Low tetrahydrobiopterin biosynthetic capacity of human monocytes is caused by exon skipping in 6-pyruvoyl tetrahydropterin synthase

Abstract

Biosynthesis of (6 R )-5,6,7,8-tetrahydro-L-biopterin (H(4)-biopterin), an essential cofactor for aromatic amino acid hydroxylases and NO synthases, is effectively induced by cytokines in most of the cell types. However, human monocytes/macrophages form only a little H(4)-biopterin, but release neopterin/7,8-dihydroneopterin instead. Whereas 6-pyruvoyl tetrahydropterin synthase (PTPS) activity, the second enzyme of H(4)-biopterin biosynthesis, is hardly detectable in these cells, PTPS mRNA levels were comparable with those of cell types containing intact PTPS activity. By screening a THP-1 cDNA library, we identified clones encoding the entire open reading frame (642 bp) as well as clones lacking the 23 bp exon 3, which results in a premature stop codon. Quantification of the two mRNA species in different cell types (blood-derived cells, fibroblasts and endothelial cells) and cell lines showed that the amount of exon-3-containing mRNA is correlated closely to PTPS activity. The ratio of exon-3-containing to exon-3-lacking PTPS mRNA is not affected by differential mRNA stability or nonsense-mediated mRNA decay. THP-1 cells transduced with wild-type PTPS cDNA produced H(4)-biopterin levels and expressed PTPS activities and protein amounts comparable with those of fibroblasts. We therefore conclude that exon 3 skipping in transcription rather than post-transcriptional mechanisms is a major cause of the low PTPS protein expression observed in human macrophages and related cell types.

Cytokine-induced metabolic effects in human adipocytes are independent of endogenous nitric oxide

“…Thus, similar to other human cells, translational mechanisms appear to block human adipocyte iNOS expression (31). In human monocytes and macrophages, the activity of existing iNOS protein is blocked by low BH 4 synthesis capacity (28). This appeared not to be the case in our adipocytes: the inflammation-induced expression of the BH 4 -synthesizing enzyme GTPCH evoked the production of both BH 4 and its side product neopterin.…”

State-of-the-art 2003 on PKU gene therapy

“…BH 4 can also be regenerated by two additional enzymes, pterin-4a-carbinolamine dehydratase and dihydropteridine reductase [42]. It turns out that only GTPCH and 6-pyruvoyltetrahydropterin synthase are highly regulated and/or not expressed constitutively in all tissues, whereas the other enzymes for BH 4 metabolism seem to be ubiquitously present [42][43][44][45].…”

Neopterin as a biomarker of immune response in cancer patients

“…Although GTP cyclohydrolase is the first enzyme in a metabolic pathway leading to the formation of 5,6,7,8-tetrahydroneopterin, the activities of enzymes distal to this step are negligible in human macrophages, thus resulting in the production of 7,8-dihydroneopterin that is rapidly oxidized to neopterin. The biologic significance of neopterin production by human macrophages is still disputed (37,38). Interestingly, human macrophages produce very little, if any, nitric oxide upon the stimulation.…”