78-kilodalton glucose-regulated protein is induced in Rous sarcoma virus-transformed cells independently of glucose deprivation

Role of Mass Spectrometry in Investigating a Novel Protein: The Example of Tumor Differentiation Factor (TDF)

Better understanding of central nervous system (CNS) molecules can include the identification of new molecules and their receptor systems. Discovery of novel proteins and elucidation of receptor targets can be accomplished using mass spectrometry (MS). We describe a case study of such a molecule, which our lab has studied using MS in combination with other protein identification techniques, such as immunohistochemistry and Western Blotting. This molecule is known as tumor differentiation factor (TDF), a recently-found protein secreted by the pituitary into the blood. TDF mRNA has been detected in brain; not heart, placenta, lung, liver, skeletal muscle, or pancreas. Currently TDF has an unclear function, and prior to our studies, its localization was only minimally understood, with no understanding of receptor targets. We investigated the distribution of TDF in the rat brain using immunohistochemistry (IHC) and immunofluorescence (IF). TDF protein was detected in pituitary and most other brain regions, in specific neurons but not astrocytes. We found TDF immunoreactivity in cultured neuroblastoma, not astrocytoma. These data suggest that TDF is localized to neurons, not to astrocytes. Our group also conducted studies to identify the TDF receptor (TDF-R). Using LC-MS/MS and Western blotting, we identified the members of the Heat Shock 70-kDa family of proteins (HSP70) as potential TDF-R candidates in both MCF7 and BT-549 human breast cancer cells (HBCC) and PC3, DU145, and LNCaP human prostate cancer cells (HPCC), but not in HeLa cells, NG108 neuroblastoma, or HDF-a and BLK CL.4 cells fibroblasts or fibroblast-like cells. These studies have combined directed protein identification techniques with mass spectrometry to increase our understanding of a novel protein that may have distinct actions as a hormone in the body and as a growth factor in the brain.

GRP78 is required for cell proliferation and protection from apoptosis in chicken embryo fibroblast cells

Chicken serum has been suggested as a supplement to promote chicken cell proliferation and development. However, the molecular mechanisms by which chicken serum stimulates chicken cell proliferation remain unknown. Here, we evaluated the effects of chicken serum supplementation on chicken embryo fibroblast (CEF) and DF-1 cell proliferation. We also sought to elucidate the molecular pathways involved in mediating the effects of chicken serum on fibroblasts and DF-1 cells by overexpression of chicken 78 kDa glucose-regulated protein (chGRP78), which is important for cell growth and the prevention of apoptosis. Our data demonstrated that the addition of 5% chicken serum significantly enhanced fibroblast proliferation. Moreover, knockdown of chGRP78 using siRNA decreased fibroblast proliferation and increased apoptosis. Based on these results, we suggest that the chGRP78-mediated signaling pathway plays a critical role in chicken serum-stimulated fibroblast survival and anti-apoptosis. Therefore, our findings have important implications for the maintenance of chicken fibroblast cells through the inhibition of apoptosis and may lead to the development of new treatments for avian disease.

Characterization of tumor differentiation factor (TDF) and its receptor (TDF-R)

Tumor differentiation factor (TDF) is an under-investigated protein produced by the pituitary with no definitive function. TDF is secreted into the bloodstream and targets the breast and prostate, suggesting that it has an endocrine function. Initially, TDF was indirectly discovered based on the differentiation effect of alkaline pituitary extracts of the mammosomatotropic tumor MtTWlO on MTW9/PI rat mammary tumor cells. Years later, the cDNA clone responsible for this differentiation activity was isolated from a human pituitary cDNA library using expression cloning. The cDNA encoded a 108-amino-acid polypeptide that had differentiation activity on MCF7 breast cancer cells and on DU145 prostate cancer cells in vitro and in vivo. Recently, our group focused on identification of the TDF receptor (TDF-R). As potential TDF-R candidates, we identified the members of the Heat Shock 70-kDa family of proteins (HSP70) in both MCF7 and BT-549 human breast cancer cells (HBCC) and PC3, DU145, and LNCaP human prostate cancer cells (HPCC), but not in HeLa cells, NG108 neuroblastoma, or HDF-a and BLK CL.4 cells fibroblasts or fibroblast-like cells. Here we review the current advances on TDF, with particular focus on the structural investigation of its receptor and on its functional effects on breast and prostate cells.

