Epigenetic upregulation of p66shc mediates low-density lipoprotein cholesterol-induced endothelial cell dysfunction

Hypercholesterolemia characterized by elevation of low-density lipoprotein (LDL) cholesterol is a major risk factor for atherosclerotic vascular disease. p66shc mediates hypercholesterolemia-induced endothelial dysfunction and atheromatous plaque formation. We asked if LDL upregulates endothelial p66shc via changes in the epigenome and examined the role of p66shc in LDL-stimulated endothelial cell dysfunction. Human LDL stimulates human p66shc promoter activity and p66shc expression in human endothelial cells. LDL leads to hypomethylation of two CpG dinucleotides and acetylation of histone 3 in the human p66shc promoter. These two CpG dinucleotides mediate LDL-stimulated p66shc promoter activity. Inhibition or knock down of DNA methyltransferases negates LDL-induced endothelial p66shc expression. p66shc mediates LDL-stimulated increase in expression of endothelial intercellular adhesion molecule-1 (ICAM1) and decrease in expression of thrombomodulin (TM). Mirroring these changes in ICAM1 and TM expression, p66shc mediates LDL-stimulated adhesion of monocytes to endothelial cells and plasma coagulation on endothelial cells. These findings indicate that LDL cholesterol upregulates human endothelial p66shc expression via hypomethylation of CpG dinucleotides in the p66shc promoter. Moreover, they show that LDL-stimulated p66shc expression mediates a dysfunctional endothelial cell surface, with proadhesive and procoagulant features.     

Antagonism of p66shc by melanoma inhibitory activity

The p66shc protein governs oxidant stress and mammalian lifespan. Here, we identify melanoma inhibitory activity (MIA), a protein secreted by melanoma cells, as a novel binding partner and antagonist of p66shc. The N-terminal collagen homology-2 (CH2) domain of p66shc binds to the Src Homology-3 (SH3)-like domain of MIA in vitro. In cells, ectopically expressed MIA and p66shc colocalize and co-precipitate. MIA also co-precipitates with the CH2 domain of p66shc in vivo. MIA expression in vivo suppresses p66shc-stimulated increase in endogenous hydrogen peroxide (H(2)O(2)), and inhibits basal and H(2)O(2)-induced phosphorylation of p66shc on serine 36 and H(2)O(2)-induced death. In human melanoma cells expressing MIA, endogenous MIA and p66shc co-precipitate. Downregulation of MIA in melanoma cells increases basal and ultraviolet radiation (UVR)-induced phosphorylation of p66shc on serine 36, augments endogenous H(2)O(2) levels, and increases their susceptibility to UVR-induced death. These findings show that MIA binds to p66shc, and suggest that this interaction antagonizes phosphorylation and function of p66shc.     

Sos-mediated activation of rac1 by p66shc

The Son of Sevenless 1 protein (sos1) is a guanine nucleotide exchange factor (GEF) for either the ras or rac1 GTPase. We show that p66shc, an adaptor protein that promotes oxidative stress, increases the rac1-specific GEF activity of sos1, resulting in rac1 activation. P66shc decreases sos1 bound to the growth factor receptor bound protein (grb2) and increases the formation of the sos1-eps8-e3b1 tricomplex. The NH(2)-terminal proline-rich collagen homology 2 (CH2) domain of p66shc associates with full-length grb2 in vitro via the COOH-terminal src homology 3 (C-SH3) domain of grb2. A proline-rich motif (PPLP) in the CH2 domain mediates this association. The CH2 domain competes with the proline-rich COOH-terminal region of sos1 for the C-SH3 domain of grb2. P66shc-induced dissociation of sos1 from grb2, formation of the sos1-eps8-e3b1 complex, rac1-specific GEF activity of sos1, rac1 activation, and oxidative stress are also mediated by the PPLP motif in the CH2 domain. This relationship between p66shc, grb2, and sos1 provides a novel mechanism for the activation of rac1.     

Opposite effects of the p52shc/p46shc and p66shc splicing isoforms on the EGF receptor-MAP kinase-fos signalling pathway.

