Changes in cellular autophagic capacity during azaserine-initiated pancreatic carcinogenesis

Cytoprotective effect of the elongation factor-2 kinase-mediated autophagy in breast cancer cells subjected to growth factor inhibition

Background

Autophagy is a highly conserved and regulated cellular process employed by living cells to degrade proteins and organelles as a response to metabolic stress. We have previously reported that eukaryotic elongation factor-2 kinase (eEF-2 kinase, also known as Ca²⁺/calmodulin-dependent protein kinase III) can positively modulate autophagy and negatively regulate protein synthesis. The purpose of the current study was to determine the role of the eEF-2 kinase-regulated autophagy in the response of breast cancer cells to inhibitors of growth factor signaling.

Methodology/Principal Findings

We found that nutrient depletion or growth factor inhibitors activated autophagy in human breast cancer cells, and the increased activity of autophagy was associated with a decrease in cellular ATP and an increase in activities of AMP kinase and eEF-2 kinase. Silencing of eEF-2 kinase relieved the inhibition of protein synthesis, led to a greater reduction of cellular ATP, and blunted autophagic response. We further showed that suppression of eEF-2 kinase-regulated autophagy impeded cell growth in serum/nutrient-deprived cultures and handicapped cell survival, and enhanced the efficacy of the growth factor inhibitors such as trastuzumab, gefitinib, and lapatinib.

Conclusion/Significance

The results of this study provide new evidence that activation of eEF-2 kinase-mediated autophagy plays a protective role for cancer cells under metabolic stress conditions, and that targeting autophagic survival may represent a novel approach to enhancing the effectiveness of growth factor inhibitors.

Immunohistochemical expression of MAP1LC3A and MAP1LC3B protein in breast carcinoma tissues

Autophagy is a protein degradation process within the cell and its deregulation has been linked to various diseases and the formation of cancer. One of the important proteins involved in the autophagy process is microtubule-associated protein 1 light chain 3 (MAP1LC3). The aims of this study were to determine the MAP1LC3A and MAP1LC3B protein expression in both normal and cancer breast tissues and to determine the relationship between the expression of these proteins and type of tissues. Immunohistochemistry assessments were carried out on tissue microarrays consisting of breast tissues. MAP1LC3A expression was detected in 52/56 of normal breast tissue cores and 65/67 of breast cancer tissue cores. MAP1LC3B expression was detected in 55/56 of normal breast tissue cores and 67/67 of breast cancer tissue cores. MAP1LC3A and MAP1LC3B protein are expressed in the majority of normal and cancer breast tissues. A large number of MAP1LC3A and MAP1LC3B positive breast cancer tissues cores have high proportion of stained cells (81-100%) as compared with normal breast tissues. However, a significantly higher number of breast cancer tissues were found to express the MAP1LC3A protein with strong immunoreactivity as compared with the normal tissues, suggesting that MAP1LC3A may play a role in breast cancer development.

Arsenic salts induced autophagic cell death and hypermethylation of DAPK promoter in SV-40 immortalized human uroepithelial cells

Arsenic is a well-known toxic and carcinogenic agent, and associated with various human malignancies, including skin, lung and bladder cancers. Paradoxically, arsenic trioxide has been used successfully in the treatment of patients with acute promyelocytic leukemia. In addition, arsenic could induce cell apoptosis or autophagy in malignant cells. However, the underlying mechanism of arsenic-induced carcinogenesis is still unclear. In this study, we demonstrated an increase of autophagosomes was produced in arsenic-treated SV-HUC-1 cells by using electron microscopy. In addition, increase of Beclin-1, an important regulator for the formation of autophagosome, protein expression in a dose-dependent manner was also found. By using methylation specific PCR, we revealed hypermethylation of CpG sites in the promoter region with decreased DAPK protein expression in arsenic-treated SV-HUC-1 cells. As epigenetic silencing of tumor suppressor genes by promoter hypermethylation has been found in a variety of malignancies including bladder cancer, our results provide new insights for the understanding of the mechanism of arsenic-induced carcinogenesis in urothelial cells.