Unfolded protein response activation contributes to chemoresistance in hepatocellular carcinoma

Colon cancer cells expressing cell surface GRP78 as a marker for reduced tumorigenicity

BACKGROUND

The glucose regulated heat shock protein 78 (GRP78) is a central regulator of ER (endoplasmic reticulum) stress due to its pro-survival property. Up regulated GRP78 expression in tumor cells has been correlated with aggressive malignancies whereas some reports have predicted an improved prognosis. Over-expression of GRP78 in the ER promotes its localization to the cell surface on several cell types including tumor cells.

METHODS

In order to elucidate whether GRP78 receptor positive and negative tumor cells manifest different properties in colorectal cancer, we first artificially separated GRP78 positive and negative sub-populations from HM7 and HCT116 cell lines using anti GRP78 antibody coated magnetic beads.

RESULTS

Only GRP78 negative cells were highly proliferative, induced significant growth in tumor size in nude mice and metastasized to the liver in a human metastatic colorectal carcinoma model in mice. In contrast, GRP78 positive cells manifested reduced proliferation, colony formation, tumor growth and liver metastases. The reduced tumorigenicity of GRP78 positive subpopulation was abrogated by silencing GRP78 expression using siRNA oligomers. In our efforts to induce cell surface GRP78, we subjected the cells to doxorubicin and taxol that increased significantly the percent of GRP78 positive population. Cells pre-incubated with doxorubicin exhibited reduced proliferation and tumor growth in mice.

CONCLUSION

This study demonstrates the significance of cell surface GRP78 in colon cancer, which may be used as a marker for reduced tumorigenicity.

A peptidic unconjugated GRP78/BiP ligand modulates the unfolded protein response and induces prostate cancer cell death

The molecular chaperone GRP78/BiP is a key regulator of protein folding in the endoplasmic reticulum, and it plays a pivotal role in cancer cell survival and chemoresistance. Inhibition of its function has therefore been an important strategy for inhibiting tumor cell growth in cancer therapy. Previous efforts to achieve this goal have used peptides that bind to GRP78/BiP conjugated to pro-drugs or cell-death-inducing sequences. Here, we describe a peptide that induces prostate tumor cell death without the need of any conjugating sequences. This peptide is a sequence derived from the cochaperone Bag-1. We have shown that this sequence interacts with and inhibits the refolding activity of GRP78/BiP. Furthermore, we have demonstrated that it modulates the unfolded protein response in ER stress resulting in PARP and caspase-4 cleavage. Prostate cancer cells stably expressing this peptide showed reduced growth and increased apoptosis in in vivo xenograft tumor models. Amino acid substitutions that destroyed binding of the Bag-1 peptide to GRP78/BiP or downregulation of the expression of GRP78 compromised the inhibitory effect of this peptide. This sequence therefore represents a candidate lead peptide for anti-tumor therapy.

Novel immunohistochemical monoclonal antibody against human glucose-regulated protein 78

Glucose-regulated protein (GRP78), an ER chaperone that belongs to the heat-shock protein (HSP) family, exist in all cells and plays important roles in maintaining cellular homeostasis. GRP78 participates in protein folding, transportation, and degradation. Lack of high affinity antibodies especially monoclonal antibodies (MAbs) suitable for Western blot and immunohistochemical staining has lagged. To gain further insight into its possible functions, we generated a novel MAb specific for hGRP78 in Western blot and immunohistochemistry and localized hGRP78 in some human cancer cell lines and cancer tissues. Immunoreactivity of GRP78 was prominent in Hela, Colo205, and A549 detected by 3F9 in Western blot analysis. 3F9 antibody recognized endogenous GRP78 in human cervical cancer, colonic cancer, esophageal cancer, and lung cancer. Thus, successful production of GRP78 monoclonal antibodies provides a new powerful tool for investigation of GRP78 function.

