A novel role for Bcl-2 associated-athanogene-1 (Bag-1) in regulation of the endoplasmic reticulum stress response in mammalian chondrocytes

BAG-1 (Bcl-2 associated athanogene-1) is a multifunctional protein, linking cell proliferation, cell death, protein folding, and cell stress. In vivo, BAG-1 is expressed in growth plate and articular cartilage, and the expression of BAG-1 is decreased with aging. Chondrocytes respond to endoplasmic reticulum (ER) stress with decreased expression of extracellular matrix proteins, and prolonged ER stress leads to chondrocyte apoptosis. Here we demonstrate for the first time that BAG-1 is involved in ER stress-induced apoptosis in chondrocytes. Induction of ER stress through multiple mechanisms all resulted in downregulation of BAG-1 expression. In addition, direct suppression of BAG-1 expression resulted in chondrocyte growth arrest and apoptosis, while stable overexpression of BAG-1 delayed the onset of ER stress-mediated apoptosis. In addition to regulating apoptosis, we also observed decreased expression of collagen type II in BAG-1 deficient chondrocytes. In contrast, overexpression of BAG-1 resulted in increased expression of collagen type II. Moreover, under ER stress conditions, the reduced expression of collagen type II was delayed in chondrocytes overexpressing BAG-1. These results suggest a novel role for BAG-1 in supporting viability and matrix expression of chondrocytes.

Multiple Signals Induce Endoplasmic Reticulum Stress in Both Primary and Immortalized Chondrocytes Resulting in Loss of Differentiation, Impaired Cell Growth, and Apoptosis

The endoplasmic reticulum is the site of synthesis and folding of secretory proteins and is sensitive to changes in the internal and external environment of the cell. Both physiological and pathological conditions may perturb the function of the endoplasmic reticulum, resulting in endoplasmic reticulum stress. The chondrocyte is the only resident cell found in cartilage and is responsible for synthesis and turnover of the abundant extracellular matrix and may be sensitive to endoplasmic reticulum stress. Here we report that glucose withdrawal, tunicamycin, and thapsigargin induce up-regulation of GADD153 and caspase-12, two markers of endoplasmic reticulum stress, in both primary chondrocytes and a chondrocyte cell line. Other agents such as interleukin-1beta or tumor necrosis factor alpha induced a minimal or no induction of GADD153, respectively. The endoplasmic reticulum stress resulted in decreased chondrocyte growth based on cell counts, up-regulation of p21, and decreased PCNA expression. In addition, perturbation of endoplasmic reticulum function resulted in decreased accumulation of an Alcian Blue positive matrix by chondrocytes and decreased expression of type II collagen at the protein level. Further, quantitative real-time PCR was used to demonstrate a down-regulation of steady state mRNA levels coding for aggrecan, collagen II, and link protein in chondrocytes exposed to endoplasmic reticulum stress-inducing conditions. Ultimately, endoplasmic reticulum stress resulted in chondrocyte apoptosis, as evidenced by DNA fragmentation and annexin V staining. These findings have potentially important implications regarding consequences of endoplasmic reticulum stress in cartilage biology.

Oxidized LDL-induced injury and apoptosis in atherosclerosis. Potential roles for oxysterols

The cell injury caused by oxidized lipoproteins was among the first findings that led to the theory that it is the oxidation of low-density lipoprotein (LDL), not just LDL concentration, that leads to arterial disease. Voluminous studies have now revealed that oxidized lipoproteins and their constituents can induce numerous effects on cells that can be construed to be atherogenic. Cell injury is but one of these, and it is these injurious effects that are the focus of this brief review. Cell injury and death appear to play multiple roles in lesion development and the toxic lipid constituents of oxidized lipoproteins, including a variety of oxysterols, are candidates for the in vivo effectors of this cytotoxicity. Recent studies have focused on the mechanisms of oxidized lipoprotein-induced cell death, whether the cells die by apoptosis or necrosis, and the identities of the toxins that induce injury. Understanding the roles of these agents in lesion development could lead to therapies that modulate cell death and inhibit lesion formation.

Expression of SET, an inhibitor of protein phosphatase 2A, in renal development and Wilms' tumor

The human gene set was originally identified as a component of the set-can fusion gene produced by a somatic translocation event in a case of acute undifferentiated leukemia. In the developing kidney, set was highly expressed in the zone of nephron morphogenesis. Recently, SET was shown to be a potent and specific inhibitor of protein phosphatase 2A, a family of major serine/threonine phosphatases involved in regulating cell proliferation and differentiation. The current study sought to define further the role of SET in the regulation of renal cell proliferation and tumorigenesis. The mRNA encoding SET was expressed at much higher levels in transformed human and rodent cell lines than in cultured renal epithelial and primary endothelial cells. Consistent with a role for SET in cell proliferation, set mRNA expression was markedly reduced in cells rendered quiescent by serum starvation, contact inhibition, or differentiation. Previous findings during renal development were extended by demonstrating that SET protein expression is also much greater in developing rat and human kidney than in fully differentiated, mature kidney. Finally, high levels of set mRNA and SET protein expression were found in Wilms' tumor, but not in renal cell carcinoma, adult polycystic kidney disease or in transitional cell carcinoma.

