The SARS-CoV-2 coronavirus encodes an essential papain-like protease domain as part of its non-structural protein (nsp)-3, namely SARS2 PLpro, that cleaves the viral polyprotein, but also removes ubiquitin-like ISG15 protein modifications as well as, with lower activity, Lys48-linked polyubiquitin. Structures of PLpro bound to ubiquitin and ISG15 reveal that the S1 ubiquitin-binding site is responsible for high ISG15 activity, while the S2 binding site provides Lys48 chain specificity and cleavage efficiency. To identify PLpro inhibitors in a repurposing approach, screening of 3,727 unique approved drugs and clinical compounds against SARS2 PLpro identified no compounds that inhibited PLpro consistently or that could be validated in counterscreens. More promisingly, noncovalent small molecule SARS PLpro inhibitors also target SARS2 PLpro, prevent self-processing of nsp3 in cells and display high potency and excellent antiviral activity in a SARS-CoV-2 infection model.
MicroRNAs are a recently discovered new class of important endogenous regulators of gene function. Aberrant regulation of microRNAs has been linked to various human diseases, most importantly cancer. Small molecule intervention of microRNA misregulation has the potential to provide new therapeutic approaches to such diseases. Here, we report the first small molecule inhibitors and activators of the liver-specific microRNA miR-122. This microRNA is the most abundant microRNA in the liver and is involved in hepatocellular carcinoma development and hepatitis C virus (HCV) infection. Our small molecule inhibitors reduce viral replication in liver cells and represent a new approach to the treatment of HCV infections. Moreover, small molecule activation of miR-122 in liver cancer cells selectively induced apoptosis through caspase activation, thus having implications in cancer chemotherapy. In addition to providing a new approach for the development of therapeutics, small molecule modifiers of miR-122 function are unique tools for exploring miR-122 biogenesis.
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