Activation of c-Jun N-terminal kinase 1 (JNK-1) after amino acid deficiency in HeLa cells

Auto-activation of c-JUN gene by amino acid deprivation of hepatocellular carcinoma cells reveals a novel c-JUN-mediated signaling pathway

Mammalian cells respond to protein or amino acid (AA) limitation by activating a number of signaling pathways, collectively referred to as the AA response (AAR), that modulate a range of cellular functions, including transcriptional induction of target genes. This study demonstrates that in hepatocellular carcinoma cells, expression of c-JUN, JUN-B, c-FOS, and FOS-B was induced by the AAR, whereas JUN-D, FRA-1, and FRA-2 were not. Of the four activated FOS/JUN members, c-JUN made the largest contribution to the induction of several known AAR target genes. For several human liver, prostate, and ovarian cell lines, the AAR-induced increase in c-JUN expression was greater in transformed cells compared with nontransformed counterparts, an effect independent of cell growth rate. Thus far, the best characterized AA-responsive genes are all transcriptionally activated by ATF4, but the AAR-dependent induction of c-JUN transcription was ATF4-independent. The increased expression of c-JUN was dependent on ATF2 and on activation of the MEK-ERK and JNK arms of the MAPK signaling pathways. Formation of c-JUN-ATF2-activated heterodimers was increased after AA limitation, and c-JUN or ATF2 knockdown suppressed the induction of c-JUN and other AAR target genes. AA deprivation triggers a feed-forward process that involves phosphorylation of existing c-JUN protein by JNK and subsequent auto-activation of the c-JUN gene by recruitment of c-JUN and ATF2 to two AP-1 sites within the proximal promoter. The results document the novel observation that AP-1 sequences within the c-JUN gene can function as transcriptional amino acid-response elements.

Negative effects of the amino acids Lys, His, and Thr on S6K1 phosphorylation in mammary epithelial cells

The role of essential amino acids (AA) on protein synthesis via the mTOR pathway was studied in murine mammary epithelial cells cultured under lactogenic conditions. Leu, Ile, and Val increased S6K1 phosphorylation compared to that measured in AA-deprived cells. Trp, Phe, and Met had no effect. Surprisingly, Lys, His, and Thr inhibited S6K1 phosphorylation in both murine and bovine mammary cells. Thr exhibited the most potent inhibition, being the only amino acid that competed with Leu's positive role. In non-deprived cells, there was no observable effect of Lys, His, or Thr on S6K1 phosphorylation at concentrations up to five times those in the medium. However, their addition as a mix revealed a synergistic negative effect. Supplementation of Lys, His, and Thr abrogated mTOR Ser 2448 phosphorylation, with no effect on Akt Ser 473-an mTORC2 target. This confirms specific mTORC1 regulation of S6K1 phosphorylation. The individual supplementation of Lys, His, and Thr maintained a low level of IRS-1 phosphorylation, which was dose-dependently increased by their combined addition. Thus, in parallel to inhibiting S6K1 activity, these AA may act synergistically to activate an additional kinase, phosphorylating IRS-1 via an S6K1-independent pathway. In cultures supplemented by Lys, His, and Thr, cellular protein synthesis decreased by up to 65%. A more pronounced effect was observed on beta-casein synthesis. These findings indicate that positive and negative signaling from AA to the mTOR pathway, combined with modulation of insulin sensitization, mediate the synthesis rates of total and specific milk proteins in mammary epithelial cells.

ATF2 is required for amino acid-regulated transcription by orchestrating specific histone acetylation

The transcriptional activation of CHOP (a CCAAT/enhancer-binding protein-related gene) by amino acid deprivation involves the activating transcription factor 2 (ATF2) and the activating transcription factor 4 (ATF4) binding the amino acid response element (AARE) within the promoter. Using a chromatin immunoprecipitation approach, we report that in vivo binding of phospho-ATF2 and ATF4 to CHOP AARE are associated with acetylation of histones H4 and H2B in response to amino acid starvation. A time course analysis reveals that ATF2 phosphorylation precedes histone acetylation, ATF4 binding and the increase in CHOP mRNA. We also show that ATF4 binding and histone acetylation are two independent events that are required for the CHOP induction upon amino acid starvation. Using ATF2-deficient mouse embryonic fibroblasts, we demonstrate that ATF2 is essential in the acetylation of histone H4 and H2B in vivo. The role of ATF2 on histone H4 acetylation is dependent on its binding to the AARE and can be extended to other amino acid regulated genes. Thus, ATF2 is involved in promoting the modification of the chromatin structure to enhance the transcription of a number of amino acid-regulated genes.

