNA expression of human U251 glioblastoma cells

“…Recently, a new member of transcription factor STAT3, has been identified to play a dual role in PTEN loss-and EGFRvIII-induced gliomas with AKT activation (17). STAT3 has proved to be connected with the function of certain miRNAs that play a functional role in oncogenesis (18)(19)(20).…”