Protein kinase CK2 inactivates PRH/Hhex using multiple mechanisms to de-repress VEGF-signalling genes and promote cell survival

Abstract

Protein kinase CK2 promotes cell survival and the activity of this kinase is elevated in several cancers including chronic myeloid leukaemia. We have shown previously that phosphorylation of the Proline-Rich Homeodomain protein (PRH/Hhex) by CK2 inhibits the DNA-binding activity of this transcription factor. Furthermore, PRH represses the transcription of multiple genes encoding components of the VEGF-signalling pathway and thereby influences cell survival. Here we show that the inhibitory effects of PRH on cell proliferation are abrogated by CK2 and that CK2 inhibits the binding of PRH at the Vegfr-1 promoter. Phosphorylation of PRH by CK2 also decreases the nuclear association of PRH and induces its cleavage by the proteasome. Moreover, cleavage of phosphorylated PRH produces a stable truncated cleavage product which we have termed PRHΔC (HhexΔC). PRHΔC acts as a transdominant negative regulator of full-length PRH by sequestering TLE proteins that function as PRH co-repressors. We show that this novel regulatory mechanism results in the alleviation of PRH-mediated repression of Vegfr-1. We suggest that the re-establishment of PRH function through inhibition of CK2 could be of value in treatment of myeloid leukaemias, as well as other tumour types in which PRH is inactivated by phosphorylation.

Protein Kinase CK2 and Angiogenesis

“…Interestingly, it was shown that phosphorylation of the Proline-Rich-Homeo domain protein PRH by CK2 inhibits the DNA binding activity of PRH. This inhibiting effect of PRH on genes encoding components of the VEGF signalling pathway is abrogated by CK2 phosphorylation [43] indicating that VEGF may be one target of CK2 in regulating angiogenesis. It was also recently shown that oxidized phospholipids induced the expression of VEGF in foetal human retinal pigment epithelial (RPE) cells [44].…”

LITAF, HHEX, and DUSP1 expression in mesenchymal stem cells from patients with psoriasis

“…In Hhex -/-mice, HHEX/PRH was found to bind to the promoter regions of key members of the VEGF signaling pathway, such as VEGF, VEGF receptor (VEGFR)1, VEGFR2, and neuropillin-1, inhibiting their transcription, preventing cell proliferation, and playing a negative regulatory role in angiogenesis (Noy et al, 2010). The negative regulatory effect of HHEX/PRH on angiogenesis can be blocked by casein kinase 2 (Noy et al, 2012). We previously reported that VEGF was highly expressed in DMSCs derived from psoriatic skin lesions (Hou et al, 2014) and that for the treatment of psoriasis, TNF-α inhibition resulted in localized inhibition of Data are reported as means ± SD.…”

Gene Expression and Regulation in Mammalian Cells - Transcription From General Aspects

“…CK2 has also been shown to inhibit the binding of PRH to DNA in cells. Ectopic over-expression of PRH in K562 cells represses transcription of the PRH target gene VEGFR-1 but this repression is lost on co-transfection with CK2α and β transgenes [48]. However, the repression of VEGFR-1 transcription by a PRH mutant in which phosphorylation of serine 163 and serine 177 is prevented by the replacement of these residues by cysteine residues is not inhibited by CK2 over-expression [48].…”

“…Ectopic over-expression of PRH in K562 cells represses transcription of the PRH target gene VEGFR-1 but this repression is lost on co-transfection with CK2α and β transgenes [48]. However, the repression of VEGFR-1 transcription by a PRH mutant in which phosphorylation of serine 163 and serine 177 is prevented by the replacement of these residues by cysteine residues is not inhibited by CK2 over-expression [48]. Quantitative chromatin immunoprecipitation (ChIP) showed that CK2 over-expression does not prevent the binding of PRH S163C,S177C to the VEGFR-1 promoter as it does with wildtype PRH [48].…”

“…PRH over-expression inhibits haematopoietic and vascular development in embryoid bodies [47] while PRH loss leads to abnormal vasculogenesis and cardiac morphogenesis [5]. PRH can inhibit the proliferation of leukaemic cells by repressing the transcription of VEGFA and other genes involved in VEGF signalling and haematopoietic and vascular biology [48]. PRH is also important in neo-angiogenesis; in endothelial cells PRH represses transcription of multiple genes that control blood vessel formation including VEGFR-1, VEGFR-2, tyrosine kinase with Ig and EGF homology domains (TIE)-1, TIE-2, and neuropilin-1 [27,49].…”