Modulation of the unfolded protein response is the core of microRNA-122-involved sensitivity to chemotherapy in hepatocellular carcinoma

The loss of microRNA-122 (miR-122) expression correlates to many characteristic properties of hepatocellular carcinoma (HCC) cells, including clonogenic survival, anchorage-independent growth, migration, invasion, epithelial-mesenchymal transition, and tumorigenesis. However, all of these findings do not sufficiently explain the oncogenic potential of miR-122. In the current study, we used two-dimensional differential in-gel electrophoresis to measure changes in the expression of thousands of proteins in response to the inhibition of miR-122 in human hepatoma cells. Several proteins that were upregulated on miR-122 inhibition were involved in the unfolded protein response (UPR) pathway. The overexpression of miR-122 resulted in the repression of UPR pathway activation. Therefore, miR-122 may act as an inhibitor of the chaperone gene expression and negatively regulate the UPR pathway in HCC. We further showed that the miR-122 inhibitor enhanced the stability of the 26S proteasome non-ATPase regulatory subunit 10 (PSMD10) through the up-regulation of its target gene cyclin-dependent kinase 4 (CDK4). This process may activate the UPR pathway to prevent chemotherapy-mediated tumor cell apoptosis. The current study suggests that miR-122 negatively regulates the UPR through the CDK4-PSMD10 pathway. The down-regulation of miR-122 activated the CDK4-PSMD10-UPR pathway to decrease tumor cell anticancer drug-mediated apoptosis. We identified a new HCC therapeutic target and proclaimed the potential risk of the therapeutic use of miR-122 silencing.

Inactivation of PTEN is responsible for the survival of Hep G2 cells in response to etoposide-induced damage

The chemo-resistance character of human hepatocellular carcinoma cells is well known but the anomalies associated with such resistance character are not completely understood. In this study, etoposide-induced signaling events in human hepatocellular carcinoma cell line, Hep G2 has been compared with Chang Liver cells, a normal human liver cell line. Hep G2 cells are resistant to etoposide when compared with Chang Liver cells. Etoposide-induced γH2AX foci in Hep G2 cells are persisted for a longer time without affecting cell cycle, indicating that Hep G2 cells are able to maintain its growth with damaged DNA. Further, Akt signaling pathway is deregulated in Hep G2 cells. The upstream negative regulator of Akt, PTEN remains inactive, as it is hyperphosphorylated in Hep G2 cells. Inhibition of PI-3K pathway by wortmannin partially reverses the etoposide-resistance character of Hep G2 cells. Either Hep G2 or Chang Liver cells when transfected with plasmid carrying active Akt (myr-Akt) become resistance towards etoposide compared to the cells transfected with empty vectors or kinase defective Akt. Transient transfection of wild type PTEN in Hep G2 cells does not change its response towards etoposide whereas Chang Liver cells become sensitive after transfection with same plasmid. These results suggest that inactivation of PTEN, which renders activation of Akt, may contribute largely for the etoposide-resistance character of Hep G2 cells.

Proteomic profiling of human hepatocellular carcinoma SMMC-7721 cells under endoplasmic reticulum stress induced with dithiothreitol

AIM: To explore new therapy targets for human hepatocellular carcinoma by proteomic profiling of human hepatocellular carcinoma SMMC-7721 cells under endoplasmic reticulum stress.

METHODS: Cultured SMMC-7721 cells were divided into two groups: experimental group and control group. The experimental group was treated with dithiothreitol (DTT, 2.5 mmol/L), while the control group was treated with equal volume of culture medium. After treatment, total cell proteins were prepared and resolved by two-dimensional electrophoresis (2-DE). The two-dimensional electrophoresis maps for the two groups of cells were analyzed using ImageMaster 2D Platinum software. Proteins that showed obvious expression alteration in the experimental group were identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS).

RESULTS: There were 844 ± 46 protein spots in the 2-DE map for the experimental group and 1 015 ± 63 protein spots for the control group. There were 593 ± 23 pairs of matched protein spots between the two groups, and the matching rate was about 71%. Most of the proteins have an isoelectric point at pH5.2-6.5 and a molecular weight of 15 000-80 000 Da. Three protein spots showed 2-fold or greater differential expression between the two groups and were identified by MALDI-TOF-MS. They were protein fem-1 homolog B, cyclin A1, and proliferation-inducing protein 44.

