Our results suggest that autophagy also participates in the clearance of the accumulated deposits. == Methods == == Cell culture and treatments == ARPE-19 human RPE cells were obtained from the American Type Culture Collection (ATCC). evaluate the morphological changes. Hsp70 levels were modulated using RNA interference and overexpression techniques. Cell viability was measured by colorimetric assay. The proteasome inhibitor MG-132 evoked the accumulation of perinuclear aggregates positive for Hsp70, ubiquitin-protein conjugates and the lysosomal membrane protein LAMP-2. Interestingly, the hsp70 mRNA depletion significantly increased cell death in conjunction with proteasome inhibition. We found that the accumulation of lysosomes was reversible: a cessation of proteasome inhibition led to clearance of the depositsviaa mechanism believed to include autophagy. The molecular chaperone Hsp70, proteasomes and autophagy have an important regulatory role in the protein turnover of human RPE cells and may thus open new avenues for understanding degenerative processes in retinal cells. Keywords: autophagosome, Hsp70, lysosome, proteasome, proteolysis, retinal pigment epithelium == Introduction == Cells and tissues are regularly exposed to different kinds of acute and chronic environmental stresses. Retinal pigment epithelial (RPE) cells must endure a high level of oxidative stress because of their high oxygen consumption, high levels of polyunsaturated lipids and long periods of exposure to light [1]. Oxidative stress refers to the progressive cellular damage caused by reactive oxygen species, which contributes to protein misfolding and evokes functional abnormalities during cellular senescence of the RPE cells [1, 2]. Sophoradin A disturbance of cellular homeostasis in the post-mitotic RPE cells is involved in the development of age-related macular degeneration (AMD), which is the most common cause of blindness in developed countries [1, 3]. The efficient removal of misfolded proteins from the cytoplasm is critical for cellular survival and adaptation. Cells can avoid the accumulation of potentially toxic misfolded protein aggregates by accessing a battery of mechanisms. One of the most important of these is the suppression of aggregate formation by heat shock proteins (Hsps), which function as molecular chaperones in cells Sophoradin [4]. The transcriptionally and post-transcriptionally regulated expression of Hsps can be promoted by many different kinds of chemical or physical stresses [510]. The Hsps are divided into different families (Hsp90, Hsp70, Hsp60, Hsp40 and small Hsps), according to their function, molecular size and cellular localization [11]. Cellular proteins are continuously synthesized and degraded during the lifespan of the cell [12]. The control of protein turnover is particularly important in post-mitotic cells, where the accumulation of malfunctioning proteins may be highly detrimental to cell survival [13]. Four different systems take care of protein degradation in eukaryotic cells: (i) proteasomes, which degrade the majority of long- and short-lived normal and abnormal intracellular proteins, (ii) mitochondrial proteases, which degrade the majority of mitochondrial proteins, (iii) calcium-activated calpains, which degrade membrane and cytoskeletal proteins and several membrane-associated enzymes and (iv) autophagy and lysosomes, which degrade whole cell organelles, membrane proteins and endocytosed proteins [1417]. During RPE cell senescence, the accumulation of lysosomal lipofuscin correlates well with the path-ogenesis of AMD [1]. Many studies have demonstrated the importance of lysosomes for proteolysis in RPE cells [1823]. The accumulated lipofuscin probably has multiple secondary detrimental effects on RPE cells. It has been shown that lipofuscin prevents cellular renewal and promotes the misfolding of intracellular proteins by increasing the sensitivity of RPE cells to oxidative stress [23, 24]. Macroautophagy, microautophagy and chaperone-mediated autophagy are Igf1 the principal pathways to deliver cytoplasmic material for lysosomal degradation in mammalian cells [25, 26]. During macroautophagy, cytoplasmic structures are segregated within autophagosomes, which deliver their contents to lysosomes, while in microautophagy, small portions of cytoplasm enter lysosomesviainvagination of the lysosomal membrane. One of the molecular chaperones, Hsc73, delivers specific proteins into lysosomes during chaperone-mediated autophagy [25, 26]. Although the connection between stress and degeneration of RPE cells is well known, the role of proteasomes and autophagy in protein turnover in RPE cells is largely unexplored. Hsps are closely linked to proteasome-mediated proteolysis, and also to lysosomal protein degradation [25, 26]. Should the Hsp-linked protein folding fail, misfolded proteins are first tagged with a polypeptide called ubiquitin (Ub) and subsequently transferred to proteasomes for Sophoradin degradation [13, 27]. In fact , it is believed that up to 8090% of all intracellular proteins are degraded by the ubiquitin-proteasome proteolysis machinery [15, 16]. However , once the capacity of Hsps to correct proteins is exceeded, or the ubiquitin-proteasome pathway experiences functional limitations, protein aggregates are formed at the cell periphery and delivered along the microtubulus networkviadynein-dependent retrograde transport to a perinuclear location. The aggregates form aggresomes, which may be cleaned through autophagy [13, 28, 29]..