Cellular stress, induced by external or internal cues, activates several well-orchestrated processes aimed at either restoring cellular homeostasis or committing to cell death. Those processes include the unfolded protein response (UPR), autophagy, hypoxia, and mitochondrial function, which are part of the global ER stress (ERS) response. When one of the ERS elements is impaired, as often occurs under pathological conditions, overall cellular homeostasis may be perturbed. Further, activation of the UPR could trigger changes in mitochondrial function or autophagy, which could modulate the UPR, exemplifying cross-talk processes. Among the numerous factors that control the magnitude or duration of these processes are ubiquitin ligases, which govern overall cellular stress outcomes. Here we summarize crosstalk among fundamental processes governing ERS responses.

The p53 tumor suppressor protein, a key regulator of cellular responses to genotoxic stress, is stabilized and activated after DNA damage. The rapid activation of p53 by ionizing radiation and radiomimetic agents is largely dependent on the ATM kinase. p53 is phosphorylated by ATM shortly after DNA damage, resulting in enhanced stability and activity of p53. The Mdm2 oncoprotein is a pivotal negative regulator of p53. In response to ionizing radiation and radiomimetic drugs, Mdm2 undergoes rapid ATM-dependent phosphorylation prior to p53 accumulation. This results in a decrease in its reactivity with the 2A10 monoclonal antibody. Phage display analysis identified a consensus 2A10 recognition sequence, possessing the core motif DYS. Unexpectedly, this motif appears twice within the human Mdm2 molecule, at positions corresponding to residues 258-260 and 393-395. Both putative 2A10 epitopes are highly conserved and encompass potential phosphorylation sites. Serine 395, residing within the carboxy-terminal 2A10 epitope, is the major target on Mdm2 for phosphorylation by ATM in vitro. Mutational analysis supports the conclusion that Mdm2 undergoes ATM-dependent phosphorylation on serine 395 in vivo in response to DNA damage. The data further suggests that phosphorylated Mdm2 may be less capable of promoting the nucleo-cytoplasmic shuttling of p53 and its subsequent degradation, thereby enabling p53 accumulation. Our findings imply that activation of p53 by DNA damage is achieved, in part, through attenuation of the p53-inhibitory potential of Mdm2.

Activation of the tumor suppressor p53 by stress and damage stimuli often correlates with induction of stress kinases, Jun-NH 2 kinase (JNK). As JNK association with p53 plays an important role in p53 stability, in the present study we have elucidated the relationship between the JNK-signaling pathway and p53 stability and activity.

Hypoxia-inducible factor-1alpha (HIF1alpha) is a central regulator of the cellular response to hypoxia. Prolyl-hydroxylation of HIF1alpha by PHD enzymes is prerequisite for HIF1alpha degradation. Here, we demonstrate that the abundance of PHD1 and PHD3 are regulated via their targeting for proteasome-dependent degradation by the E3 ubiquitin ligases Siah1a/2, under hypoxia conditions. Siah2 null fibroblasts exhibit prolonged PHD3 half-life, resulting in lower levels of HIF1alpha expression during hypoxia. Significantly, hypoxia-induced HIF1alpha expression was completely inhibited in Siah1a/2 null cells, yet could be rescued upon inhibition of PHD3 by RNAi. Siah2 targeting of PHD3 for degradation increases upon exposure to even mild hypoxic conditions, which coincides with increased Siah2 transcription. Siah2 null mice subjected to hypoxia displayed an impaired hyperpneic respiratory response and reduced levels of hemoglobin. Thus, the control of PHD1/3 by Siah1a/2 constitutes another level of complexity in the regulation of HIF1alpha during hypoxia.

The tumour suppressor p53 is important in the cell decision to either arrest cell cycle progression or induce apoptosis in response to a variety of stimuli. p53 post-translational modifications and association with other proteins have been implicated in the regulation of its stability and transcriptional activities. Here we report that, on DNA damage, p53 interacts with Pin1, a peptidyl-prolyl isomerase, which regulates the function of many proteins involved in cell cycle control and apoptosis. The interaction is strictly dependent on p53 phosphorylation, and requires Ser 33, Thr 81 and Ser 315. On binding, Pin1 generates conformational changes in p53, enhancing its transactivation activity. Stabilization of p53 is impaired in UV-treated Pin1(-/-) cells owing to its inability to efficiently dissociate from Mdm2. As a consequence, a reduced p53-dependent response was detected in Pin1(-/-) cells, and this correlates with a diminished transcriptional activation of some p53-regulated genes. Our results suggest that, following stress-induced phosphorylation, p53 needs to form a complex with Pin1 and to undergo a conformational change to fulfil its biological roles.

