Repositioning of Verrucosidin, a Purported Inhibitor of Chaperone Protein GRP78, as an Inhibitor of Mitochondrial Electron Transport Chain Complex I

Abstract

Verrucosidin (VCD) belongs to a group of fungal metabolites that were identified in screening programs to detect molecules that preferentially kill cancer cells under glucose-deprived conditions. Its mode of action was proposed to involve inhibition of increased GRP78 (glucose regulated protein 78) expression during hypoglycemia. Because GRP78 plays an important role in tumorigenesis, inhibitors such as VCD might harbor cancer therapeutic potential. We therefore sought to characterize VCD’s anticancer activity in vitro. Triple-negative breast cancer cell lines MDA-MB-231 and MDA-MB-468 were treated with VCD under different conditions known to trigger increased expression of GRP78, and a variety of cellular processes were analyzed. We show that VCD was highly cytotoxic only under hypoglycemic conditions, but not in the presence of normal glucose levels, and VCD blocked GRP78 expression only when glycolysis was impaired (due to hypoglycemia or the presence of the glycolysis inhibitor 2-deoxyglucose), but not when GRP78 was induced by other means (hypoxia, thapsigargin, tunicamycin). However, VCD’s strictly hypoglycemia-specific toxicity was not due to the inhibition of GRP78. Rather, VCD blocked mitochondrial energy production via inhibition of complex I of the electron transport chain. As a result, cellular ATP levels were quickly depleted under hypoglycemic conditions, and common cellular functions, including general protein synthesis, deteriorated and resulted in cell death. Altogether, our study identifies mitochondria as the primary target of VCD. The possibility that other purported GRP78 inhibitors (arctigenin, biguanides, deoxyverrucosidin, efrapeptin, JBIR, piericidin, prunustatin, pyrvinium, rottlerin, valinomycin, versipelostatin) might act in a similar GRP78-independent fashion will be discussed.

Mitochondrial Respiratory Chain Inhibitors Involved in ROS Production Induced by Acute High Concentrations of Iodide and the Effects of SOD as a Protective Factor

“…Cillero-Pastor et al used 10 and 50 μ g/mL Rotenone to culture human chondrocytes, to investigate mRNA expression [ 33 ]. 100 nM Rotenone was treated to hypoglycemic cells to express the inhibitory effect [ 34 ]. There is ROS production in the presence of Rotenone when digitonin protein solubilizates with glycerol-3-phosphate (GP) [ 35 ].…”

Pharmacological induction of cell surface GRP78 contributes to apoptosis in triple negative breast cancer cells

“…We demonstrated that GRP78 negative expression on breast cancer tumor cells correlated with the absence of ER, PR and HER2 receptors in this TNBC. Although we focused our study to the MDAMB468 TNBC line, others also described low levels of GRP78 expression in TNBC cells lines such as MDAMB231 [ 29 ], HCC1937 and BT-20 [ 30 , 31 ]. We first evaluated the effect of doxorubicin and taxotere on cell surface GRP78 expression in the human breast cancer cell line, since we have previously shown that these drugs induced cell surface GRP78 expression in human colon cancer cells [ 32 ].…”

Glucose-regulated proteins in cancer: molecular mechanisms and therapeutic potential

“…This implies that agents that selectively block the stress induction of GRP78 will affect tumours that require a high level of GRP78 and spare normal organs. Natural compounds with anti-cancer properties that suppress GRP78 induction have been reported, however, they exert pleiotrophic effects 5,120 . Specific cancers also express microRNAs that can act cooperatively to suppress GRP78 translation (Figure 3) and reverse chemoresistance 121 .…”