# RNase L Induces Expression of A Novel Serine/Threonine Protein Kinase, DRAK1, to Promote Apoptosis  
[PERSON]  
1, [PERSON]  
1, [PERSON], [PERSON] and [PERSON]  
1 Department of Biological Sciences, University of Toledo, 2801 West Bancroft Street, Toledo, OH 43606, USA 1  
Footnote 1: email: {mdpi.com/journals/ijms/2019/201  
activated by dsRNA and convert cellular ATP into 2'-5'-oligoadenylate oligomers (2-5A) that bind to the ubiquitously expressed latent endoribonuclease, Ribonuclease L (RNase L) [11]. Binding of RNase L inactive monomers to 2-5A, causes conformational change and dimerization to produce an active nuclease that cleaves single-stranded viral and cellular RNAs after UpUp or UpAp residues, producing small dsRNA products with roles in transcriptional activation of antiviral and antitumor genes, amplifying IFN production, activating inflammosomes and promoting switch from autophagy to apoptosis [12; 13; 14; 15; 16]. Activation of RNase L leads to mitochondrial release of cytochrome C and caspase-dependent apoptosis involving JNK activity in a cell-type specific manner [17; 18]. Unexpectedly, RNase L transcriptionally induced antiviral and antitumor genes [12]. Among the genes that were transcriptionally induced by RNase L, death-associated protein kinase-related apoptosis-inducing protein kinase 1 (DRAK1) was a unique member of the DAP-kinase family, and its role in cell death by IFN signaling was unknown [12].  
DRAK1, also known as STK17A, is a member of the death-associated protein kinase (DAPK) family of proteins and shares homology with DRAK2 (STK17B). Both proteins share homology with other DAPK members in the kinase domain, however, the regulatory domains are distinct, suggesting involvement in cell death induced by different stimuli [19]. The role of DRAK1 in inducing apoptosis varies in different cell types. Overexpression in NIH3T3 cells induced morphological changes characteristic of apoptosis [19]. In testicular cancer cells, DRAK1 is induced in a p53-dependent manner by cisplatin and induces cell death by regulating reactive oxygen species [20]. Interestingly, DRAK1 was overexpressed in glioblastoma and promoted cell proliferation, migration and resistance to genotoxic agents [21; 22]. DRAK1 is overexpressed and localized in the cytoplasm in head and neck squamous cell carcinomas (HNSCC) and negatively regulates the tumor suppressor function of TGF-\(\beta\) by binding to Smad3 which is required for Transforming Growth Factor Beta (TGF-\(\beta\)) function [23]. Recently, DRAK1 was shown to localize in the nucleus and translocate to the cytoplasm in response to Protein Kinase C (PKC) activation and interact with p53 and mitochondrial protein, Adenine Nucleotide Translocator 2 (ANT2), suggesting regulation at multiple levels [24].  
In this study, we show that DRAK1, and not DRAK2, is induced by RNase L and IFN signaling. Ectopic expression of DRAK1 in prostate cancer cells is sufficient to induce apoptosis in the absence of RNase L activation. Our studies show that DRAK1 induces JNK activity and cell death involving Bax translocation to the mitochondria accompanied by loss of mitochondrial membrane potential and cleavage of caspase 3 and PARP. Finally, we show that both DRAK1 nuclear localization and kinase activity are required for inducing apoptosis. These studies identify DRAK1 as one of the factors that contributes to cell death in response to RNase L activation.