Reduced oxygen tension (hypoxia) induces a 3-fold increase in stability of mRNA for tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis, in the pheochromocytoma (PC12) clonal cell line. To investigate the possibility that RNA-protein interactions are involved in mediating this increase in stability, RNA gel shift assays were performed using different fragments of labeled TH mRNA and the S-100 fraction of PC12 cytoplasmic protein extracts. We identified a sequence within the 3‘-untranslated region of TH mRNA that binds cytoplasmic protein. RNase T1 mapping revealed that the protein was bound to a 28 nucleotide long sequence that is located between bases 1551-1579 of TH mRNA. Moreover, protein binding to this fragment was prevented with an antisense oligonucleotide directed against bases 1551-1579 and subsequent RNase H digestion. This fragment of the 3‘-untranslated region of TH mRNA is rich in pyrimidine nucleotides, and the binding of cytoplasmic protein to this fragment was reduced by competition with other polypyrimidine sequences including poly(C) but not poly(U) polymers. The binding of the protein to TH mRNA was increased when cytoplasmic proteins were extracted from PC12 cells exposed to hypoxia (5% O2) for 24 h. Electrophoresis of the UV cross-linked RNA-protein complex on SDS-polyacrylamide gel electrophoresis revealed a complex of 74 kDa. The potential role of this protein-TH mRNA interaction in regulation of TH mRNA stability during hypoxia is discussed.
Abstract: Acute exposure to hypoxia causes a release of adenosine (ADO) that is inversely related to the O 2 levels in oxygen‐sensitive pheochromocytoma (PC12) cells. In the current study, chronic exposure (48 h) of PC12 cells to moderate hypoxia (5% O 2 ) significantly enhanced the release of ADO during severe, acute hypoxia (1% O 2 ). Investigation into the intra‐ and extracellular mechanisms underpinning the secretion of ADO in PC12 cells chronically exposed to hypoxia revealed changes in gene expression and activities of several key enzymes associated with ADO production and metabolism, as well as the down‐regulation of a nucleoside transporter. Decreases in the enzymatic activities of ADO kinase and ADO deaminase accompanied by an increase in those of cytoplasmic and ecto‐5′‐nucleotidases bring about an increased capacity to produce intra‐ and extracellular ADO. This increased potential to generate ADO and decreased capacity to metabolize ADO indicate that PC12 cells shift toward an ADO producer phenotype during hypoxia. The reduced function of the rat equilibrative nucleoside transporter rENT 1 also plays a role in controlling extracellular ADO levels. The hypoxia‐induced alterations in the ADO metabolic enzymes and the rENT 1 transporter seem to increase the extracellular concentration of ADO. The biological significance of this regulation is unclear but is likely to be associated with modulating cellular activity during hypoxia.
Reduced arterial oxygen tension (i.e. hypoxia) is a powerful physiological stimulus that induces synthesis and release of dopamine from O2-sensitive (type I) cells in the mammalian carotid bodies. We reported recently that hypoxia stimulates gene expression for tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis in type I cells of the carotid body. Efforts to identify the mechanisms regulating TH gene expression in O2-sensitive cells during hypoxia have been hampered by the lack of an appropriate model cell culture system. Here we report that TH gene expression in the rat pheochromocytoma cell line (PC12) is regulated during hypoxia in a manner similar to that measured in carotid body type I cells. PC12 cells might therefore be useful as an experimental model for identifying the molecular mechanisms that regulate TH gene expression during hypoxia. Nuclear runoff assays revealed that transcription of the wild type TH gene was enhanced during exposures to hypoxia lasting 12 h. Chloramphenicol acetyltransferase assays with constructs that contained different fragments of TH promoter revealed that the regulatory sequences that mediate the hypoxia-induced increase in transcription are located between bases -272 and +27 of the TH gene. Findings from experiments in which transcription was inhibited either with actinomycin D or 5,6-dichloro-1-D-ribofuranosylbenzimidazole, as well as pulse-chase experiments using 4-thiouridine showed that the half-life of TH mRNA was substantially increased during hypoxia. Thus, in the present paper we show that TH gene expression in PC12 cells during hypoxia is regulated by increases in both the rate of TH gene transcription and TH mRNA stability.
The present study was undertaken to determine if gene expression for tyrosine hydroxylase (TH), the rate limiting enzyme in the biosynthesis of catecholamines, is regulated in the carotid body, sympathetic ganglia and adrenal medulla by hypoxia. We found that a reduction in oxygen tension from 21% to 10% caused a substantial increase (200% at 1 hour and 500% at 6 hours exposure) in the concentration of TH mRNA in carotid body type I cells but not in either the sympathetic ganglia or adrenal gland. In addition, we found that hypercapnia, another natural stimulus of carotid body activity, failed to enhance TH mRNA in type I cells. Removal of the sensory and sympathetic innervation of the carotid body failed to prevent the induction of TH mRNA by hypoxia in type I cells. Our results show that TH gene expression is regulated by hypoxia in the carotid body but not in other peripheral catecholamine synthesizing tissue and that the regulatory mechanism is intrinsic to type I cells.
A complete understanding of the neural mechanisms responsible for the chemoreceptor and baroreceptor reflexes requires precise knowledge of the locations and chemical phenotypes of higher-order neurons within these reflex pathways. In the present study, the protein product (Fos) of the c-fos protooncogene was used as a metabolic marker to trace central neural pathways following activation of carotid sinus nerve afferent fibers. In addition, immunohistochemical double-labeling techniques were used to define the chemical phenotypes of activated neurons. Both electrical stimulation of the carotid sinus nerve and physiological stimulation of the carotid bodies by hypoxia induced Fos-like immunoreactivity in catecholaminergic neurons containing tyrosine hydroxylase or phenylethanolamine-N-methyltransferase in the ventrolateral medulla oblongata and, to a lesser degree, in the dorsal vagal complex. Tyrosine hydroxylase/Fos colocalization was also observed in the locus coeruleus and the A5 noradrenergic cell group in pons. Many serotoninergic neurons in nucleus raphe pallidus, nucleus raphe magnus, and along the ventral medullary surface contained Fos-like immunoreactivity. In pons and midbrain, Fos-like immunoreactivity was observed in the lateral parabrachial and Kölliker-Fuse nuclei, the inferior colliculus, the cuneiform nucleus, and in the vicinity of the Edinger-Westphal nucleus, but no catecholaminergic or serotoninergic colocalization was observed in these regions. Although Fos-labeled cells were observed within and lateral to the dorsal raphe nucleus, few were catecholaminergic or serotoninergic. This study further defines a potential central neuroanatomical substrate for the chemoreceptor and/or baroreceptor reflexes.