Osmotic stress, a proinflammatory signal in Caco-2 cells

Hyper- (450 mOsm/l) and hypoosmotic exposure (150 mOsm/l) of Caco-2 cells, a human intestinal epithelial cell line, induced a twofold- and a fivefold increase in the production of IL-8, a constitutively expressed cytokine, respectively. This was observed both in the presence or in the absence of added proinflammatory cytokines and the stimulatory effect of osmotic stress was additive to that induced by the cytokines. Thus, IL-8 production appeared minimal around isoosmolarity, i.e. 300 mOsm/l. Concerning the signalling pathway involved, specific inhibition of p38- or p42/44 MAP kinases decreased the IL-8 production by about 30% independently of the osmotic condition used. Inhibition of c-jun-NH2-terminal kinase (JNK) by using both dicoumarol and SP600125 totally inhibited the stimulatory effect of hypoosmolarity. Moreover, hypoosmolarity induced an about threefold increase in JNK activity demonstrating that JNK was specifically involved in the effect of hypoosmolarity on IL-8 production. This is not the case for hyperosmolarity. Such an effect of osmotic stress was not restricted to IL-8, but was also observed on the production of IL-6, a non-constitutively expressed cytokine. Again, IL-6 production appeared minimal in isoosmotic condition. Taken together, these results demonstrate that osmotic stress is a proinflammatory signal in Caco-2 cells and suggest that an osmosensor might specifically exist in intestinal epithelial cells.

Induction of CHOP expression by amino acid limitation requires both ATF4 expression and ATF2 phosphorylation

The CHOP gene is transcriptionally induced by amino acid starvation. We have previously identified a genomic cis-acting element (amino acid response element (AARE)) involved in the transcriptional activation of the human CHOP gene by leucine starvation and shown that it binds the activating transcription factor 2 (ATF2). The present study was designed to identify other transcription factors capable of binding to the CHOP AARE and to establish their role with regard to induction of the gene by amino acid deprivation. Electrophoretic mobility shift assay and transient transfection experiments show that several transcription factors that belong to the C/EBP or ATF families bind the AARE sequence and activate transcription. Among all these transcription factors, only ATF4 and ATF2 are involved in the amino acid control of CHOP expression. We show that inhibition of ATF2 or ATF4 expression impairs the transcriptional activation of CHOP by amino acid starvation. The transacting capacity of ATF4 depends on its expression level and that of ATF2 on its phosphorylation state. In response to leucine starvation, ATF4 expression and ATF2 phosphorylation are increased. However, induction of ATF4 expression by the endoplasmic reticulum stress pathway does not fully activate the AARE-dependent transcription. Taken together our results demonstrate that at least two pathways, one leading to ATF4 induction and one leading to ATF2 phosphorylation, are necessary to induce CHOP expression by amino acid starvation. This work was extended to the regulation of other amino acid regulated genes and suggests that ATF4 and ATF2 are key components of the amino acid control of gene expression.

Expression of spermidine/spermine N1-acetyltransferase in HeLa cells is regulated by amino acid sufficiency

The effect of amino acids on the regulation of the expression of spermidine/spermine N(1)-acetyltransferase (SSAT), the key enzyme of polyamine catabolism, was studied in HeLa cells. When compared with similar exposure to complete medium, deprivation of arginine, methionine or leucine gave rise to a time-dependent, slowly reversible increase in the cellular level of SSAT mRNA that started to be significant after 8, 12 or 16h and reached four-, five- and two-fold after 16h, respectively. Experiments utilizing (i) constructs containing fragments of the SSAT promoter linked to a luciferase reporter gene or (ii) actinomycin D (Act-D)-treated cells indicated that the increase in the SSAT mRNA level was due to an augmentation in gene transcription and message stability after omission of one of the polyamine precursor amino acids. By contrast, SSAT mRNA stabilisation was only observed when leucine was the omitted amino acid. Amino acid deprivation was also found to cause increased intracellular activity of SSAT concurrent with changes in the cell polyamine content, namely increased putrescine but decreased spermine levels. Furthermore, stable expression of a dominant negative mutant of stress-activated protein kinase/extracellular signal-regulated protein kinase (SAPK/ERK) kinase 1 in HeLa cells was found to inhibit the increase in SSAT mRNA by amino acid deprivation. The data suggest that c-Jun N-terminal kinase/SAPK (JNK/SAPK) may be involved in the amino acid-dependent regulation of SSAT expression.

Dietary protein modifies hepatic gene expression associated with oxidative stress responsiveness in growing pigs

Understanding the basis for differences in nutrient requirements and for nutrient effects on health and performance requires an appreciation of the links between nutrition and gene expression. We developed and applied molecular probes to characterize diet-associated postabsorptive hepatic gene expression in growing pigs chronically fed protein-restricted diets based on either casein (CAS) or soy protein isolate (SPI). Eighty-eight expressed sequence tags (ESTs) were identified on the basis of diet-related changes in expression, by using an mRNA differential display method. Expression profiling based on transcription analysis by real-time reverse transcriptase-polymerase chain reaction showed that the SPI diet significantly changed the pattern of gene expression as compared with the CAS diet and allowed identification of coregulated genes. The expression of six genes involved in the metabolism of stress response (glutathione S-transferase, peptide methionine sulfoxide reductase, apolipoprotein A-I, organic anion transport polypeptide 2, calnexin, heat shock transcription factor 1) exhibited significant changes in the transcription level and indicated an increased oxidative stress response in pigs fed the SPI diet. Hierarchical clustering of gene expression data of all 33 ESTs analyzed across 14 pigs fed the two different diets resulted in clustering of genes related to the oxidative stress response with genes related to the regulation of gene expression and neuronal signaling.