CONCLUSION: Three differentially expressed proteins in SMMC-7721 cells under endoplasmic reticulum stress were identified and may be useful molecular targets for the diagnosis and treatment of hepatocellular carcinoma.

The unfolded protein response in human corneal endothelial cells following hypothermic storage: implications of a novel stress pathway

Human corneal endothelial cells (HCEC) have become increasingly important for a range of eye disease treatment therapies. Accordingly, a more detailed understanding of the processing and preservation associated stresses experienced by corneal cells might contribute to improved therapeutic outcomes. To this end, the unfolded protein response (UPR) pathway was investigated as a potential mediator of corneal cell death in response to hypothermic storage. Once preservation-induced failure had begun in HCECs stored at 4°C, it was noted that necrosis accounted for the majority of cell death but with significant apoptotic involvement, peaking at several hours post-storage (4-8h). Western blot analysis demonstrated changes associated with apoptotic activation (caspase 9, caspase 3, and PARP cleavage). Further, the activation of the UPR pathway was observed through increased and sustained levels of ER folding and chaperone proteins (Bip, PDI, and ERO1-Lα) in samples experiencing significant cell death. Modulation of the UPR pathway using the specific inhibitor, salubrinal, resulted in a 2-fold increase in cell survival in samples experiencing profound cold-induced failure. Furthermore, this increased cell survival was associated with increased membrane integrity, cell attachment, and decreased necrotic cell death populations. Conversely, addition of the UPR inducer, tunicamycin, during cold exposure resulted in a significant decrease in HCEC survival during the recovery period. These data implicate for the first time that this novel cell stress pathway may be activated in HCEC as a result of the complex stresses associated with hypothermic exposure. The data suggest that the targeted control of the UPR pathway during both processing and preservation protocols may improve cell survival and function of HCEC thus improving the clinical utility of these cells as well as whole human corneas.

Unfolded protein response in cancer: the physician's perspective

The unfolded protein response (UPR) is a cascade of intracellular stress signaling events in response to an accumulation of unfolded or misfolded proteins in the lumen of the endoplasmic reticulum (ER). Cancer cells are often exposed to hypoxia, nutrient starvation, oxidative stress and other metabolic dysregulation that cause ER stress and activation of the UPR. Depending on the duration and degree of ER stress, the UPR can provide either survival signals by activating adaptive and antiapoptotic pathways, or death signals by inducing cell death programs. Sustained induction or repression of UPR pharmacologically may thus have beneficial and therapeutic effects against cancer. In this review, we discuss the basic mechanisms of UPR and highlight the importance of UPR in cancer biology. We also update the UPR-targeted cancer therapeutics currently in clinical trials.

Oncostatin M up-regulates the ER chaperone Grp78/BiP in liver cells

OSM, a cytokine of the IL-6-type cytokine family, regulates inflammatory processes (like the acute phase response), tissue remodeling, angiogenesis, cell differentiation and proliferation. Inflammation is discussed to favor carcinogenesis and the inflammatory cytokine OSM was lately described to up-regulate HIF-1α, whose up-regulation is also observed in many cancers. In this study we demonstrate that OSM, and to a lesser degree IL-6, induces the expression of Grp78/BiP, an ER chaperone associated with tumor development and poor prognosis in cancer. In contrast, IFN-γ or TNF-α had no effect on Grp78 expression. The up-regulation seems to be specific to liver cells, as it occurs in hepatocytes and hepatoma cells but not in prostate, melanoma, breast or kidney cells. OSM does not lead to up-regulation of Grp94, enhanced XBP-1 mRNA splicing or phosphorylation of eIF2α, indicating that it is not associated to a general ER stress response. Analysis of the underlying mechanism showed that Grp78 is up-regulated by transcriptional processes which are to the greater part, though not completely, dependent on MEK/Erk activation.