Silencing of HSP90 affects miRNA profile in monocyte-like cells

Heat shock protein 90 (HSP90), a highly conserved chaperone, is crucial for the stability and function of many proteins, including oncogenic proteins involved in tumorigenesis (e.g., antiapoptotic proteins, transcription factors, signal-transduction proteins, Tyr-kinase receptors, etc.). The inhibition of HSP90 has been shown to be a promising therapy approach with clinical relevance for the treatment of specific tumour types. On the other hand, microRNAs (miRNAs), small, non-coding RNAs (i.e., 20-23 nucleotides) involved in the control of gene expression, are recognised as key players in the pathogenesis of human malignancies, but also as potential biomarkers and therapeutic targets. However, no data has been obtained on the effects of the suppression of HSP90 expression on miRNA profile yet. This study investigated the miRNA profile in monocyte-like cells after exposure to specific small interference RNA (siRNA) directed to HSP90 mRNA. The reduction of Hsp90 mRNA expression by 70% and 65% was achieved following 24 h and 48 h of transfection of human acute monocytic leukemia THP-1 cells with 10 nM siRNA respectively, as determined by the TaqMan real-time PCR quantification. ApoTox Glo assay revealed that the silencing neither affected cell viability nor showed cytotoxic or pro-apoptotic effect. The expression patterns of microRNAs were determined using Agilent microRNA arrays on biological duplicate time-courses. Scrambled RNA transfected cells served as controls. The obtained data suggest profound changes in miRNA profile in cells with significantly reduced HSP90 expression. In other words, after 24 h, the expression of 3 miRNAs was reduced (i.e., miR-195, miR-221, miR-224), while the expression of 20 miRNAs significantly increased, especially of let-7b and let-7f, miR-15a, miR-4270, miR-769, miR-933. After 48 h the study observed a reduced expression of 14 miRNAs (e.g., miR-1, miR-125b, miR133b, miR145, miR-451, miR-517a and b) and increased expression of 10 miRNAs (e.g., the most significantly of miR-1234, miR-135a, miR-146a, miR-374, and miR-4298). The understanding of the biological meaning of observed changes in miRNA profile after the suppression of HSP90 expression, as well as molecular mechanisms underlying these events, remain to be detailed. These data could contribute to the better understanding of mode of action of the therapeutic HSP90 inhibitors, but also reveal new potential therapeutic targets.