Expression of deoxynucleoside kinases and 5'-nucleotidases in mouse tissues: implications for mitochondrial toxicity
Svetlana N Rylova1, Saeedeh Mirzaee, Freidoun Albertioni
1Department of Anatomy, Physiology and Biochemistry, Section of Veterinary Medical Biochemistry, SLU, The Biomedical Center, P.O. Box 575, SE-751 23 Uppsala, Sweden.
Abstract:
Anti-HIV nucleoside therapy can result in mitochondrial toxicity affecting muscles, peripheral nerves, pancreas and adipose tissue. The cytosolic deoxycytidine kinase (dCK; EC 2.7.1.74) and thymidine kinase (TK1; EC 2.7.1.21), the mitochondrial thymidine kinase (TK2) and deoxyguanosine kinase (dGK; EC 2.7.1.113) as well as 5'-deoxynucleotidases (5'-dNT; EC 3.1.3.5) are enzymes that control rate-limiting steps in formation of intracellular and intra-mitochondrial nucleotides. The mRNA levels and activities of these enzymes were determined in mouse tissues, using real-time PCR and selective enzyme assays. The expression of mRNA for all these enzymes and the mitochondrial deoxynucleotide carrier was detected in all tissues with a 5-10-fold variation. TK1 activities were only clearly detected in spleen and testis, while TK2, dGK and dCK activities were found in all tissues. dGK activities were higher than any other dNK in all tissues, except spleen and testis. In skeletal muscle dGK activity was 5-fold lower, TK2 and dCK levels were 10-fold lower as compared with other tissues. The variation in 5'-dNT activities was about eight-fold with the highest levels in brain and lowest in brown fat. Thus, the salvage of deoxynucleosides in muscles is 5-10-fold lower as compared to other non-proliferating tissues and 100-fold lower compared to spleen. These results may help to explain tissue specific toxicity observed with nucleoside analogs used in HIV treatment as well as symptoms in inherited mitochondrial TK2 deficiencies.
Insights
Nucleoside analog drugs used for HIV can cause mitochondrial toxicity. Enzyme levels in mouse tissues reveal lower deoxynucleoside salvage in muscles, explaining tissue-specific drug side effects.
Area of Science:
- Biochemistry
- Pharmacology
- Mitochondrial Biology
Background:
- Nucleoside analog therapies for HIV can lead to mitochondrial toxicity.
- This toxicity affects various tissues, including muscles, nerves, pancreas, and adipose tissue.
- Enzymes controlling nucleotide formation are crucial for understanding these effects.
Purpose of the Study:
- To investigate the mRNA levels and activities of key enzymes involved in deoxynucleoside metabolism across different mouse tissues.
- To correlate enzyme expression and activity with tissue-specific mitochondrial toxicity observed in nucleoside analog treatments.
- To understand the basis of symptoms in inherited mitochondrial thymidine kinase 2 (TK2) deficiencies.
Main Methods:
- Real-time PCR was used to determine mRNA levels of enzymes like deoxycytidine kinase (dCK), thymidine kinase 1 (TK1), mitochondrial thymidine kinase (TK2), deoxyguanosine kinase (dGK), and 5'-deoxynucleotidases (5'-dNT).
- Selective enzyme assays were employed to measure the activities of these enzymes.
- Mitochondrial deoxynucleotide carrier expression was also assessed.
Main Results:
- All studied enzymes and the mitochondrial deoxynucleotide carrier showed detectable mRNA expression in all tissues, with 5-10 fold variations.
- TK1 activity was primarily found in spleen and testis, while TK2, dGK, and dCK activities were present in all tissues.
- Deoxyguanosine kinase (dGK) activity was highest in most tissues, except spleen and testis.
- Skeletal muscle exhibited significantly lower dGK (5-fold), TK2, and dCK (10-fold) activities compared to other tissues.
- 5'-deoxynucleotidase activities varied eight-fold, with the highest in brain and lowest in brown fat.
- Overall deoxynucleoside salvage in muscles was 5-10 fold lower than in other non-proliferating tissues and 100-fold lower than in spleen.
Conclusions:
- Tissue-specific variations in deoxynucleoside salvage enzymes, particularly lower levels in skeletal muscle, likely contribute to the observed toxicity of nucleoside analogs used in HIV treatment.
- These findings provide insights into the mechanisms underlying tissue-specific side effects of anti-HIV therapies.
- The results may also help explain the clinical manifestations of inherited mitochondrial TK2 deficiencies.