Calcium mediates expression of stress-response genes in prostaglandin A2-induced growth arrest

We have explored the mechanisms involved in the induction of five stress-response genes (heme oxygenase [HO], c-fos, Egr-1, gadd153, and HSP70) in human diploid fibroblasts growth-arrested by treatment with the antiproliferative prostaglandin A2 (PGA2). The kinetics of c-fos and Egr-1 induction were found to be rapid with maximum expression occurring within 60 min of treatment, whereas maximum expression of HO, gadd153, and HSP70 occurred between 4 and 8 h of treatment. Nuclear run-on assays and measurements of mRNA clearance in the presence of actinomycin D demonstrated that increases in both the rates of gene transcription and/or mRNA stability contribute to the genetic response to PGA2. Although the mechanisms responsible for increasing the mRNA levels differ for the individual genes, additional experiments provided evidence that alterations in intracellular calcium ([Ca2+]i) levels were important in initiating the genetic response to PGA2. PGA2 treatment resulted in a rapid increase in [Ca2+]i with the dose-response relationship for Ca2+ mobilization consistent with that seen for the induction of all five genes. [Ca2+]i chelators that attenuate Ca2+ mobilization by PGA2 also blocked the mRNA induction by PGA2 treatment. Density-inhibited confluent cells were less responsive than proliferating subconfluent cells with respect to Ca2+ mobilization after PGA2 treatment. This was correlated with a lower level of gene induction. These studies support the hypothesis that increased Ca2+ mobilization is an early and central event in the signal transduction pathway (or pathways) mediating the activation of genes in response to PGA2 treatment.

Regulation of the C/EBP-related gene gadd153 by glucose deprivation

gadd153 encodes a CCAAT/enhancer-binding protein (C/EBP)-related protein that lacks a functional DNA-binding domain. Since the gadd153 protein is capable of heterodimerizing with other C/EBPs, gadd153 may function as a negative regulator of these transcription factors. Here we examined the role of glucose in regulating gadd153 expression. We found that glucose deprivation markedly induces gadd153 mRNA levels in both HeLa and 3T3-L1 cells and that addition of D-(+)-glucose resulted in a rapid decrease of gadd153 mRNA. Similar induction and reversal of gadd153 expression were observed at the protein level. Because C/EBP alpha appears to play an important role in regulating genes involved in adipogenesis and energy metabolism, we examined gadd153 expression during the differentiation of 3T3-L1 preadipocytes and as a function of glucose utilization in differentiated adipocytes. Using a standard differentiation protocol that consisted of hormonal stimulation for 2 days followed by medium changes every 2 days thereafter, we observed that both C/EBP alpha and gadd153 mRNAs were elevated. However, C/EBP alpha induction occurred on day 3, while gadd153 expression was not seen until day 4, when the cells were fully differentiated. Frequent addition of fresh medium to the cells during the differentiation process, as well as supplementation of medium with glucose, reduced gadd153 expression without preventing C/EBP alpha expression or interfering with cellular differentiation. Thus, gadd153 expression is not essential for the process of adipocyte differentiation but is significantly influenced by the availability of glucose to the cell.

Decreased striatal D2 dopamine receptor mRNA synthesis during aging

Decreases in striatal dopamine receptors during aging are related to deficits in motor functions, and are highly correlated with the D2 receptor subtype. Some receptor loss is attributed to cell death, while the remainder is thought to be related to synthetic capacity. We discuss here our findings, for isolated rat striatal nuclei, which indicate a 50% overall decrease in the synthesis of the dopamine D2 mRNA in aged nuclei.

Calcium ionophore A23187 induces expression of the growth arrest and DNA damage inducible CCAAT/enhancer-binding protein (C/EBP)-related gene, gadd153. Ca2+ increases transcriptional activity and mRNA stability

gadd153 is a CCAAT/enhancer-binding protein (C/EBP)-related gene whose expression is induced in response to growth arrest and DNA damage. This investigation explored the possibility that Ca2+ might play a role in regulating expression of gadd 153. We have demonstrated that treatment of HeLa cells with the calcium ionophores A23187 and ionomycin leads to the induction of gadd153 mRNA. The induction was rapid; increases in mRNA were detected by 90 min of treatment, and near maximum levels were achieved within 5-h exposure to A23187. Elevated mRNA levels resulted from both an increase in the rate of gadd153 transcription and an increase in the stability of the gadd153 mRNA. The response was not dependent on protein kinase C nor was it coupled to c-fos expression. Buffering intracellular and extracellular Ca2+ by combined treatment with BAPTA-AM (acetoxymethyl ester form of bis(aminophenoxy)ethane N,N'-tetraacetic acid) and EGTA prevented the induction of gadd153 mRNA by A23187. In addition, these treatments prevented the induction of gadd153 mRNA in response to the DNA damaging agent methyl methanesulfonate. We conclude that intracellular Ca2+ plays a role in regulating gadd153 expression. More specifically, Ca2+ likely plays a role in the induction of gadd153 mRNA following DNA damage.

