Interleukin-6: A Critical Cytokine in Cancer Multidrug Resistance

Multidrug resistance (MDR) is a phenomenon through which tumor cells develop resistance against the cytotoxic effects of various structurally and mechanistically unrelated chemotherapeutic agents. The most consistent feature in MDR is overexpression and/or overactivity of ATP-dependent drug efflux transporters. Other mechanisms such as overexpression of drug-detoxifying enzymes and alterations in pro-survival or pro-death signaling pathways are also responsible for MDR. Inflammatory mediators including interleukin-6 (IL-6) play important roles in various events during inflammation and are also involved in development and progression of several types of cancers. Mounting evidence has suggested a crosstalk between IL-6 and MDR in cancer, highlighting the role of IL-6 in chemotherapy response, and the potential opportunity to control MDR through modulation of IL-6 expression. Upregulation of IL-6 has been shown to promote MDR through activation of Janus kinases (JAK)/signal transducer and activator of transcription 3 (STAT3), phosphatidylinositol-3 kinase (PI3K)/protein kinase B (Akt), and Ras-MAPK (mitogen-activated protein kinase) pathways. Activation of the aforementioned pathways changes the expression pattern of several genes involved in proliferation, survival and cell cycle regulation, thus facilitating MDR. Conversely, IL-6 inhibition using different strategies (antibodies, siRNA, and antisense transfection) has been shown to improve tumor responsiveness and mitigate MDR in different cancer cell lines. This review focuses on the in vitro, experimental and clinical findings on the role of IL-6 in MDR, and potential therapeutic opportunities arising from this role of IL-6.