Autophagy in mammalian cells

Autophagy is a regulated process for the degradation of cellular components that has been well conserved in eukaryotic cells. The discovery of autophagy-regulating proteins in yeast has been important in understanding this process. Although many parallels exist between fungi and mammals in the regulation and execution of autophagy, there are some important differences. The pre-autophagosomal structure found in yeast has not been identified in mammals, and it seems that there may be multiple origins for autophagosomes, including endoplasmic reticulum, plasma membrane and mitochondrial outer membrane. The maturation of the phagophore is largely dependent on 5’-AMP activated protein kinase and other factors that lead to the dephosphorylation of mammalian target of rapamycin. Once the process is initiated, the mammalian phagophore elongates and matures into an autophagosome by processes that are similar to those in yeast. Cargo selection is dependent on the ubiquitin conjugation of protein aggregates and organelles and recognition of these conjugates by autophagosomal receptors. Lysosomal degradation of cargo produces metabolites that can be recycled during stress. Autophagy is an important cellular safeguard during starvation in all eukaryotes; however, it may have more complicated, tissue specific roles in mammals. With certain exceptions, autophagy seems to be cytoprotective, and defects in the process have been associated with human disease.

Autophagy in the stress-induced myocardium

Cardiovascular disease is a leading cause of death worldwide, particularly in Western societies. During an ischaemic insult, ventricular pressure from the heart is diminished as a result of cardiac myocyte death by necrosis and apoptosis. Autophagy is a process whereby cells catabolise intracellular proteins in order to generate ATP in times of stress such as nutrient starvation and hypoxia. Emerging evidence suggests that autophagy plays a positive role in cardiac myocyte survival during periods of cellular stress performing an important damage limitation function. By promoting cell survival, cardiac myocyte loss is reduced thereby minimising the potential of heart failure. In contrast, it has been reported that autophagy can also be a form of cell death. By considering the various animal models of autophagy, we examine the role of the Signal Transducers and Activator of Transcription (STAT) proteins in the autophagic response. Additionally we review the role of the tumour suppressor, p53 and its family member p73 and their potential role in the autophagic response.

Autophagy in the stress-induced myocardium

Cardiovascular disease is a leading cause of death worldwide, particularly in Western societies. During an ischaemic insult, ventricular pressure from the heart is diminished as a result of cardiac myocyte death by necrosis and apoptosis. Autophagy is a process whereby cells catabolise intracellular proteins in order to generate ATP in times of stress such as nutrient starvation and hypoxia. Emerging evidence suggests that autophagy plays a positive role in cardiac myocyte survival during periods of cellular stress performing an important damage limitation function. By promoting cell survival, cardiac myocyte loss is reduced thereby minimising the potential of heart failure. In contrast, it has been reported that autophagy can also be a form of cell death. By considering the various animal models of autophagy, we examine the role of the Signal Transducers and Activator of Transcription (STAT) proteins in the autophagic response. Additionally we review the role of the tumour suppressor, p53 and its family member p73 and their potential role in the autophagic response.

Activation of Src protein tyrosine kinase plays an essential role in urocortin-mediated cardioprotection

Urocortin is a 40 amino acid peptide of the corticotrophin-releasing factor (CRF) family that is synthesized and released by cardiac myocytes. Endogenous urocortin expression is increased during ischemia/reperfusion (I/R) and addition of exogenous urocortin reduces cell death caused by I/R injury. Studies have also showed that the protective action of urocortin is mediated by the activation of ERK1/2. We discovered that a non-receptor tyrosine kinase, Src, is involved in the urocortin-induced activation of ERK1/2 in mouse atrial HL-1 myocytes. The selective Src family kinase inhibitor, PP2, reduced the urocortin-induced phosphorylation of ERK1/2, and so did the expression of a dominant-negative mutant of Src in transfected HL-1 cells. Inhibition of Src by PP2 also reduced urocortin's protective effects in HL-1 cells after hypoxia/reoxygenation (H/R), as assessed by flow cytometry and caspase-3 activation assay. Titration studies indicated that as little as 10(-8)M urocortin was sufficient to induce Src activation. Maximal phosphorylation/activation of Src and ERK1/2 were both detected after 5 min incubation with urocortin. These effects of urocortin were largely mediated by CRF receptor-1, although a minor contribution of CRF receptor-2 cannot be excluded. Here we report for the first time that short-term treatment with urocortin causes rapid phosphorylation of Src, and that the urocortin-activated Src kinase serves as an upstream modulator of ERK1/2 activation, playing an essential role in urocortin-mediated cardioprotection.

Effects of cathepsins B and L inhibition on postischemic protein alterations in the brain

The effects of selective inhibition of cathepsins B and L on postischemic protein alterations in the brain were investigated in a rat model of middle cerebral artery occlusion (MCAO). Cathepsin B activity increased predominantly in the subcortical region of the ischemic hemisphere where the levels of collapsing mediator response protein 2, heat shock cognate 70 kDa protein, 60 kDa heat shock protein, protein disulfide isomerase A3 and albumin, were found to be significantly elevated. Postischemic treatment with Cbz-Phe-Ser(OBzl)-CHN(2), cysteine protease inhibitor 1 (CP-1), reduced infarct volume, neurological deficits and cathepsin B activity as well as the amount of heat shock proteins and albumin found in the brain. Our data strongly suggests that the decrease in heat shock protein levels and the significant reduction of serum albumin leakage into the brain following acute treatment with CP-1 is indicative of less secondary ischemic damage, which ultimately, is related to less cerebral tissue loss and improved neurological recovery of the animals.