Nerve growth factor-induced increase in biopterin level of pheochromocytoma PC12.

神经生长因子（NGF）是各种神经元存活和发育所必需的蛋白质。在培养系统中，NGF诱导嗜铬细胞瘤PC12由嗜铬细胞样表型向交感神经元样细胞分化。本研究发现NGF可使PC12细胞中的四氢生物蝶呤（tetrahydrobiopterin, BPH4）瞬时升高。然后探讨了NGF的作用机制及该现象的生物学意义。作用机制：(1)ngf诱导的BPH4升高被放线菌素D （AD）和环己亚胺（CH）阻断。(2)被认为是BPH4生物合成调控酶的GTP环水解酶（GTPCH）活性也在NGF作用下瞬间升高。在时间和剂量依赖性实验中，GTPCH活性和BPH4水平对NGF有相似的反应。NGF对GTPCH的作用也被AD和'CH阻断。这些结果表明，NGF通过RNA和蛋白质的合成增加GTPCH的量，从而增加BPH4水平。(3)蛋白激酶抑制剂H-7和H-8在K_i值附近的浓度对蛋白激酶A、蛋白激酶G和蛋白激酶C的作用没有影响。(4)二丁基环AMP （dBcAMP）也能提高GTPCH活性和BPH4水平，但NGF的作用机制似乎与dBcAMP不同。生物学意义：研究了BPH4类似物四氢-6-甲基蝶呤（MPH4）对名为SMP的核蛋白磷酸化的影响，其中SMP的磷酸化是由NGF刺激的。MPH4抑制NGF刺激的SMP磷酸化，提示BPH4可能是NGF作用的调节剂。然而，需要进一步的研究来阐明NGF诱导的BPH4瞬时升高的生物学作用。

Nerve growth factor (NGF) is a protein required for the survival and development of various neurons. In culture system, NGF induces differentiation of pheochromocytoma PC12 into sympathetic neuron-like cells from chromaffin cell-like phenotype. In this study, tetrahydrobiopterin (BPH4) in PC12 cells are found to be transiently increased by NGF. Then the mechanism of this NGF action and the biological meaning of the phenomenon are investigated.1. Mechanism of action: (1) NGF-induced increase in BPH4 was blocked by actinomycin D (AD) and cycloheximide (CH). (2) The activity of GTP cyclohydrolase (GTPCH), which is thought to be a regulatory enzyme in BPH4 biosynthesis, was also transiently increased by NGF. The GTPCH activity and BPH4 level showed quite similar responses to NGF in the time course and dose dependence experiments. The NGF effect on GTPCH was also blocked by AD and 'CH. These results suggest that NGF increase BPH4 level by increasing the amount of GTPCH via synthesis of RNA and of protein. (3) Protein kinase inhibitors, H-7 and H-8, of the concentrations around K_i values for protein kinase A, protein kinase G and protein kinase C did not affect the NGF action. (4) Dibutyryl cyclic AMP (dBcAMP) also increased the GTPCH activity and BPH4 level, but the action mechanism of NGF seems to be different from that of dBcAMP.2. Biological meaning: Effect of an analog of BPH4, tetrahydro-6-methylpterin (MPH4), on the phosphorylation of a nuclear protein named SMP, of which phosphorylation is stimulated by NGF, was examined. MPH4 inhibited the NGF-stimulated phosphorylation of SMP, suggesting a possible function of BPH4 as a modulator of NGF action. Further study, however, should be needed to clarify the biological role of transient increase in BPH4 induced by NGF.