Phosphoproteins in stress-induced disease

The integrated stress response (ISR) is an evolutionarily conserved homeostatic program activated by specific pathological states. These include amino acid deprivation, viral infection, iron deficiency, and the misfolding of proteins within the endoplasmic reticulum (ER), the so-called ER stress. Although apparently disparate, each of these stresses induces phosphorylation of a translation initiation factor, eIF2α, to attenuate new protein translation while simultaneously triggering a transcriptional program. This is achieved by four homologous stress-sensing kinases: GCN2, PKR, HRI, and PERK. In addition to these kinases, mammals possess two specific eIF2α phosphatases, GADD34 and CReP, which play crucial roles in the recovery of protein synthesis following the initial insult. They are not only important in embryonic development but also appear to play important roles in disease, particularly cancer. In this chapter, we discuss each of the eIF2α kinases, in turn, with particular emphasis on their regulation and the new insights provided by recent structural studies. We also discuss the potential for developing novel drug therapies that target the ISR.

Changes in the porcine peripheral blood mononuclear cell proteome induced by infection with highly virulent classical swine fever virus

Leukopenia and immunosuppression are characteristic clinical manifestations of classical swine fever and peripheral blood mononuclear cells (PBMCs) are major targets of classical swine fever virus. To investigate proteomic expression changes in swine PBMCs during lethal CSFV infection, proteins of PBMCs from five lethally CSFV-infected pigs were resolved by two-dimensional electrophoresis followed by mass spectrometry. Quantitative intensity analysis revealed that 66 protein spots showed altered expression, 44 of which were identified as 34 unique proteins by MALDI-TOF-MS/MS. Cellular functions of these proteins included cytoskeletal, energy metabolism, protein translation and processing, antioxidative stress, heat shock and blood clotting. Western blot analysis confirmed the upregulation of annexin A1 and downregulation of cofilin. Identification of these changed levels of expression provides an understanding at the molecular level of the response of in vivo target cells to CSFV infection and of the pathogenic mechanisms of leukopenia and immunosuppression induced by the virus.

Proteome analysis of the Atlantic salmon (Salmo salar) cell line SHK-1 following recombinant IFN-γ stimulation

Type II IFN exists as a single molecule (IFN-gamma) in contrast to type I IFN, of which there are a number of different forms. IFN-gamma is involved both directly and indirectly in antiviral activity, stimulation of bactericidal activity, antigen presentation and activation of macrophages. Recently IFN-gamma was cloned from a salmonid fish, the rainbow trout and a functional recombinant protein produced exhibited IFN-gamma activity. This recombinant IFN-gamma was used to stimulate an Atlantic salmon cell line, SHK-1, to monitor the changes in protein expression by proteomic analysis 24 h after stimulation compared to unstimulated control cells. An SHK-1 cell proteome map was developed and proteins altered in abundance by the IFN-gamma stimulation were identified. Under the analytical conditions used, 22 proteins were found to be altered in abundance, 15 increased and 7 decreased. Several proteins were excised from the gel and identified, following trypsin digestion and MALDI-MS/MS/LC-MS and database interrogation. Transcriptional analysis of five mRNAs encoding proteins increased in abundance by IFN-gamma in the proteome analysis was determined by real-time PCR. We assessed the correlation between gene expression and change in abundance of proteins for these genes.

Recruitment of Hsp70 chaperones: a crucial part of viral survival strategies

Virus proliferation depends on the successful recruitment of host cellular components for their own replication, protein synthesis, and virion assembly. In the course of virus particle production a large number of proteins are synthesized in a relatively short time, whereby protein folding can become a limiting step. Most viruses therefore need cellular chaperones during their life cycle. In addition to their own protein folding problems viruses need to interfere with cellular processes such as signal transduction, cell cycle regulation and induction of apoptosis in order to create a favorable environment for their proliferation and to avoid premature cell death. Chaperones are involved in the control of these cellular processes and some viruses reprogram their host cell by interacting with them. Hsp70 chaperones, as central components of the cellular chaperone network, are frequently recruited by viruses. This review focuses on the function of Hsp70 chaperones at the different stages of the viral life cycle emphasizing mechanistic aspects.

