Variation in the relative synthesis of some proteins in mammalian cells exposed to hypertonic medium

Requirement for the eIF4E binding proteins for the synergistic down-regulation of protein synthesis by hypertonic conditions and mTOR inhibition

The protein kinase mammalian target of rapamycin (mTOR) regulates the phosphorylation and activity of several proteins that have the potential to control translation, including p70S6 kinase and the eIF4E binding proteins 4E-BP1 and 4E-BP2. In spite of this, in exponentially growing cells overall protein synthesis is often resistant to mTOR inhibitors. We report here that sensitivity of wild-type mouse embryonic fibroblasts (MEFs) to mTOR inhibitors can be greatly increased when the cells are subjected to the physiological stress imposed by hypertonic conditions. In contrast, protein synthesis in MEFs with a double knockout of 4E-BP1 and 4E-BP2 remains resistant to mTOR inhibitors under these conditions. Phosphorylation of p70S6 kinase and protein kinase B (Akt) is blocked by the mTOR inhibitor Ku0063794 equally well in both wild-type and 4E-BP knockout cells, under both normal and hypertonic conditions. The response of protein synthesis to hypertonic stress itself does not require the 4E-BPs. These data suggest that under certain stress conditions: (i) translation has a greater requirement for mTOR activity and (ii) there is an absolute requirement for the 4E-BPs for regulation by mTOR. Importantly, dephosphorylation of p70S6 kinase and Akt is not sufficient to affect protein synthesis acutely.

Effect of hyperosmolality on alkaline phosphatase and stress-response protein 27 of MCF-7 breast cancer cells

MCF-7, a continuous cell line derived from a human breast carcinoma, exhibits very low alkaline phosphatase (ALP) activity. The enzyme is heat-stable and is inhibited by L-phenylalanine and L-phenylalanylglycylglycine, but not by L-homoarginine, 1-bromotetramisole, or levamisole. These data indicate that MCF-7 produces term-placental ALP, the oncodevelopmental enzyme form inappropriately expressed by a variety of human tumors. In contrast to human cancer cells that produce this enzyme monophenotypically, ALP activity of MCF-7 cells is not significantly increased by glucocorticoids or sodium butyrate. By comparison, exposure to hyperosmolality causes a striking increase in enzyme activity. Cycloheximide blocks this effect. The results obtained with cell-free assays were confirmed by cytochemical and immunocytochemical assays on whole cells. Because some of the agents tested in the enzyme modulation experiments affect cell proliferation, their possible effect on two stress-response proteins (srp 27 and srp 72) was also examined; specific immunocytochemical assays were used. These tests revealed that neither protein is affected by glucocorticoids; that sodium butyrate has no effect on srp 27, but alters the intracellular distribution of srp 72; and that hyperosmolality, while not significantly affecting srp 72, causes an increase in srp 27.

Inhibitors of protein synthesis cause increased hexose transport in cultured human fibroblasts by a mechanism other than transporter translocation

We have investigated the effect of various inhibitors of protein synthesis on hexose transport in human skin fibroblasts using 2-deoxy-D-glucose (2-DG) and 3-0-methyl-D-glucose (3-OMG) to measure hexose transport. Exposure of glucose-fed, serum-free cultures to cycloheximide (CHX) (50 micrograms/ml) for 6 h resulted in increased 2-DG transport (3.81 +/- .53 vs. 6.62 +/- .88 nmoles/mg protein/2 min; n = 9) and 3-OMG transport (1.36 +/- .66 vs. 3.18 +/- .83 nmoles/mg protein/30 sec; n = 4) in the CHX exposed group. Under these conditions inhibition of protein synthesis was greater than 90%. This CHX induced transport increase was time dependent (approaching maximum within 1 h of exposure to CHX) and related to an increase in the Vmax of hexose transport in the CHX exposed group (18.4 +/- 2.4 vs. 4.8 +/- 1.1 nmoles 2-DG/mg protein/min) with no difference in the transport Km (1.55 +/- .63 vs. 2.92 +/- .59 mM). Further, the CHX induced increase in hexose transport was reversible. Exposure of human fibroblasts to inhibitors of protein synthesis with different mechanisms of action (e.g., puromycin, pactamycin, or CHX) all generated hexose transport increases in a concentration-dependent fashion correlating with their increasing inhibitory effects on protein synthesis. Nucleotidase enriched (i.e., plasma membrane) fractions of control and CHX-exposed cells showed no differences in D-glucose inhibitable cytochalasin B binding activity. Further, quantitative Western analysis of nucleotidase enriched fractions indicated CHX exposure resulted in no significant increase in glucose transporter mass compared with control plasma membrane fractions. Glucose deprived cells, however, which exhibited increased sugar transport comparable to the CHX-exposed group, did show increased glucose transporter mass in the plasma membrane fraction. The data indicate that inhibitors of protein synthesis can cause a significant elevation in hexose transport and that the hexose transporter mass in the isolated plasma membrane fractions did not reflect the whole cell transport change. It is suggested that a mechanism other than glucose transporter translocation to the plasma membrane may be involved in causing this sugar transport increase.

Modulation of alkaline phosphatases in LoVo, a human colon carcinoma cell line

LoVo, a continuous cell line derived from a human colon carcinoma produces two alkaline phosphatases: the heat-labile, L-homoarginine-insensitive, intestinal form, characteristic of its tissue of origin and the heat-stable, term-placental form, ectopically produced by a variety of tumors. Under basal conditions the activity levels of both enzymes are similar. Hyperosmolality and sodium butyrate induce increased levels of activity of the two alkaline phosphatases in a disparate fashion; whereas hyperosmolality augments the activity of both to the same extent, the effect of butyrate is more pronounced on the activity of the intestinal enzyme. When the two inducers are combined, induction of term-placental alkaline phosphatase is additive and that of the intestinal enzyme is synergistic. The effect of hyperosmolality is blocked by cycloheximide, and induction by sodium butyrate is inhibited by thymidine, cordycepin and cycloheximide. The known alkaline phosphatase inducer, prednisolone, has no effect on the enzymes of LoVo cells. Our results suggest that in these tumor cells the activity levels of the closely homologous term-placental and intestinal alkaline phosphatases appear to be independently controlled.

Inhibition of protein synthesis by diphtheria toxin induces a peculiar pattern of synthesized protein species

The irreversible inhibition of overall protein synthesis induced by diphtheria toxin in mammalian cells is accompanied by a peculiar pattern of synthesized protein species during the early stages of cell intoxication. This pattern closely resembles patterns of synthesized protein species observed in cells treated with reversible inhibitors of protein synthesis acting at different levels. Synthesized proteins are in the Mr range of 70-100 and 20-40 kDa. A 50-kDa protein, the most represented species, exhibits a peptide map identical to that of the 50-kDa species observed in cells treated with cycloheximide and hypertonic medium. The synthesis of these proteins is apparently regulated at both transcriptional and translational levels.