A mitogen-responsive promoter region that is synergistically activated through multiple signalling pathways

The emerging roles of MAPK-AMPK in ferroptosis regulatory network

Ferroptosis, a newform of programmed cell death, driven by peroxidative damages of polyunsaturated-fatty-acid-containing phospholipids in cellular membranes and is extremely dependent on iron ions, which is differs characteristics from traditional cell death has attracted greater attention. Based on the curiosity of this new form of regulated cell death, there has a tremendous progress in the field of mechanistic understanding of ferroptosis recent years. Ferroptosis is closely associated with the development of many diseases and involved in many diseases related signaling pathways. Not only a variety of oncoproteins and tumor suppressors can regulate ferroptosis, but multiple oncogenic signaling pathways can also have a regulatory effect on ferroptosis. Ferroptosis results in the accumulation of large amounts of lipid peroxides thus involving the onset of oxidative stress and energy stress responses. The MAPK pathway plays a critical role in oxidative stress and AMPK acts as a sensor of cellular energy and is involved in the regulation of the energy stress response. Moreover, activation of AMPK can induce the occurrence of autophagy-dependent ferroptosis and p53-activated ferroptosis. In recent years, there have been new advances in the study of molecular mechanisms related to the regulation of ferroptosis by both pathways. In this review, we will summarize the molecular mechanisms by which the MAPK-AMPK signaling pathway regulates ferroptosis. Meanwhile, we sorted out the mysterious relationship between MAPK and AMPK, described the crosstalk among ferroptosis and MAPK-AMPK signaling pathways, and summarized the relevant ferroptosis inducers targeting this regulatory network. This will provide a new field for future research on ferroptosis mechanisms and provide a new vision for cancer treatment strategies. Video Abstract.

PGC-1alpha is coupled to HIF-1alpha-dependent gene expression by increasing mitochondrial oxygen consumption in skeletal muscle cells

Mitochondrial biogenesis occurs in response to increased cellular ATP demand. The mitochondrial electron transport chain requires molecular oxygen to produce ATP. Thus, increased ATP generation after mitochondrial biogenesis results in increased oxygen demand that must be matched by a corresponding increase in oxygen supply. We found that overexpression of peroxisome proliferator-activated receptor-gamma coactivator 1alpha (PGC-1alpha), which increases mitochondrial biogenesis in primary skeletal muscle cells, leads to increased expression of a cohort of genes known to be regulated by the dimeric hypoxia-inducible factor (HIF), a master regulator of the adaptive response to hypoxia. PGC-1alpha-dependent induction of HIF target genes under physiologic oxygen concentrations is not through transcriptional coactivation of HIF or up-regulation of HIF-1alpha mRNA but through HIF-1alpha protein stabilization. It occurs because of intracellular hypoxia as a result of increased oxygen consumption after mitochondrial biogenesis. Thus, we propose that at physiologic oxygen concentrations, PGC-1alpha is coupled to HIF signaling through the regulation of intracellular oxygen availability, allowing cells and tissues to match increased oxygen demand after mitochondrial biogenesis with increased oxygen supply.

Synthetic lethal screening identifies compounds activating iron-dependent, nonapoptotic cell death in oncogenic-RAS-harboring cancer cells

We screened small molecules to identify two compounds, which we named RSL3 and RSL5, that have increased lethality in the presence of oncogenic RAS. Counter screening with biologically active compounds defined aspects of the mechanism of action for RSL3 and RSL5, such as a nonapoptotic, MEK-dependent, and iron-dependent oxidative cell death. Erastin, a previously reported compound with RAS-selective lethality, showed similar properties. RNA interference experiments targeting voltage-dependent anion channel 3 (VDAC3), a target of erastin, demonstrated that RSL5 is a scaffold that acts through VDACs to activate the observed pathway. RSL3 activated a similar death mechanism but in a VDAC-independent manner. We found that cells transformed with oncogenic RAS have increased iron content relative to their normal cell counterparts through upregulation of transferrin receptor 1 and downregulation of ferritin heavy chain 1 and ferritin light chain.

