Auto-activation of c-JUN Gene by Amino Acid Deprivation of Hepatocellular Carcinoma Cells Reveals a Novel c-JUN-mediated Signaling Pathway

Abstract

Limitation of dietary protein/amino acid (AA)2 availability negatively impacts general health in mammals, particularly in pregnancy (1) and early development (2), and can be a contributing factor in the progression of diseases associated with cachexia, such as cancer (3). Conversely, dietary protein limitation is beneficial to some patients with renal disease (4) and a diet deficient in a single essential AA can lead to life span extension (5, 6). Mammalian cells monitor AA levels and respond to AA deprivation by altering the expression of a wide variety of genes (7). The mechanisms by which these events occur are not fully understood, but protein/AA deprivation activates an AA response (AAR) (8, 9), which is a collection of signal transduction pathways, the best studied of which is the general control nonderepressible 2 (GCN2) kinase pathway. The kinase activity of GCN2 is activated by binding uncharged tRNA when there is an increase abundance of any one of the uncharged tRNA molecules. The GCN2 kinase phosphorylates the translation initiation factor eIF2␣ on serine 51, which leads to suppression of general protein synthesis but promotes a paradoxical increase in translation of selected mRNA species, including activating transcription factor 4 (ATF4) (10, 11). ATF4 triggers increased transcription by binding to CCAAT enhancer-binding proteinactivating transcription factor (C/EBP-ATF) sequences that function as AA-response elements (AARE) (8, 9). AA depletion also activates Raf-1 through dephosphorylation by protein phosphatase 2A (12, 13). The enhanced Raf-1 kinase activity triggers the MEK/ERK signaling cascade, which in turn stimulates autophagy in an AA-dependent manner via a G␣-interacting protein (G␣13) (13,14).Similarly, an AA-dependent signaling pathway has been described that involves activation of a G-protein-coupled receptor (G␣12) followed by Rac1-MEKK1-MKK7-JNK2 signaling and ultimately terminates in ATF2 phosphorylation by JNK2 (15). JNK3 is primarily expressed in brain, but both JNK1 and JNK2 are ubiquitous and exhibit broad substrate specificity, as reviewed by Bode and Dong (16). Because knock-out mice for each are viable and show no major deficiencies, they are often considered to have overlapping specificity, but the validity of that assumption remains open for debate. A principal substrate for the JNKs is c-Jun. JNK2 appears to bind to c-Jun with greater affinity and to target it for degradation, whereas JNK1 may be more effective in phosphorylating c-Jun leading to activation and stabilization (17). It has been proposed that the opposing effects of the JNK1 and JNK2 on the cell cycle and cell proliferation may be the consequence of opposite actions on c-Jun (18). However, Jaeschke et al. (19) have presented evidence that JNK1 and JNK2 are both positive regulators of c-Jun. It is clear that the precise functions of the JNKs will require * This work was supported, in whole or in part, by National Institutes of Health Grant DK-092062 (to M. S. K.

Elevated cJUN expression and an ATF/CRE site within the ATF3 promoter contribute to activation of ATF3 transcription by the amino acid response

“…The PCR primers used for ATF3 were as follows: for steady-state mRNA, forward 5=-GAAGAGCTGAGGTTTGCCATCCA-3=, reverse 5=-TGATTCCA-GCGCAGAGGACATC-3=; for transcription activity, forward 5=-CCCTCGGGGTGTCCATCACAAA-3=, reverse 5=-TGGCTGCGA-GCGAAACATG-3=. The primers for steady-state cJUN and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA were described previously (10).…”

“…Accumulation of ATF4 protein activates a broad spectrum of downstream target genes (17), including ATF3 (22,28). In addition to the GCN2/eIF2/ATF4 pathway, mitogenactivated protein kinase (MAPK) pathways are activated by the AAR, which leads to phosphorylation/activation of critical transcription factors such as ATF2 (7) and cJUN (10).…”

“…The "AP-1" family of transcriptional regulators includes complexes composed of homo-or heterodimers of cJUN, JUN-B, JUN-D, cFOS, FOS-B, Fra1, and Fra2 (31). Some of the JUN/FOS proteins are induced during the AAR (30), and the increase in cJUN expression occurs by a process that is independent of ATF4 signaling (10). ATF/CRE elements have been identified within the ATF3 gene (16,37), and in particular, an ATF/CRE site (nt Ϫ93/Ϫ85) exists upstream of the ATF3 CARE site (nt Ϫ23/Ϫ15).…”

Inhibition of GTPase Rac1 in Endothelium by 6-Mercaptopurine Results in Immunosuppression in Nonimmune Cells: New Target for an Old Drug

“…2A). Downstream of activated JNK are the transcription factors c-Jun (24)(25)(26)(27) and ATF2 (also known as cAMP-dependent transcription factor) (24,27,28).…”

The potential involvement of the cofactor of BRCA1 in hepatocellular carcinoma pathogenesis

“…This temporary overexpression is needed for priming hepatocytes for migration and tissue invasion [15] . In contrast, the c-Jun protein is involved in tumor cell survival and apoptosis [16] . A mechanism, by which c-Jun modulates tumorigenesis, is by suppressing the famous cell death regulator p53.…”