Shc proteins are targets of activated tyrosine kinases and are implicated in the transmission of activation signals to Ras. The p46shc and p52shc isoforms share a C-terminal SH2 domain, a proline- and glycine-rich region (collagen homologous region 1; CH1) and a N-terminal PTB domain. We have isolated cDNAs encoding for a third Shc isoform, p66shc. The predicted amino acid sequence of p66shc overlaps that of p52shc and contains a unique N-terminal region which is also rich in glycines and prolines (CH2). p52shc/p46shc is found in every cell type with invariant reciprocal relationship, whereas p66shc expression varies from cell type to cell type. p66shc differs from p52shc/p46shc in its inability to transform mouse fibroblasts in vitro. Like p52shc/p46shc, p66shc is tyrosine-phosphorylated upon epidermal growth factor (EGF) stimulation, binds to activated EGF receptors (EGFRs) and forms stable complexes with Grb2. However, unlike p52shc/p46shc it does not increase EGF activation of MAP kinases, but inhibits fos promoter activation. The isolated CH2 domain retains the inhibitory effect of p66shc on the fos promoter. p52shc/p46shc and p66shc, therefore, appear to exert different effects on the EGFR-MAP kinase and other signalling pathways that control fos promoter activity. Regulation of p66shc expression might, therefore, influence the cellular response to growth factors.     

The mammalian longevity-associated gene product p66shc regulates mitochondrial metabolism

Previous studies have determined that mice with a homozygous deletion in the adapter protein p66(shc) have an extended life span and that cells derived from these mice exhibit lower levels of reactive oxygen species. Here we demonstrate that a fraction of p66(shc) localizes to the mitochondria and that p66(shc-/-) fibroblasts have altered mitochondrial energetics. In particular, despite similar cytochrome content, under basal conditions, the oxygen consumption of spontaneously immortalized p66(shc-/-) mouse embryonic fibroblasts were lower than similarly maintained wild type cells. Differences in oxygen consumption were particularly evident under chemically uncoupled conditions, demonstrating that p66(shc-/-) cells have a reduction in both their resting and maximal oxidative capacity. We further demonstrate that reconstitution of p66(shc) expression in p66(shc-/-) cells increases oxygen consumption. The observed defect in oxidative capacity seen in p66(shc-/-) cells is partially offset by augmented levels of aerobic glycolysis. This metabolic switch is manifested by p66(shc-/-) cells exhibiting an increase in lactate production and a stricter requirement for extracellular glucose in order to maintain intracellular ATP levels. In addition, using an in vivo NADH photobleaching technique, we demonstrate that mitochondrial NADH metabolism is reduced in p66(shc-/-) cells. These results demonstrate that p66(shc) regulates mitochondrial oxidative capacity and suggest that p66(shc) may extend life span by repartitioning metabolic energy conversion away from oxidative and toward glycolytic pathways.     

Evaluation of methods for determining 6-hydroxydopamine cytotoxicity

Summary The toxic effects of 6-hydroxydopamine on the human neuroblastoma cell line SK-N-SH-SY5Y (SY5Y) and the Chinese hamster ovary (CHO) cell line were measured with five viability assays. Four of the assays (attachment efficiency, plating efficiency, amino acid incorporation into acid-precipitable proteins, and Trypan Blue dye exclusion) showed higher drug susceptibility in SY5Y cells than CHO cells. Only growth inhibition (proliferation index) gave results indicating greater sensitivity in CHO cells. Over a time span of 48 hr, injured cell populations lost vital functions in the following order: attachment ability, amino acid incorporation, proliferative capacity, and dye exclusion. Recovery of each of the functions occurred in sublethally injured populations. Monitoring the extinction and recovery of vital functions permitted the accurate determination of a drug concentration (30 μg/ml) selectively toxic for SY5Y cells. A strong correlation was noted between relative values for amino acid incorporation 3 hr after drug treatment, attachment efficiency at 24 hr, and dye exclusion at 24 and 48 hr. We concluded that Trypan Blue dye exclusion and amino acid incorporation were suitable methods for comparing the effects of cytotoxins on different cell lines, provided they were performed at the appropriate time after treatment with the toxin. The combined techniques yield both population and individual cell data, are simple to do, and are applicable to nondividing cell populations. This work was supported by an NIH National Research Service Award GM07204 to E. T. C., a gift from the Lola-Wright Foundation, NINCDS Grants NS14034 and NS15234, Robert Welch Grant H698, and an RCDA (NS00213) to J. R. P.     

