Transient Up-Regulation of Liver Mitochondrial Thymidine Kinase Activity in Proliferating Mitochondria

Abstract

Administration of the fatty acid analogue tetradecylthioacetic acid (TTA) to rodents up-regulates peroxisomal and mitochondrial lipid-metabolizing enzymes and induces a proliferation of these organelles in hepatocytes. We show here that male NMRI mice fed a diet containing 0.3% (w/w) TTA revealed a transient two-fold increase in the incorporation of [3H]thymidine into the liver mtDNA followed by a 1.6-fold increase in the content of mtDNA. In addition, a transient three-fold increase in the mitochondrial thymidine kinase (TK2) activity and a slight increase in the DNA polymerase gamma activity was observed, indicating that the TTA induced mitochondrial proliferation is linked to an up-regulation of the mitochondrial thymidine kinase activity.

Tight Binding of Deoxyribonucleotide Triphosphates to Human Thymidine Kinase 2 Expressed in Escherichia coli. Purification and Partial Characterization of Its Dimeric and Tetrameric Forms

“…Nevertheless, by using a specific forward primer directed to this 5′-sequence, we were able to clone the cytosolic isoform again, from Hep2 cells. Thus, it is likely that at least two different forms of the enzyme actually exist in a cell, i.e., a mitochondrial and a cytosolic isoform, as observed for mouse dGK (45) and previously suggested for hTK2 (3,7,10). In fact, the hTK2 gene shows multiple transcripts in most tissues (8,29), and the two isoforms may originate by alternative splicing of alternative first exons, with the hTK2 gene consisting of more exons (for reference, see http://www.ncbi.nlm.nih.gov/IEB/Research/ Acembly/av.cgi?db)human&c)Gene&l)TK2) than the 10 initially proposed (6).…”

“…hTK2 has generally been considered to be located in the mitochondria as for the mouse enzyme (36,44). However, a cytoplasmic localization of the human enzyme has also been suggested (3,7,10). In agreement with such a proposal, a full-length hTK2 cDNA, containing an initiation codon and lacking a mitochondrial leader sequence, was cloned (8), and the recombinant protein was shown to be predominantly localized to the cytosol on expression in mammalian cells as a GFP fusion protein (9).…”

“…TK1 and dCK have been shown to be cytosolic enzymes, although dCK may enter the nucleus under certain conditions (2). By contrast, dGK and TK2 are predominantly mitochondrial enzymes (3)(4)(5)(6), but TK2 may also be present in the cytosol (3,(7)(8)(9)(10). Furthermore, the expression of TK1 is strictly correlated to the S phase in contrast to the other three enzymes, which are constitutively expressed.…”

Kinetic Analysis and Ligand-Induced Conformational Changes in Dimeric and Tetrameric Forms of Human Thymidine Kinase 2

“…Together with other deoxynucleoside kinases, TK2 is responsible for the initial and rate-limiting step of the salvage of deoxyribonucleosides in the cell, which can be further phosphorylated to triphosphates by nucleoside mono- and diphosphate kinases and eventually incorporated into DNA (reviewed in ref 1 ). TK2 is predominantly a mitochondrial enzyme ( − ) but may also be present in the cytosol ( 3 , − and is the only dThd phosphorylating enzyme expressed in nonproliferating tissues such as liver, brain, and muscle. TK2 is thus important in providing precursors for DNA repair and/or mitochondrial DNA (mtDNA) replication in resting cells, where de novo synthesis is undetectable (reviewed in ref 1 ).…”

Biochemical Characterization of Human Guanylate Kinase and Mitochondrial Thymidine Kinase: Essential Enzymes for the Metabolic Activation of Nucleoside Analog Prodrugs

“…TK2 is predominantly found in mitochondria, but it may also be present in cytosol [139,227,228]. In fact, hTK2 shows multiple transcripts in most tissues, and the two isoforms (mitochondrial and cytosolic) may originate by alternative splicing of the alternative first exons in the hTK2 gene which consists of more than 10 exons [139].…”