Increased sensitivity to dextran sodium sulfate colitis in IRE1beta-deficient mice

The epithelial cells of the gastrointestinal tract are exposed to toxins and infectious agents that can adversely affect protein folding in the endoplasmic reticulum (ER) and cause ER stress. The IRE1 genes are implicated in sensing and responding to ER stress signals. We found that epithelial cells of the gastrointestinal tract express IRE1beta, a specific isoform of IRE1. BiP protein, a marker of ER stress, was elevated in the colonic mucosa of IRE1beta(-/-) mice, and, when exposed to dextran sodium sulfate (DSS) to induce inflammatory bowel disease, mutant mice developed colitis 3-5 days earlier than did wild-type or IRE1beta(+/-) mice. The inflammation marker ICAM-1 was also expressed earlier in the colonic mucosa of DSS-treated IRE1beta(-/-) mice, indicating that the mutation had its impact early in the inflammatory process, before the onset of mucosal ulceration. These findings are consistent with a model whereby perturbations in ER function, which are normally mitigated by the activity of IRE1beta, participate in the development of colitis.

Polypyrimidine tract-binding protein inhibits translation of bip mRNA

Translation initiation of human Bip mRNA is directed by an internal ribosomal entry site (IRES) located in the 5' non-translated region. No trans-acting factor possibly involved in this process has as of yet been identified. For the encephalomyocarditis virus and other picornaviruses, polypyrimidine tract-binding protein (PTB) has been found to enhance the translation through IRES elements, probably by interaction with the IRES structure. Here, we report that PTB specifically binds to the central region (nt 50-117) of the Bip 5' non-translated region. Addition of purified PTB to rabbit reticulocyte lysate and overexpression of PTB in Cos-7 cells selectively inhibited Bip IRES-dependent translation. On the other hand, depletion of endogenous PTB or addition of an RNA interacting with PTB enhanced the translational initiation directed by Bip IRES. These suggest that PTB can either enhance or inhibit IRES-dependent translation depending on mRNAs.

Lead targets GRP78, a molecular chaperone, in C6 rat glioma cells

Exposure to potentially neurotoxic levels of lead (Pb) occurs in about 9% of American children under 6 years of age. Astroglia in the brain serve as a Pb depot, sequestering Pb and preventing its contact with the more sensitive neurons. Astroglia have the capacity to adapt to Pb exposure, and as such are able to tolerate relatively high intracellular Pb accumulation. This tolerance mechanism has yet to be defined in biochemical terms. In the present study, we present evidence that glucose-regulated protein (GRP78), a molecular chaperone in the ER, participates directly or indirectly in the tolerance mechanism. Exposure of cultured C6 rat glioma cells, an astroglia-like cell line, to 1 microM Pb acetate for 1 week raised the intracellular levels of two proteins, one of which was identified by sequence analysis as GRP78. GRP78 accumulation started within 1 day and progressed with time of exposure. Studies in vitro showed that GRP78 bound tightly to affinity columns with Pb(2+) as the affinity ligand and bound weakly when either Zn(2+) or Ni(2+) replaced the Pb(2+). The reduced form of GSH and BSA did not compete with GRP78 to chelate Pb(2+). However, the heavy metal binding domain (HMB) of Menkes protein competed with GRP78 for chelating Pb(2+). The data provide evidence that GRP78 may be a component of the Pb tolerance mechanism through its direct interaction with Pb(2+). Its increased synthesis could be part of the adaptive response to Pb exposure.

Expression of cell death regulatory genes and limited apoptosis induction in avian blastodermal cells

Apoptosis is a well-established cellular mechanism for selective cell deletion during development. However, little is known about the expression of an apoptotic pathway and its role in determining the relative sensitivity of the early, pre-gastrula, avian embryo to stress-induced cell death. We examined the sensitivity of avian blastodermal cells to engage in apoptosis upon exposure to etoposide, a topoisomerase II-inhibitor that rapidly and efficiently induces apoptosis in many cell types. We found that while the blastodermal cells are capable of engaging in apoptosis, they are highly resistant to such induction with respect to both concentration of drug required and length of exposure, even when compared to a tumor cell line with a known multi-drug resistant phenotype. Additionally, we assessed the expression of several candidate regulatory genes in blastodiscs from infertile eggs (i.e., maternal RNA transcripts), blastodermal cells immediately following oviposition, and various stages of early development up to gastrulation. This analysis revealed that several genes whose products have anti-apoptotic activity, including bcl-2, bcl-xL, hsp70, grp78 and the glutathione S-transferases, are expressed as early as the stage 1 embryo in the newly oviposited egg. These transcripts are also found in the infertile blastodisc, suggesting a role for maternally derived transcripts in the protection of the oocyte and zygote. Significantly, constitutive levels of hsp70 mRNA exceeded those of the other anti-apoptotic genes in the blastodermal cells. These results contribute to an emerging picture of stress resistance at the earliest stages of avian embryo development which involves multiple anti-apoptotic genes that act at different regulatory points in the apoptotic cascade.