Metabolic rewiring is an established hallmark of cancer, but the details of this rewiring at a systems level are not well characterized. Here we acquire this insight in a melanoma cell line panel by tracking metabolic flux using isotopically labeled nutrients. Metabolic profiling and flux balance analysis were used to compare normal melanocytes to melanoma cell lines in both normoxic and hypoxic conditions. All melanoma cells exhibited the Warburg phenomenon; they used more glucose and produced more lactate than melanocytes. Other changes were observed in melanoma cells that are not described by the Warburg phenomenon. Hypoxic conditions increased fermentation of glucose to lactate in both melanocytes and melanoma cells (the Pasteur effect). However, metabolism was not strictly glycolytic, as the tricarboxylic acid (TCA) cycle was functional in all melanoma lines, even under hypoxia. Furthermore, glutamine was also a key nutrient providing a substantial anaplerotic contribution to the TCA cycle. In the WM35 melanoma line glutamine was metabolized in the "reverse" (reductive) direction in the TCA cycle, particularly under hypoxia. This reverse flux allowed the melanoma cells to synthesize fatty acids from glutamine while glucose was primarily converted to lactate. Altogether, this study, which is the first comprehensive comparative analysis of metabolism in melanoma cells, provides a foundation for targeting metabolism for therapeutic benefit in melanoma.Metabolism in cancer cells differs from that of normal nonproliferative cells. Perhaps the most common variation from the norm in cancer metabolism is "aerobic glycolysis" or the Warburg effect. Under the Warburg effect, metabolism of glucose is largely fermentative rather than respiratory, with increased production of lactate, in normal atmospheric oxygen conditions (1). This is also associated with increased uptake of glucose, a common characteristic of cancers detectable in tumors in patients via 18 F-deoxyglucose-PET (2). However, the extent to which the Warburg effect represents a rebalancing of metabolism (increasing fermentation while decreasing respiration) versus an amplification of metabolism (increasing fermentation while maintaining, or even increasing, respiration) is the subject of debate (3, 4). The Warburg effect contrasts with the Pasteur effect, in that the latter describes the switch from fermentation to respiration when oxygen is plentiful, and its reversal when oxygen is limiting (5), while the Warburg effect describes fermentative activity of cancer cells irrespective of oxygen. In the progression of tumors, cancer cells are subject to a range of oxygen concentrations, and low oxygen induces hypoxia-inducible factor (HIF), 2 which leads to a metabolic rewiring of cancer cells, resulting in a more glycolytic metabolism (6). Therefore, cancer cells may potentially demonstrate both Warburg and Pasteur effects. Furthermore, beyond glycolysis, altered oncogene expression has strong effects on other branches of central carbon metabolism. For insta...

Activation of the transcription factor NF-kappa B in response to proinflammatory stimuli requires the phosphorylation-triggered and ubiquitin-dependent degradation of the NF-kappa B inhibitor, I kappa B alpha. Here, we show the in vitro reconstitution of the phosphorylation-dependent ubiquitination of I kappa B alpha with purified components. ROC1, a novel SCF-associated protein, is recruited by cullin 1 to form a quatemary SCFHOS-ROC1 holenzyme (with Skp1 and the beta-TRCP homolog HOS). SCFHOS-ROC1 binds IKK beta-phosphorylated I kappa B alpha and catalyzes its ubiquitination in the presence of ubiquitin, E1, and Cdc34. ROC1 plays a unique role in the ubiquitination reaction by heterodimerizing with cullin 1 to catalyze ubiquitin polymerization.

Lysine acetylation of the tumor suppressor protein p53 in response to a wide variety of cellular stress signals is required for its activation as a transcription factor that regulates cell cycle arrest, senescence, or apoptosis. Here, we report that the conserved bromo-domain of the transcriptional coactivator CBP (CREB binding protein) binds specifically to p53 at the C-terminal acetylated lysine 382. This bromodomain/acetyl-lysine binding is responsible for p53 acetylation-dependent coactivator recruitment after DNA damage, a step essential for p53-induced transcriptional activation of the cyclin-dependent kinase inhibitor p21 in G1 cell cycle arrest. We further present the three-dimensional nuclear magnetic resonance structure of the CBP bromodomain in complex with a lysine 382-acetylated p53 peptide. Using structural and biochemical analyses, we define the molecular determinants for the specificity of this molecular recognition.