Enhancer sequences are regulatory regions that greatly increase transcription of certain eukaryotic genes. An immunoglobulin heavy-chain variable gene segment is moved from a region lacking enhancer activity to a position adjacent to the known heavy-chain enhancer early in B-cell maturation. In lymphoid cells, the heavy-chain and SV40 enhancers bind a common factor essential for enhancer function. In contrast, fibroblast cells contain a functionally distinct factor that is used by the SV40 but not by the heavy-chain enhancer. The existence of different factors in these cells may explain the previously described lymphoid cell specificity of the heavy-chain enhancer.
Tumor necrosis factor-alpha (TNF; cachectin), a peptide secreted from stimulated macrophages, mediates some of the metabolic derangements in inflammatory and neoplastic disorders. To determine whether TNF is responsible for the changes in hypothalamic-pituitary-thyroid (HPT) function in nonthyroid illnesses, we administered synthetic human TNF to male Sprague-Dawley rats. The rats were given TNF or saline (control; both pair fed and nonpair fed) iv (six to eight per group). HPT function was tested 8 h after administration of 200 micrograms TNF/kg BW, 8 h after 5 days of 150 micrograms TNF/kg BW, and 8 h after a 3-day series of 50, 200, and 800 micrograms TNF/kg BW. The single injection of 200 micrograms TNF/kg significantly reduced (all P less than 0.05) serum TSH, T4, free T4, T3, and hypothalamic TRH compared to the corresponding hormone levels in saline-injected control rats. Serum TSH and hypothalamic TRH recovered to normal levels after 5 days of 150 micrograms/kg TNF treatment. With the increasing daily doses of TNF, serum TSH and hypothalamic TRH fell significantly. Hepatic 5'-deiodinase activity was reduced after 1 day of TNF treatment, but increased after the 3-day series of injections. TNF treatment reduced pituitary TSH beta mRNA, but did not affect alpha-subunit mRNA. TNF treatment also reduced thyroid 125I uptake and reduced thyroidal release of T4 and T3 in response to bovine TSH, but did not change the TSH response to TRH. TNF treatment reduced the binding of pituitary TSH to Concanavalin-A, indicating that it alters the glycosylation of TSH. The TSH with reduced affinity for this lectin had reduced biological activity when tested in cultured FRTL-5 rat thyroid cells. In vitro, TNF inhibited 125I uptake by cultured FRTL-5 rat thyroid cells and blocked the stimulation of [3H]thymidine uptake by these cells. The data indicate that TNF acts on the HPT axis at multiple levels and suggest that TNF is one of the mediators responsible for alterations in thyroid function tests in patients with nonthyroidal illnesses.
hCG stimulates thyroid function, but it has been suggested that it is impurities in commercial hCG preparations or a variant of hCG that are responsible for the thyrotropic activity. In this study, we tested the thyrotropic activity of purified and commercial hCG and compared its action with that of bovine TSH (bTSH) in cultured rat FRTL-5 cells in regard to stimulation of iodide uptake, activation of adenylate cyclase, and synthesis of DNA. Iodide uptake was measured after incubation of the cells for 48-72 h with the test hormones, followed by a 40-min incubation with 0.1 microCi Na125I and 10 mumol/L carrier NaI; the 125I in the washed cells was counted. Adenylate cyclase was measured after incubation of the cells with the test stimulators for 3 h in hypotonic medium by RIA of cAMP in the medium. DNA synthesis was measured after incubation of the cells with the test substances for 24 h, followed by addition of [3H]thymidine for 3 h and then measuring the incorporation of [3H]thymidine into the cells. Both purified and commercial hCG produced a dose-related increase in iodide uptake. The relative potency of commercial hCG was 0.024 microU bTSH/U hCG and that of purified hCG was 0.042 microU bTSH/U hCG; compared with human TSH, the potency of purified hCG was 0.72 microU/U hCG. hCG caused a dose-related increment of adenylate cyclase and [3H]thymidine incorporation. The effect of hCG on iodide uptake and [3H]thymidine incorporation was additive with that of bTSH; hCG was not an antagonist of TSH in these cultured rat thyroid cells. We conclude that hCG has intrinsic thyrotropic activity in FRTL-5 cells in regard to stimulation of iodide uptake, activation of adenylate cyclase, and stimulation of DNA synthesis.
Chromatin from chicken reticulocytes and mouse Ehrlich ascites tumor cells has been extracted with 2 M NaCl, leaving a portion of the DNA still complexed with a fraction of nonhistones (designated M3, since it can be dissociated from DNA in solutions of 3 M NaCl containing 5 M urea). The DNA complexed with M3, separated from the bulk DNA by centrifugation, was found to contain sequences poorly represented in bulk DNA. Specifically we found that DNA--M3 complexes isolated from chicken reticulocyte chromatin were enriched in globin gene sequences by 20-fold relative to unfractionated DNA and by over 1000-fold relative to DNA rendered free of protein following the extraction of chromatin with 2 M NaCl. We have therefore isolated DNA fractions complexed with M3 which are enriched in specific sequences as may be determined by M3.
A B ST RA C T We have used hamster insulinoma tumor (HIT) cells, an insulin-secreting tumor cell line, to investigate modulation of the Na/K-ATPase and of the ATP-sensitive K channel (KATe) by the sulfonylurea glyburide. Membrane proteins from cells cultured in RPMI with 11 mM glucose have at least two glyburide receptor populations, as evidenced by high and low binding affinity constants, (Kd = 0.96 and 91 nM, respectively). In these cells KATP channel activity was blocked by low glyburide concentrations, IC50 = 5.4 nM. At 12.5 nM glyburide the inhibition developed slowly, "r = 380 s, and caused reduction of channel activity by 75%. At higher concentrations, however, inhibition occurred at a fast rate, -r = 42 s at 100 nM, and was almost complete. Na/K-ATPase activity measured enzymatically and electrophysiologically was also suppressed by glyburide, but higher concentrations were needed, ICs0 = 20-40 nM. Inhibition occurred rapidly, T = 30 s at 50 nM, when maximum, activity was reduced by 40%. By contrast, cells cultured in RPMI supplemented with 25 mM glucose exhibit a single receptor population binding glyburide with low affinity, Kd = 68 nM. In these cells inhibition of the Na/K-ATPase by the sulfonylurea was similar to that observed in cells cultured in 11 mM glucose, but KAT P channel inhibition was markedly altered. Inhibition occurred only at high concentrations of glyburide and at a fast rate; maximum inhibition was observed at ~100 nM. Based on these data, we propose that glyburide binding to the high affinity site affects primarily KAT P channel activity, while interaction with the low affinity site inhibits both Na/K-ATPase and KAT P channel activities. The latter observation suggests possible functional interactions between the Na/K-ATPase and the KATP channel.
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