Heat shock proteins in retinal neurogenesis: identification of the PM1 antigen as the chick Hsc70 and its expression in comparison to that of other chaperones

While the role of heat shock proteins under experimental stress conditions is clearly characterized, their expression in unstressed cells and tissues and their functions in normal cell physiology, besides their chaperone action, remain largely undetermined. We report here the identification in chicken of the antigen recognized by the monoclonal antibody PM1 [Hernández-Sánchez et al. (1994) Eur. J. Neurosci., 6,1801-1810] as the noninducible chaperone heat-shock cognate 70 (Hsc70). Its identity was determined by partial peptide sequencing, immuno-crossreactivity and two-dimensional gel-electrophoresis. In addition, we examined its expression during chick embryo retinal neurogenesis. The early widespread Hsc70 immunostaining corresponding to most, if not all, of the neuroepithelial cells becomes restricted to a subpopulation of these cells in the peripheral retina as development proceeds. On the other hand, retinal ganglion cells, differentiating in the opposite central-to-peripheral gradient, retained Hsc70 immunostaining. Other molecular chaperones, the heat-shock proteins Hsp40, Hsp60 and Hsp90, did not seem to compensate the loss of Hsc70. They also showed decreasing immunostaining patterns as neurogenesis proceeds, although distinctive from that of Hsc70, whereas Hsp70 was not detected in the embryonic retina. This precise cellular and developmental regulation of Hsc70, a generally considered constitutive molecular chaperone, in unstressed embryos, together with the expression of other chaperones, provides new tools and a further insight on neural precursor heterogeneity, and suggests possible specific cellular roles of chaperone function during vertebrate neurogenesis.

Isolation and characterization of a cDNA encoding a Xenopus immunoglobulin binding protein, BiP (grp78)

We have isolated a full-length cDNA clone encoding a Xenopus laevis immunoglobulin binding protein (BiP; also called glucose-regulated protein or grp78). The Bip cDNA sequence includes an open reading frame of 1,965 bp encoding a 655 amino acid protein with an N-terminal hydrophobic leader sequence and a C-terminal KDEL tetrapeptide which has been found in other lumenal proteins of the endoplasmic reticulum. The 3' untranslated region contains a polyadenylation and an adenylation control element (ACE) as well as a putative mRNA instability sequence. The Xenopus BiP amino acid sequence displayed high identity with BiP from other vertebrates including chicken (91.3%), rat (90.7%), and human (89.9%). Northern hybridization analysis demonstrated that BiP mRNA was present constitutively in the Xenopus A6 kidney epithelial cell line and that BiP mRNA levels could be enhanced by treatment of the cells with galactose-free media, 2-deoxyglucose, 2-deoxygalactose, glucosamine, tunicamycin, heat shock, dithiothreitol, and the calcium ionophore, A23187. Finally, while BiP mRNA was detected in all of the adult tissues examined, the relative level of BiP mRNA differed dramatically between organs. For example, relatively high levels of BiP mRNA were detected in liver with moderate levels in testis, ovary and heart and reduced levels in eye and muscle tissue.

Alteration of in vivo DNA synthesis in the alpha globin locus of chick embryo fibroblasts due to in vivo activity of Rous sarcoma virus pp60src

Globin gene transcription is activated in chick embryo fibroblasts (CEF) transformed by Rous sarcoma virus (RSV). To determine whether this activation is correlated with a change in the replication of the alpha D globin locus we used a nuclear runoff replication assay. Density labeling of nuclei showed that replication of the alpha D globin gene in normal and RSV-transformed CEF (RSV-CEF) is in the transcriptional direction. However, in RSV-CEF the signal from a specific restriction fragment between the alpha D and alpha A globin genes was lower in the replicated DNA. This decrease was selective in that the signal of this restriction fragment was not diminished in the unreplicated DNA in the same nuclei. When CEF transformed by temperature sensitive RSV were grown at the restrictive temperature for pp60src activity the loss of transformed cell morphology correlated with the reappearance of the replicated alpha globin DNA fragments in their normal stoichiometry. By comparison, histone H5 gene expression was not seen in CEF or in RSV-CEF, and H5 gene replication was not perturbed by viral transformation. These results suggest that the viral pp60src protein can activate transcription of the alpha D globin gene and change the replication of alpha globin chromatin.