p66Shc mediates anoikis through RhoA

Detachment of parenchymal cells from a solid matrix switches contextual cues from survival to death during anoikis. Marked shape changes accompany detachment and are thought to trigger cell death, although a working model to explain the coordination of attachment sensation, shape change, and cell fate is elusive. The constitutive form of the adapter Shc, p52Shc, confers survival properties, whereas the longer p66Shc signals death through association with cytochrome c. We find that cells that lack p66Shc display poorly formed focal adhesions and escape anoikis. However, reexpression of p66Shc restores anoikis through a mechanism requiring focal adhesion targeting and RhoA activation but not an intact cytochrome c-binding motif. This pathway stimulates the formation of focal adhesions and stress fibers in attached cells and tension-dependent cell death upon detachment. p66Shc may thus report attachment status to the cell by imposing a tension test across candidate anchorage points, with load failure indicating detachment.     

High levels of p66shc and intracellular ROS in permanently arrested early embryos

A high incidence of permanent embryo arrest occurs during the first week of in vitro development. We hypothesize that this developmental arrest event is regulated by the stress adaptor protein p66shc, a genetic determinant of life span in mammals, which regulates ROS metabolism, apoptosis, and cellular senescence. The aim of this study was to assess the relationship between intracellular oxidative stress levels with the incidence of embryo arrest and the expression of senescent-associated genes in embryos produced under different oxygen tensions. Embryos cultured under 20% oxygen conditions showed approximately 10-fold increase in oxidative stress, 2-fold increase in the percentage of 2- to 4-cell arrest, and significantly lower developmental capabilities compared to embryos cultured under a 5% oxygen environment. Quantification by real-time PCR and by semiquantitative immunofluorescence showed significantly higher p66shc mRNA and protein levels, respectively, in embryos cultured in 20% versus those cultured in 5% oxygen atmosphere. No significant changes in p53 mRNA and protein levels were detected among embryos derived from both oxygen tensions. Taken together, these results demonstrate that p66shc, but not p53, is significantly more abundant in an embryo population that exhibits higher frequencies of embryo arrest and quantities of intracellular ROS. These results further substantiate that p66shc and oxidative stress are associated with a p53-independent embryonic arrest event for in vitro-produced embryos.     

Expression profiles of p53 and p66shc during oxidative stress-induced senescence in fetal bovine fibroblasts

Somatic cells undergo a permanent cell cycle arrest, called cellular senescence, after a limited number of cell divisions in vitro. Both the tumor suppressor protein p53 and the stress-response protein p66(shc) are suggested to regulate the molecular events associated with senescence. This study was undertaken to investigate the effect of different oxygen tensions and oxidative stress on cell longevity and to establish the role of p53 and p66(shc) in cells undergoing senescence. As a model of cellular senescence, primary fetal bovine fibroblasts were cultured in either 20% O(2) or 5% O(2) atmospheres until senescence was reached. Fibroblasts cultured under 20% O(2) tension underwent senescence after 30 population doublings (PD), whereas fibroblasts cultured under 5% O(2) tension did not exhibit signs of senescence. Oxidative stress, as measured by protein carbonyl content, was significantly elevated in senescent cells compared to their younger counterparts and to fibroblasts cultured under 5% O(2) at the same PD. p53 mRNA gradually decreased in 20% O(2) cultured fibroblasts until senescence was reached, whereas p53 protein levels were significantly increased as well as p53 phosphorylation on serine 20, suggesting that p53 might be stabilized by posttranslational modifications during senescence. Senescence was also associated with high levels of p66(shc) mRNA and protein levels, while the levels remained low and stable in dividing fibroblasts under 5% O(2) atmosphere. Taken together, our results show an effect of oxidative stress on the replicative life span of fetal bovine fibroblasts as well as an involvement of p53, serine 20-p53 phosphorylation and p66(shc) in senescence.     

Alteration of p66shc is associated with endothelial dysfunction in the abdominal aortic coarctation of rats

To examine the role of p66shc in endothelial dysfunction, we investigated the endothelium-dependent relaxation, protein expression and superoxide production in abdominal aortic coarctation rats. Endothelium-dependent relaxation to acetylcholine was impaired only in the aortic segments above the aortic coarctation (35.0+/-7.1% vs. 86.6+/-6.0% for sham control at 1 microM Ach). The aortic segments exposed to increased blood pressure showed a decreased phosphorylation of endothelial nitric oxide synthase, an increased phosphorylation of p66shc, and an increased superoxide production. Angiotensin II elicited a significantly increased phosphorylation of p66shc in the endothelial cells. Taken together, these findings suggest that the increased phosphorylation of p66shc is one of the important mediators in the impaired endothelium-dependent relaxation of aortic coarctation rats.     

