ABSTRACT
The activity of the protein kinase (CK2) is enhanced in vitro by the binding of polyamines to the CK2beta regulatory subunit. The overexpression of ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, also elevates CK2 kinase activity in primary keratinocytes and tissues of K6/ODC transgenic mice. In an effort to better characterize the mechanisms by which polyamines may affect CK2 in vivo, we constructed a transfectable CK2 substrate cDNA consisting of the enhanced green fluorescence protein appended with a canonical CK2 phosphorylation sequence (EGFP-S). In contrast to unmodified EGFP, the EGFP-S protein was extensively phosphorylated by CK2, and this phosphorylation was stimulated by the polyamine spermine in a dose-dependent manner. The in vivo phosphorylation of EGFP-S was examined in cell lines which inducibly express either wild-type CK2 holoenzyme or a CK2 holoenzyme which contains activating mutations in the polyamine-binding region of its CK2beta regulatory subunit. Neither the overexpression of ODC in either cell line nor the mutation of the CK2beta subunit conferred an increase in CK2 kinase activity as measured by the in vivo phosphorylation of EGFP-S. Rather, our data indicate that polyamines increase total CK2 kinase activity through increases in steady-state levels of both CK2alpha and CK2beta subunits. The overexpression of ODC resulted in a 3-fold increase in steady-state levels of both exogenous and endogenous CK2 transcripts but did not increase the half-life of wild-type or mutated CK2 protein. These data suggest that the regulation of intracellular CK2 by the polyamines may occur through mechanisms distinct from the direct stimulation of CK2 by polyamines in vitro as previously described.
Subject(s)
Casein Kinase II/drug effects , Casein Kinase II/metabolism , Polyamines/pharmacology , Casein Kinase II/chemistry , Cell Line , Cells, Cultured , Ecdysone/pharmacology , Enzyme Activation/drug effects , Enzyme Stability , Green Fluorescent Proteins/chemistry , Humans , Ornithine Decarboxylase/biosynthesis , Phosphorylation , Protein Subunits/chemistry , RNA, Messenger/chemistry , Recombinant Proteins/chemistryABSTRACT
Our previous studies have shown that the overexpression of ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, increases the enzymatic activity of the polyamine-responsive enzyme casein kinase 2 (CK2). Because CK2 is known to preferentially associate with the nuclear matrix in response to other trophic stimuli, we investigated the effects of ODC overexpression on CK2 localisation and on the CK2-mediated phosphorylation of a known CK2 substrate, the nucleolar phosphoprotein B23. Immunofluorescence analysis of CK2 and B23 in primary keratinocytes revealed that ODC overexpression resulted in the colocalisation of CK2 with B23 at the nucleolar borders. ODC overexpression also increased CK2 kinase activity 2-fold at the nuclear matrix, a response which could be abrogated by treatment of K6/ODC transgenic keratinocytes with the ODC inhibitor alpha-difluoromethylornithine (DFMO). Levels of B23 protein were also elevated in ODC-overexpressing cells compared to normal cells or transgenic cells treated with DFMO. This increase in protein level was neither due to an increase in steady-state mRNA levels, nor was it due to increased stability of B23 protein. Phosphorylation of B23 was also increased in ODC-overexpressing cells, and this increased phosphorylation could be blocked by treatment of the cells with the CK2 kinase inhibitors apigenin or 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). These data suggest that B23 may be a downstream effector of polyamines via phosphorylation by the protein kinase CK2.
