TLR4-mediated inflammation promotes KSHV-induced cellular transformation and tumorigenesis by activating the STAT3 pathway

Toll-like receptors (TLR) are conserved immune sensors mediating antimicrobial and antitumoral responses, but recent evidence implicates them in promoting carcinogenesis in certain cancers. Kaposi's sarcoma (KS) is caused by infection of Kaposi's sarcoma-associated herpesvirus (KSHV) and is characterized by uncontrolled neoangiogenesis and inflammation. Here we show that TLR4 is upregulated in KSHV-infected spindle tumor cells in human KS lesions. In a model of KSHV-induced cellular transformation, KSHV upregulated expression of TLR4, its adaptor MyD88, and coreceptors CD14 and MD2. KSHV induction of TLR4 was mediated by multiple viral microRNAs. Importantly, the TLR4 pathway was activated constitutively in KSHV-transformed cells resulting in chronic induction of IL-6, IL-1β and IL-18. Accordingly, IL-6 mediated constitutive activation of the STAT3 pathway, an essential event for uncontrolled cellular proliferation and transformation. TLR4 stimulation with lipopolysaccharides or live bacteria enhanced tumorigenesis while TLR4 antagonist CLI095 inhibited it. These results highlight an essential role of the TLR4 pathway and chronic inflammation in KSHV-induced tumorigenesis, which helps explain why HIV-infected patients, who frequently suffer from opportunistic bacterial infections and metabolic complications, frequently develop KS.

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