Novel Roles for Protein Kinase Cδ-dependent Signaling Pathways in Acute Hypoxic Stress-induced Autophagy

Macroautophagy, a tightly orchestrated intracellular process for bulk degradation of cytoplasmic proteins or organelles, is believed to be essential for cell survival or death in response to stress conditions. Recent observations indicate that autophagy is an adaptive response in cells subjected to prolonged hypoxia. However, the signaling mechanisms that activate autophagy under acute hypoxic stress are not clearly understood. In this study, we show that acute hypoxic stress by treatment with 1% O-2 or desferroxamine, a hypoxia-mimetic agent, of cells renders a rapid induction of LC3-II level changes and green fluorescent protein-LC3 puncta accumulation, hallmarks of autophagic processing, and that this process involves protein kinase C delta (PKC delta), and occurs prior to the induction of BNIP3 (Bcl-2/adenovirus E1B 19-kDa interacting protein 3). Interestingly, hypoxic stress leads to a rapid and transient activation of JNK in Pa-4 or mouse embryo fibroblast cells. Acute hypoxic stress-induced changes in LC3-II level and JNK activation are attenuated in Pa-4 cells by dominant negative PKC delta KD or in mouse embryo fibroblast/PKC delta-null cells. Intriguingly, the requirement of PKC delta is not apparent for starvation-induced autophagy. The importance of PKC delta in hypoxic stress-induced adaptive responses is further supported by our findings that inhibition of PKC delta-facilitated autophagy by 3-methyladenine or Atg5 knock-out renders a greater prevalence of cell death following prolonged desferroxamine treatment, whereas PKC delta- or JNK1-deficient cells exhibit resistance to extended hypoxic exposure. These results uncover dual roles of PKC delta-dependent signaling in the cell fate determination upon hypoxic exposure.