Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:2.4.2.8 (hypoxanthine-guanine phosphoribosyltransferase)
 2,527 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

HeLA H23 cells are a mutant female human tumor cell line harboring defective hypoxanthine phosphoribosyltransferase (HPRT; IMP-pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) as a result of a mutation that alters the isoelectric point of the enzyme (G. Milman, E. Lee, G. S. Changas, J. R. McLaughlin, and J. George, Jr., Proc. Natl. Acad. Sci. USA 73:4589-4592, 1976). As shown by Milman et al. and confirmed by us here, rare HAT+ revertants arise spontaneously at 1.9 X 10(-8) frequency and express both mutant and wild-type polypeptides. Thus, the H23 mutant also carries a silent wild-type HPRT allele that is activated in revertants. To test whether the silent allele was activated via hypomethylation of genomic DNA, H23 cells were treated with inhibitors of DNA methylation, and revertants were scored by HAT or azaserine selection. At an optimal dose of 5 microM 5-azacytidine, the reversion frequency was increased about 50-fold when assayed by HAT selection and over 1,000-fold when assayed by azaserine selection. HAT+ and azaserine revertants were heterozygous for HPRT, expressing both wild-type and mutant HPRT polypeptides. Like spontaneous revertants, they contained active HPRT enzyme and were genetically unstable, reverting at about 10(-4) frequency. Similar results were found after treatment with N-methyl-N'-nitro-N-nitrosoguanidine, a DNA-alkylating agent and potent inhibitor of mammalian DNA methylation. By contrast, the DNA-ethylating agent, ethyl methanesulfonate (EMS), did not increase the HAT+ reversion frequency; it did, however, increase the frequency by which H23 revertants heterozygous for HPRT reverted to 6-thioguanine resistance. Of nine EMS revertants, seven lacked HPRT activity and had a substantially reduced expression of the wild-type polypeptide. These observations support the hypothesis that DNA methylation plays an important role in human X-chromosome inactivation and that EMS can inactivate gene expression by promoting enzymatic methylation of genomic DNA as found previously for the prolactin gene in GH3 rat pituitary tumor cells (R. D. Ivarie and J. A. Morris, Proc. Natl. Acad. Sci. USA 79:2967-2970, 1982; R. D. Ivarie, J. A. Morris, and J. A. Martial, Mol. Cell. Biol. 2:179-189, 1982).
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PMID:Activation of a nonexpressed hypoxanthine phosphoribosyltransferase allele in mutant H23 HeLa cells by agents that inhibit DNA methylation. 243 Dec 68

The clonal rat pituitary tumor cell line GH4C1 secretes PRL but does not respond to dopamine, a physiological inhibitor of PRL. In an attempt to generate a dopamine-responsive cell line, GH4C1 cells, which lack the enzyme hypoxanthine-guanine phosphoribosyltransferase, were fused with cells from the pituitary glands of lactating rats to generate cell hybrids. The GH4C1 cells were fused with dispersed normal pituitary cells by either chemical fusion in 40% polyethylene glycol or electrofusion. The fused cells were grown in medium with hypoxanthine, aminopterin, and thymidine (HAT) for 4 weeks to select for hybrid cells. Control fusions between GH4C1 cells only or normal cells only did not produce viable colonies. Of 36 HAT-selected colonies, 3 responded to 10 nM bromocryptine (BCR) with inhibition of TRH-stimulated PRL release. These hybrid colonies had an inhibitory response similar to that of normal pituitary cells in culture. Both TRH- and vasoactive intestinal peptide-stimulated PRL release were inhibited to basal levels by 10 nM BCR, with an IC50 for BCR of approximately 0.25 nM. Basal hormone release was not inhibited by BCR. The BCR-sensitive hybrid cells grew more slowly than the parental GH4C1 line both in culture and when passaged in female Wistar-Furth rats. The response of the hybrid cells to the dopamine agonist and the characteristic of slow growth were lost after 9 months of continuous culture and after freezing cells. The parental GH4C1 cells were grown in female Wistar-Furth rats, the resulting tumors were dissociated, and the cells were grown in culture. This resulted in a brief establishment of the dopamine response. Stimulated PRL and GH release from freshly dispersed GH4C1 tumor cells was inhibited by BCR at concentrations from 0.1-10 nM, and spiroperidol reversed the inhibition. The inhibitory response to the dopaminergic agonist was lost quickly as the cells were carried in culture. These results demonstrate that GH4C1 cells may have the genetic information necessary for dopaminergic inhibition of PRL synthesis, but that the dopamine response is not observed under standard tissue culture conditions.
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PMID:Transient dopaminergic inhibition of prolactin release from hybrid cells derived by fusion of normal rat pituitary and GH4C1 tumor cells. 312 81