guanosine-triphosphate and Testicular-Neoplasms

In most experiments done in cell-free systems, the LH/CG-induced desensitization of the ovarian LH/CG-responsive adenylyl cyclase has been reported to be dependent on GTP. Little is known, however, about the molecular basis of this phenomenon or about the FSH-induced desensitization of the FSH-responsive adenylyl cyclase. We report here that, contrary to most previous findings, ATP is required for desensitization of the LH/CG- and FSH-responsive adenylyl cyclase in human kidney cells stably transfected with the complementary DNAs for the rat LH/CG or FSH receptor. This requirement does not seem to be peculiar to transfected cells because under our experimental conditions ATP is also preferred over GTP for the human CG-induced desensitization of the LH/CG-responsive adenylyl cyclase in highly purified plasma membranes from MA-10 Leydig tumor cells. Maximal desensitization of both FSH- and LH/CG-sensitive adenylyl cyclase in membranes from the transfected cells was achieved with millimollar concentrations of Mg2+ and ATP and did not appear to correlate with activation of the enzyme. In both of these systems, GTP, uridine triphosphate, and cytidine triphosphate were not able to substitute for ATP. In MA-10 membranes, however, there was some desensitization even without added nucleotide triphosphates, and ATP was more potent than GTP. Last, desensitization of the gonadotropin-sensitive adenylyl cyclase could not be explained by a decrease in the functional activities of stimulatory guanine nucleotide binding protein or of the catalytic moiety of the enzyme. A change in the functional properties of the gonadotropin receptors appears to be the most likely mechanism for desensitization.

Topics: Adenosine Triphosphate; Adenylyl Cyclases; Cell Line; Cell Membrane; Chorionic Gonadotropin; Follicle Stimulating Hormone; Guanosine Triphosphate; Humans; Kidney; Kinetics; Leydig Cell Tumor; Luteinizing Hormone; Magnesium Chloride; Male; Receptors, LH; Recombinant Proteins; Ribonucleotides; Testicular Neoplasms; Transfection; Tumor Cells, Cultured

Cell free desensitization of a tumour Leydig cell plasma membrane adenylate cyclase has been demonstrated in the presence of guanine nucleotides. In experiments in which the membranes were pre-incubated with various nucleotides and LH, it was shown that this decreased adenylate cyclase activity was dependent on the presence of GTP and occurred both in the presence and absence of ATP. While pre-treatment with LH alone appeared to enhance subsequent adenylate cyclase activity, this hormone was able to potentiate the desensitizing effect of GTP. The desensitizing effect of GTP was not inhibited by sodium fluoride. In contrast, the GTP analogue p(NH)ppG (guanosine 5'beta, gamma-imido triphosphate) caused a persistent activation of the adenylate cyclase. GMP and guanosine also initially inhibited the adenylate cyclase activity, but this was entirely reversed by p(NH)ppG plus LH. GDP in addition to GTP caused desensitization but this was only partially reversed by p(NH)ppG plus LH. It is proposed that in similarity with the ovary (Bockaert et al. 1976; Ezra & Salomon 1982a) desensitization of Leydig tumour cell plasma membrane adenylate cyclase may involve a GTP-mediated phosphorylation step.

Topics: Adenylyl Cyclases; Animals; Cell-Free System; Cyclic AMP; Guanosine Triphosphate; Guanylyl Imidodiphosphate; Leydig Cell Tumor; Luteinizing Hormone; Male; Sodium Fluoride; Testicular Neoplasms

Topics: Adenosine Triphosphate; Animals; Cytosine Nucleotides; Diethylstilbestrol; DNA; DNA-Directed RNA Polymerases; Guanosine Triphosphate; Liver; Male; Mice; Mice, Inbred BALB C; Neoplasms, Experimental; Nucleic Acid Hybridization; RNA; Testicular Neoplasms; Testis; Tritium; Uracil Nucleotides; Uridine