Liver iron transport

The liver plays a central role in iron metabolism. It is the major storage site for iron and also expresses a complex range of molecules which are involved in iron transport and regulation of iron homeostasis. An increasing number of genes associated with hepatic iron transport or regulation have been identified. These include transferrin receptors (TFR1 and 2), a ferrireductase (STEAP3), the transporters divalent metal transporter-1 (DMT1) and ferroportin (FPN) as well as the haemochromatosis protein, HFE and haemojuvelin (HJV), which are signalling molecules. Many of these genes also participate in iron regulatory pathways which focus on the hepatic peptide hepcidin. However, we are still only beginning to understand the complex interactions between liver iron transport and iron homeostasis. This review outlines our current knowledge of molecules of iron metabolism and their roles in iron transport and regulation of iron homeostasis.

Inhibition of transferrin receptor 1 transcription by a cell density response element

TfR1 (transferrin receptor 1) mediates the uptake of transferrin-bound iron and thereby plays a critical role in cellular iron metabolism. Its expression is coupled to cell proliferation/differentiation and controlled in response to iron levels and other signals by transcriptional and post-transcriptional mechanisms. It is well established that TfR1 levels decline when cultured cells reach a high density and in the present study we have investigated the underlying mechanisms. Consistent with previous findings, we demonstrate that TfR1 expression is attenuated in a cell-density-dependent manner in human lung cancer H1299 cells and in murine B6 fibroblasts as the result of a marked decrease in mRNA content. This response is not associated with alterations in the RNA-binding activity of iron regulatory proteins that are indicative of a transcriptional mechanism. Reporter assays reveal that the human TfR1 promoters contains sequences mediating cell-density-dependent transcriptional inhibition. Mapping of the human and mouse TfR1 promoters identified a conserved hexa-nucleotide 5'-GAGGGC-3' motif with notable sequence similarity to a previously described element within the IGF-2 (insulin-like growth factor-2) promoter. We show that this motif is necessary for the formation of specific complexes with nuclear extracts and for cell-density-dependent regulation in reporter gene assays. Thus the TfR1 promoter contains a functional 'cell density response element' (CDRE).

Expression of constitutively active 4EBP-1 enhances p27Kip1 expression and inhibits proliferation of MCF7 breast cancer cells

BACKGROUND: Eukaryotic initiation factor 4E (eIF4E) is essential for cap-dependent initiation of translation. Cell proliferation is associated with increased activity of eIF4E and elevated expression of eIF4E leads to tumorigenic transformation. Many tumors express very high levels of eIF4E and this may be a critical factor in progression of the disease. In contrast, overexpression of 4EBP, an inhibitor of eIF4E, leads to cell cycle arrest and phenotypic reversion of some transformed cells. RESULTS: A constitutively active form of 4EBP-1 was inducibly expressed in the human breast cancer cell line MCF7. Induction of constitutively active 4EBP-1 led to cell cycle arrest. This was not associated with a general inhibition of protein synthesis but rather with changes in specific cell cycle regulatory proteins. Cyclin D1 was downregulated while levels of the CDK inhibitor p27Kip1 were increased. The levels of cyclin E and CDK2 were unaffected but the activity of CDK2 was significantly reduced due to increased association with p27Kip1. The increase in p27Kip1 did not reflect changes in p27Kip1 mRNA or degradation rates. Rather, it was associated with enhanced synthesis of the protein, even though 4EBP-1 is expected to inhibit translation. This could be explained, at least in part, by the ability of the p27Kip1 5'-UTR to mediate cap-independent translation, which was also enhanced by expression of constitutively active 4EBP-1. CONCLUSIONS: Expression of active 4EBP-1 in MCF7 leads to cell cycle arrest which is associated with downregulation of cyclin D1 and upregulation of p27Kip1. Upregulation of p27Kip1reflects increased synthesis which corresponds to enhanced cap-independent translation through the 5'-UTR of the p27Kip1 mRNA.

