Tanaka K, Kaufman S, Milstien S
Laboratory of Neurochemistry, National Institute of Mental Health, Bethesda, MD 20892.
Proc Natl Acad Sci U S A. 1989 Aug;86(15):5864-7. doi: 10.1073/pnas.86.15.5864.
The only known role for 6(R)-5,6,7,8-tetrahydrobiopterin (BH4) is as the cofactor for the aromatic amino acid hydroxylases. However, BH4 has been shown to be synthesized by cells that do not contain any hydroxylase activity, suggesting that it may have still undiscovered functions. Our finding of much higher levels of BH4 and GTP cyclohydrolase, the first enzyme of de novo BH4 biosynthesis, in rat reticulocytes compared to mature erythrocytes raised the possibility that BH4 might play a role in erythrocyte maturation. We have now demonstrated, by using murine erythroleukemia (MEL) cells as a model for erythrogenesis, that BH4 synthesis is required for proliferation of these cells. Inhibition of BH4 biosynthesis in rapidly dividing MEL cells with N-acetylserotonin, a potent inhibitor of sepiapterin reductase, the terminal enzyme in the BH4 biosynthetic pathway, results in inhibition of DNA synthesis and mitogenesis without induction of hemoglobin synthesis. The inhibition of DNA synthesis is reversed by repletion of cellular BH4 levels with sepiapterin, a pterin that is readily taken up by the cells and converted to BH4 by the sequential reductions of sepiapterin reductase and dihydrofolate reductase. Treatment of MEL cells with hexamethylene bisacetamide, an inducer of differentiation, results in a decrease in BH4 synthesis accompanied by a cessation of growth and concomitant hemoglobin synthesis. The inhibition of proliferation induced by hexamethylene bisacetamide can be reversed by maintaining high intracellular levels of BH4, which also decreases the amount of hemoglobin. The mechanism of the BH4 effect has not yet been elucidated, but it appears as though BH4 synthesis is more intimately linked with cell proliferation than with the differentiation process.
6(R)-5,6,7,8-四氢生物蝶呤（BH4）唯一已知的作用是作为芳香族氨基酸羟化酶的辅助因子。然而，已证明不含任何羟化酶活性的细胞也能合成BH4，这表明它可能还有未被发现的功能。我们发现，与成熟红细胞相比，大鼠网织红细胞中BH4和GTP环水解酶（从头合成BH4的第一种酶）的水平要高得多，这增加了BH4可能在红细胞成熟过程中发挥作用的可能性。我们现在以鼠红细胞白血病（MEL）细胞作为红细胞生成的模型进行研究，结果表明这些细胞的增殖需要BH4的合成。用N-乙酰血清素（一种强效的蝶呤还原酶抑制剂，蝶呤还原酶是BH4生物合成途径中的末端酶）抑制快速分裂的MEL细胞中的BH4生物合成，会导致DNA合成和有丝分裂受到抑制，而不会诱导血红蛋白的合成。用蝶酰三谷氨酸补充细胞内的BH4水平可逆转DNA合成的抑制作用，蝶酰三谷氨酸是一种蝶呤，细胞很容易摄取它，并通过蝶呤还原酶和二氢叶酸还原酶的相继还原作用转化为BH4。用六亚甲基双乙酰胺（一种分化诱导剂）处理MEL细胞，会导致BH4合成减少，同时生长停止并伴随血红蛋白合成。维持细胞内高浓度的BH4可逆转六亚甲基双乙酰胺诱导的增殖抑制作用，这也会减少血红蛋白的量。BH4作用的机制尚未阐明，但似乎BH4的合成与细胞增殖的联系比与分化过程的联系更为密切。
