Human VRK2 modulates apoptosis by interaction with Bcl-xL and regulation of BAX gene expression

Abstract

VRK2 is a novel Ser-Thr kinase whose VRK2A isoform is located in the endoplasmic reticulum and mitochondrial membranes. We have studied the potential role that VRK2A has in the regulation of mitochondrial-mediated apoptosis. VRK2A can regulate the intrinsic apoptotic pathway in two different ways. The VRK2A protein directly interacts with Bcl-xL, but not with Bcl-2, Bax, Bad, PUMA or Binp-3L. VRK2A does not compete with Bax for interaction with Bcl-xL, and these proteins can form a complex that reduces apoptosis. Thus, high VRK2 levels confer protection against apoptosis. In addition, VRK2 knockdown results in an increased expression of BAX gene expression that is mediated by its proximal promoter, thus VRK2A behaves as a negative regulator of BAX. Low levels of VRK2A causes an increase in mitochondrial Bax protein level, leading to an increase in the release of cytochrome C and caspase activation, detected by PARP processing. VRK2A loss results in an increase in cell death that can be detected by an increase in annexinV+ cells. Low levels of VRK2A increase cell sensitivity to induction of apoptosis by chemotherapeutic drugs like camptothecin or doxorubicin. We conclude that VRK2A protein is a novel modulator of apoptosis.

Deletion of the Vaccinia Virus B1 Kinase Reveals Essential Functions of This Enzyme Complemented Partly by the Homologous Cellular Kinase VRK2

“…For example, VRK2 has been reported to modulate MAP kinase signaling pathways at the ER, in part via an interaction with the KSR scaffolding protein (41,42). Additionally, VRK2 appears to act as an apoptosis regulator in some studies, inhibiting cell death via interaction with Bcl-xL or by altering BAX gene expression (18). An investigation of these pathways in conjunction with structure/function studies of VRK2 are ongoing in our laboratory.…”

“…For example, VRK1 is located predominantly in the nucleus, while the major isoform of VRK2 is associated with the endoplasmic reticulum (ER) and nuclear envelope (11,14). Interestingly, both VRK1 and VRK2 possess strong catalytic activity found to modulate cellular processes, including mitosis and apoptosis, via multiple substrates (15)(16)(17)(18). It has also been demonstrated that VRK2 limits cell death during Epstein-Barr virus (EBV) infection (19) and delays myxoma virus replication in a breast cancer cell line (20), thus providing other examples that VRKs may regulate pathways important during viral infection.…”

Vaccinia-Related Kinase 2 Controls the Stability of the Eukaryotic Chaperonin TRiC/CCT by Inhibiting the Deubiquitinating Enzyme USP25

“…VRK2 also promotes cancer cell invasion by elevating the transcriptional activity of nuclear factor of activated T cells 1 (NFAT1) by VRK2-mediated phosphorylation (17). In addition, VRK2 prevents apoptosis by interacting with Bcl-x L , an antiapoptotic Bcl-2 homology (BH) domain protein, which is involved in BAX gene expression and regulates mitochondrial function (18). Recently, VRK2 function in the brain has been noted in psychiatric disorders, including schizophrenia and epilepsy.…”

Vaccinia-Related Kinase 2 Mediates Accumulation of Polyglutamine Aggregates via Negative Regulation of the Chaperonin TRiC

“…It seems that a subpopulation of VRK2 is localized in the outer mitochondrial membrane. VRK2 was reported to regulate apoptosis through interaction with Bcl-xL in the outer mitochondrial membrane (25). Although VRK2 was observed in the nuclear membrane, it did not perfectly overlap the lamin B (an inner nuclear envelope protein marker).…”

“…VRK2 also inhibits MEK/extracellular signal-regulated kinase signal transduction by interacting with the KSR1 scaffold protein in breast cancer cell lines, which leads to a signaling imbalance (23). In addition, VRK2 enhances cell survival through interacting with BHRF1, an antiapoptotic Epstein-Barr virus protein (24), and Bcl-xL, an antiapoptotic BH (Bcl-2 homology) domain protein (25). Because VRK2 has a sequence similar to that of the catalytic domain of vaccinia-related kinase 1 (VRK1), VRK2 also phosphorylates histone H3 (26), p53 (21), and barrier to autointegration factor (BAF) (27), which supposedly compensates for the VRK1 function.…”