Control of cyclin-dependent kinase inhibitor p27 expression by cap-independent translation

p27 is a key regulator of cell proliferation through inhibition of G(1) cyclin-dependent kinase (CDK) activity. Translation of the p27 mRNA is an important control mechanism for determining cellular levels of the inhibitor. Nearly all eukaryotic mRNAs are translated through a mechanism involving recognition of the 5' cap by eukaryotic initiation factor 4E (eIF4E). In quiescent cells eIF4E activity is repressed, leading to a global decline in translation rates. In contrast, p27 translation is highest during quiescence, suggesting that it escapes the general repression of translational initiation. We show that the 5' untranslated region (5'-UTR) of the p27 mRNA mediates cap-independent translation. This activity is unaffected by conditions in which eIF4E is inhibited. In D6P2T cells, elevated cyclic AMP levels cause a rapid withdrawal from the cell cycle that is correlated with a striking increase in p27. Under these same conditions, cap-independent translation from the p27 5'-UTR is enhanced. These results indicate that regulation of internal initiation of translation is an important determinant of p27 protein levels.

The cyclin-dependent kinase inhibitor p27Kip1 is localized to the cytosol in Swiss/3T3 cells

p27Kip1 plays an important role in cell cycle progression by negatively regulating the activity of cyclin-Cdk complexes. To understand how p27Kip1 functions, the level and subcellular location of p27Kip1 in Swiss/3T3 cells following serum stimulation of quiescent cells was examined. Surprisingly, p27Kip1 was observed exclusively in the cytosol throughout G1 and into early S phase. However, as expected, p27Kip1 in the cytosolic fraction was greatly reduced following serum stimulation and reached very low levels by late G1. The decline in the level of p27Kip1 corresponded in time to an increase in the nuclear level of both Cdk2 and cyclin E. In quiescent 3T3 cells Cdk2 was inactive and co-precipitated with p27Kip1. After serum stimulation, both nuclear and cytosolic Cdk2 was activated and this corresponded to the decline in p27Kip1. Overexpression of p27Kip1 allowed accumulation of the inhibitor in the nucleus but inhibited entry of Cdk2 into the nucleus following serum stimulation. The subcellular localization of p27Kip1 was also examined in a variety of other mammalian cells. In all the cell lines examined the preponderance of p27Kip1 was found in the cytosolic fraction. However, a substantial level of nuclear p27Kip1 was observed for several cell lines. In a primary mixed glial cell culture p27Kip1 was localized to the nucleus. The results suggest that cytosolic p27Kip1 has a functional role in regulating cell cycle progression, possibly through inhibiting transport of cyclin E-Cdk 2 complexes into the nucleus.

A DNA-binding activity, TRAC, specific for the TRA element of the transferrin receptor gene copurifies with the Ku autoantigen

We have previously described purification and characterization of a nuclear protein, TREF, which interacts specifically with the transcriptional control element, TRA, of the human transferrin receptor (TR) gene. In this report we show that TREF can be separated into two functionally distinct DNA-binding activities. The first DNA-binding activity (TRAC) is highly specific for the 8-bp element TRA and the related Escherichia coli cAMP receptor binding site. This motif is homologous to the phorbol 12-tetradecanoate 13-acetate- and cAMP-responsive elements of eukaryotic genes and the regulatory proximal sequence elements of the U1 small nuclear RNA gene and is also present in the promoter of the Drosophila melanogaster yolk protein factor 1 gene. In striking contrast, the second activity exhibits high affinity for the ends of double-stranded DNA in a sequence-unspecific manner and is attributable to the heterodimeric Ku autoantigen. Notably, transcription of Ku is induced during mid-late G0/G1 with kinetics similar to the TR gene. Ku is a highly abundant nuclear protein possessing nonspecific affinity for the ends of DNA, whose biological role remains to be elucidated. A transcriptional role for this protein has been proposed, however, on the basis of studies attributing DNA sequence-specific binding activity, notably for TRA-like sequences described above, directly to the Ku heterodimer. The observation that Ku-mediated nonspecific DNA-binding activity copurifies with the TRA-specific activity, TRAC, clearly has implications for these and related studies. The unusual properties of TRAC activity and its relationship, if any, with the enigmatic Ku protein, are discussed.