Oncotarget 2013; 4: 560-571 560 www.impactjournals.com/oncotarget www.impactjournals.com/oncotarget/ Oncotarget, April, Vol.4, No 4 Characterizing the convergence of protein kinase CK2 and caspase-3 reveals isoform-speciic phosphorylation of caspase-3 by CK2α’: implications for pathological roles of CK2 in promoting cancer cell survival. Jacob P. Turowec 1,* , Greg Vilk 1,* , Michelle Gabriel 1 , David W. Litchield 1 1 Department of Biochemistry, Schulich School of Medicine, Western University, London, ON, Canada. * These authors contributed equally to this work Correspondence to: David W. Litchield, email: litchi@uwo.ca Keywords: Protein kinase CK2, isozyme speciic substrate, caspase-3 Received: March 25, 2013 Accepted: March 29, 2013 Published: March 31, 2013 This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. AbstrAct: Protein kinase CK2 has emerged as a promising candidate for the treatment of a number of cancers. This enzyme is comprised of two catalytic subunits (CK2 and/ or CK2α′) that form complexes with homodimers of regulatory CK2β subunits. While catalytic and regulatory CK2 subunits are generally expressed at similar levels to form tetrameric complexes, asymmetric expression of CK2 subunits has been associated with various forms of cancer and the enhanced survival of cancer cells. To elucidate mechanisms responsible for regulation of cancer cell survival by CK2, we recently employed computational and experimental strategies that revealed widespread overlap between sites for CK2 phosphorylation and caspase cleavage. Among candidates with overlapping CK2 and caspase cleavage sites was caspase-3 that is phosphorylated by CK2 to prevent its activation by upstream caspases. To elucidate the precise relationship between CK2 and caspase-3, we modulated expression of individual CK2 subunits and demonstrated that CK2α′ exhibits a striking preference for caspase-3 phosphorylation in cells as compared to CK2α and that CK2β exhibits the capacity to abolish caspase-3 phosphorylation. Since caspase-3 represents the irst CK2 substrate selectively phosphorylated by CK2α′ in cells, our work highlights divergent functions of the diferent forms of CK2. Given the involvement of CK2 in a diverse series of biological events and its association with various cancers, this work has important implications for identifying pathological roles of distinct forms of CK2 that could instruct eforts to selectively target individual CK2 subunits for therapy. INtrODUctION Protein kinase CK2 is a ubiquitously expressed, highly conserved, constitutively active protein Ser/Thr kinase implicated in a plethora of cellular functions, including cancer progression [1]. The enzyme is generally comprised of two catalytic (CK2α or CK2α′) subunits that form a holoenzyme complex when bound with two regulatory CK2β subunits [2]. High levels of transcript expression [3, 4], as well as increased protein and kinase activity are associated with the pathological functions of CK2 in a number of human tumors and lymphomas (reviewed in [5]). Indeed, a causative role for CK2 in transformation was observed in mice where tissue-speciic overexpression of CK2α in mammary and T-cells increased cancer burden [6, 7]. Interest in CK2 as a target for cancer therapeutics continues to mount with the number of patents iled for CK2 inhibitors increasing signiicantly in recent years [8] and at least one CK2 inhibitor, CX- 4945, entering clinical trials [9]. Implicit in understanding how CK2 inhibition can be most successfully translated to the clinic will be a careful dissection of CK2 function in disease to distinguish between physiological roles that promote survival of healthy cells and pathological roles that enable the inappropriate growth and survival of diseased cells. Interestingly, recent studies have revealed