Induction of HSP70 gene expression by the antiproliferative prostaglandin PGA2: a growth-dependent response mediated by activation of heat shock transcription factor

Prostaglandins (PG) of the A series are potent inhibitors of cell proliferation. Recently, it was shown that PGA2-induced growth arrest was associated with the increased synthesis of stress proteins encoded by the HSP70 gene family. In this study, we have examined the molecular basis for this increases HSP70 expression. Northern (RNA) blot analysis and nuclear run-on assays demonstrated that induction of high levels of HSP70 mRNA results from an increase in the rate of transcription. High-level induction is specific to the HSP70 family of heat shock proteins and is rapid, reversible, dose dependent, and specific for PGs capable of growth-arresting HeLa cells. In addition, the response was found to be highly dependent on the growth state of the cells, as induction occurs in growing but not in confluent nongrowing cell populations. Induction is dependent on the activation of heat shock factor. Cycloheximide pretreatment, which inhibits the antiproliferative effects of PGA2, prevents activation of the heat shock factor and induction of HSP70 mRNA by PGA2. These results support a role for HSP70 in mediating the antiproliferative effects of PGA2.

Cell growth inhibition by prostaglandin A2 results in elevated expression of gadd153 mRNA

Treatment of Hela cells with prostaglandin A2 (PGA2) resulted in a marked inhibition of cell proliferation which was associated with a significant induction of gadd153 mRNA, a member of a novel class of genes associated with growth arrest and DNA damage. Induction of gadd153 mRNA was specific to prostaglandins capable of arresting cell growth and was dose-dependent with the maximum effect seen at 36 microM PGA2. Induction was rapid, occurring within 2-4 h and reaching a maximum by 8 h. These effects were reversible as removal of PGA2 resulted in a rapid decline in gadd153 mRNA levels coincident with resumption of cell growth. PGA2 induction of gadd153 mRNA was completely prevented by the presence of actinomycin D at a concentration sufficient to block transcription and was partially inhibited (50%) by the protein synthesis inhibitor cycloheximide. The presence of the protein kinase inhibitor 2-aminopurine decreased the PGA2 induction of gadd153 mRNA by greater than 90%, suggesting that cellular kinases play a role in the induction of gadd153 by PGA2. Thus PGA2-mediated growth arrest provides a useful model to further define the role of gadd153 in the negative control of cell growth.

The effect of hypoxia on traumatic head injury in rats: alterations in neurologic function, brain edema, and cerebral blood flow

We evaluated the effects of early posttraumatic hypoxia on neurologic function, magnetic resonance images (MRI), brain tissue specific gravities, and cerebral blood flow (CBF) in head-injured rats. By itself, an hypoxic insult (PaO2 40 mm Hg for 30 min) had little effect on any measure of cerebral function. After temporal fluid-percussion impact injury, however, hypoxia significantly increased morbidity. Of rats subjected to impact (4.9 +/- 0.3 atm) plus hypoxia, 71% had motor weakness contralateral to the impact side 24 h after injury, while only 29% of rats subjected to impact alone had demonstrable weakness (p less than 0.05). Lesions observed on MR images 24 h after injury were restricted to the impact site in rats with impact injury alone, but extensive areas with longer T1 relaxation times were observed throughout the ipsilateral cortex in rats with impact injury and hypoxic insult. Brain tissue specific gravity measurements indicated that much more widespread and severe edema developed in rats with impact injury and hypoxia. [14C]Iodoantipyrine autoradiography performed 24 h after injury showed that there was extensive hypoperfusion of the entire ipsilateral cortex in rats with impact injury and hypoxia. These results show that large areas of impact-injured brain are extremely vulnerable to secondary insults that can irreparably damage neural tissue, and provide experimental evidence for the observed adverse effects of hypoxia on outcome after human head injury.

Selective changes in local cerebral glucose utilization induced by phenobarbital in the rat

Alterations in cerebral metabolic activity were measured after different doses of phenobarbital. Local cerebral glucose utilization was determined in 58 brain regions with the use of the [14C]deoxyglucose technique in 3-month-old Fischer 344 rats, at 1 h after the ip administration of saline or of phenobarbital. Whole brain glucose utilization declined in a dose-related manner by 4%, 13%, 33%, 35%, and 56% after phenobarbital 18, 60, 180, 300, and 600 mg/kg, respectively. The number of regions significantly affected (P less than 0.05) increased from 7 to 95% of the regions examined between doses of 18 to 600 mg/kg. Metabolism decreased in all significantly affected regions except the interpeduncular nucleus, where it was increased. In a separate group of rats, the number of falls per 5 min from a constantly rotating cylinder was measured at subanesthetic doses of phenobarbital. Doses of drug that affected performance on the rotating cylinder (18 and 60 mg/kg) reduced glucose utilization in brain regions involved with motor performance, including the red nucleus, vestibular nucleus, substantia nigra, and deep layers of the superior colliculus, whereas cerebral cortical regions were not altered significantly. The results demonstrate that phenobarbital reduces cerebral glucose utilization, in a dose-dependent manner, in most brain regions and affects subcortical regions of the motor system significantly before reducing metabolism in the cerebral cortex.