The p70 tumor necrosis factor receptor mediates cytotoxicity.

Tumor necrosis factor alpha (TNF) selectively kills tumor cells, but this specificity is not clearly understood. Two distinctly different cell surface receptors (TNFRs), proteins of 55 kd (p55) and 70-80 kd (p70), mediate TNF action. Mouse TA1 cells are not killed by human (h) TNF, but are killed by mouse (m) TNF alone. Since the mouse p70 TNFR is recognized only by mTNF, these results implicate p70 receptor action in TA1 cell killing. Human HeLa cells have mainly the p55 receptor and are not killed by hTNF alone. When transfected with the human p70 TNFR, HeLa p70 die within 24 hr. HeLa p70 cells also show reduced c-fos and manganous superoxide dismutase induction by TNF. NIH 3T3 mouse fibroblasts are sensitive to only mTNF, but overexpression of the human p70 receptor causes cell death by hTNF and increased sensitivity to mTNF. These results provide a direct function for the p70 TNFR in TNF-induced cytotoxicity.     

Preconditioning regulation of bcl-2 and p66shc by human NOS1 enhances tolerance to oxidative stress.

Preconditioning stress induced by a transient ischemia may increase brain tolerance to oxidative stress, and the underlying neuroprotective mechanisms are not well understood. In a series of experiments, we found that endogenous nitric oxide (NO), S-nitrosoglutathione (GSNO), and antioxidants blocked serum deprivation-induced oxidative stress and apoptosis in human neuroblastoma cells. Similar to nuclear redox factor-1 (Ref-1), mRNA of human neuronal nitric oxide synthase (hNOS1) was maximally up-regulated within 2 h after oxidative stress and down-regulated by NO/GSNO and hydroxyl radical (OH) scavenger. A brief preconditioning stress induced by serum deprivation for 2 h caused a delayed increase in the expression of hNOS1 protein and the associated formation of NO and cGMP, which in turn decreased OH generation and stress-related cell death. In addition to inhibiting caspase-3 through a dithiothreitol-sensitive S-nitrosylation process, preconditioning stress concomitantly up-regulated the expression of the anti-apoptotic bcl-2 protein and down-regulated the p66shc adaptor protein. This beneficial cytoprotective process of preconditioning stress is mediated by newly synthesized NO because it can be suppressed by the inhibition of hNOS1 and guanylyl cyclase. Therefore, the constitutive isoform of hNOS1 is dynamically redox-regulated to meet both functional and compensatory demands of NO for gene regulation, antioxidant defense, and tolerance to oxidative stress.     

Phosphorylation of the C subunit (p66) of human DNA polymerase delta

Of the four subunits constituting DNA polymerase δ, subunit C or p66 has been shown to mainly mediate polymerase interaction with PCNA, an auxiliary factor that greatly enhances DNA polymerase δ processivity on primed DNA templates. Here, we provide evidence that a highly conserved region located between amino acids 384 and 399 in the C-terminus of p66 is phosphorylated, most probably by Protein kinase CK2, and that another region, most probably located within the PCNA interacting domain in its extreme C-terminus, regulates its interaction with PCNA. Phosphorylation of p66 is associated with its co-localization with large subunit of DNA polymerase δ, p125, and PCNA, to the insoluble chromatin fraction at the beginning of S-phase. Taken together, the results provide evidence that concurrent phosphorylation events in p66 may positively and negatively regulate its activity and interactions with other components of the replisome during the cell cycle.     

p66shc inhibits pro-survival epidermal growth factor receptor/ERK signaling during severe oxidative stress in mouse renal proximal tubule cells

