Background: Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) is the etiologic agent of KS, the most common AIDS-related malignancy. The majority of KS tumor cells harbor latent KSHV virus but only a small percentage undergoes spontaneous lytic replication. Viral reactivation from latency is crucial for the pathogenesis and development of KS, but the cellular mechanisms underlying the switch between viral latency and replication are not well understood.
Methods: The level of let-7 miRNAs and MAP4K4 in KSHV infected 293T cells were quantified by real-time PCRs. Let-7 expression was silenced by the miRNA sponge technique. In let-7a transfected 293T cells, the expression of MAP4K4 was measured by real-time PCR and western blot. Luciferease expression was employed to examine the effect of let-7a on the 3'-untranslated region (UTR) of the MAP4K4 gene in 293T cells. Real-time PCR was used to quantify the KSHV copy numbers in BC-3 cells in which the expression of let-7a and/or MAP4K4 were altered. Finally, ERK, JNK and p38 protein production and their phosphorylation status were detected by western blots in let-7a or MAP4K4 transfected BCBL-1 cells.
Results: The expression of microRNA let-7 was dramatically decreased in KSHV infected 293T cells, but that of MAP4K4 was increased significantly. Let-7a is physically associated with and targets the MAP4K4 3'UTR, and inhibits MAP4K4 expression at both mRNA and protein levels. MAP4K4 stimulates KSHV reactivation from latency, whereas let-7a inhibits the function of MAP4K4 by reversing the function of MAP4K4 on JNK, phospho-JNK and phospho-ERK1/2 levels.
Conclusion: Our results establish that let-7a specifically suppresses MAP4K4 expression, and further inhibits KSHV reactivation by interfering with the function of MAP4K4 on the MAPK pathway, highlighting let-7a as a potential treatment for KS.