The fully executed epidermal growth factor receptor (EGFR)/Ras/MEK/ERK pathway serves a pro-survival role in renal epithelia under moderate oxidative stress. We and others have demonstrated that during severe oxidative stress, however, the activated EGFR is disconnected from ERK activation in cultured renal proximal tubule cells and also in renal proximal tubules after ischemia/reperfusion injury, resulting in necrotic death. Studies have shown that the tyrosine-phosphorylated p46/52 isoforms of the ShcA family of adaptor proteins connect the activated EGFR to activation of Ras and ERK, whereas the p66(shc) isoform can inhibit this p46/52(shc) function. Here, we determined that severe oxidative stress (after a brief period of activation) terminates activation of the Ras/MEK/ERK pathway, which coincides with ERK/JNK-dependent Ser(36) phosphorylation of p66(shc). Isoform-specific knockdown of p66(shc) or mutation of Ser(36) to Ala, but not to Asp, attenuated severe oxidative stress-mediated ERK inhibition and cell death in vitro. Also, severe oxidative stress (unlike ligand stimulation and moderate oxidative stress, both of which support survival) increased binding of p66(shc) to the activated EGFR and Grb2. This binding dissociated the SOS1 adaptor protein from the EGFR-recruited signaling complex, leading to termination of Ras/MEK/ERK activation. Notably, Ser(36) phosphorylation of p66(shc) and its increased binding to the EGFR also occurred in the kidney after ischemia/reperfusion injury in vivo. At the same time, SOS1 binding to the EGFR declined, similar to the in vitro findings. Thus, the mechanism we propose in vitro offers a means to ameliorate oxidative stress-induced cell injury by either inhibiting Ser(36) phosphorylation of p66(shc) or knocking down p66(shc) expression in vivo.     

Role of reactive oxygen species in extracellular signal-regulated protein kinase phosphorylation and 6-hydroxydopamine cytotoxicity

A number of reports indicate the potential for redox signalling via extracellular signal-regulated protein kinases (ERK) during neuronal injury. We have previously found that sustained ERK activation contributes to toxicity elicited by 6-hydroxydopamine (6-OHDA) in the B65 neuronal cell line. To determine whether reactive oxygen species (ROS) play a role in mediating ERK activation and 6-OHDA toxicity, we examined the effects of catalase, superoxide dismutase (SOD1), and metalloporphyrin antioxidants ('SOD mimetics') on 6-OHDA-treated cells. We found that catalase and metalloporphyrin antioxidants not only conferred protection against 6-OHDA but also inhibited development of sustained ERK phosphorylation in both differentiated and undifferentiated B65 cells. However, exogenously added SOD1 and heat-inactivated catalase had no effect on either toxicity or sustained ERK phosphorylation. This correlation between antioxidant protection and inhibition of 6-OHDA-induced sustained ERK phosphorylation suggests that redox regulation of ERK signalling cascades may contribute to neuronal toxicity.     

p66shc negatively regulates insulin-like growth factor I signal transduction via inhibition of p52shc binding to Src homology 2 domain-containing protein tyrosine phosphatase substrate-1 leading to impaired growth factor receptor-bound protein-2 membrane recruitment

Our previous studies have indicated an essential role of p52shc in mediating IGF-I activation of MAPK in smooth muscle cells (SMC). However, the role of the p66 isoform of shc in IGF-I signal transduction is unclear. In the current study, two approaches were employed to investigate the role of p66shc in mediating IGF-I signaling. Knockdown p66shc by small interfering RNA enhanced IGF-I-stimulated p52shc tyrosine phosphorylation and growth factor receptor-bound protein-2 (Grb2) association, resulting in increased IGF-I-dependent MAPK activation. This was associated with enhanced IGF-I-stimulated cell proliferation. In contrast, knockdown of p66shc did not affect IGF-I-stimulated IGF-I receptor tyrosine phosphorylation. Overexpression of p66shc impaired IGF-I-stimulated p52shc tyrosine phosphorylation and p52shc-Grb2 association. In addition, IGF-I-dependent MAPK activation was also impaired, and SMC proliferation in response to IGF-I was inhibited. IGF-I-dependent cell migration was enhanced by p66shc knockdown and attenuated by p66shc overexpression. Mechanistic studies indicated that p66shc inhibited IGF-I signal transduction via competitively inhibiting the binding of Src homology 2 domain-containing protein tyrosine phosphatase-2 (SHP-2) to SHP substrate-1 (SHPS-1), leading to the disruption of SHPS-1/SHP-2/Src/p52shc complex formation, an event that has been shown previously to be essential for p52shc phosphorylation and Grb2 recruitment. These findings indicate that p66shc functions to negatively regulate the formation of a signaling complex that is required for p52shc activation in response to IGF-I, thus leading to attenuation of IGF-I-stimulated cell proliferation and migration.