title: Changes in the proteolytic activities of proteasomes and lysosomes in human fibroblasts produced by serum withdrawal, amino-acid deprivation and confluent conditions.
abstract: The contribution of the main proteolytic pathways to the degradation of long-lived proteins in human fibroblasts grown under different conditions was investigated. The effects of various commonly used pharmacological inhibitors of protein degradation were first analysed in detail. By choosing specific inhibitors of lysosomes and proteasomes, it was observed that together both pathways accounted for 80% or more of the degradation of cell proteins. With lysosomal inhibitors, it was found that serum withdrawal or amino-acid deprivation strongly stimulated macroautophagy but not other lysosomal pathways, whereas confluent conditions had no effect on macroautophagy and slightly activated other lysosomal pathways. Prolonged (24 h) serum starvation of confluent cultures strongly decreased the macroautophagic pathway, whereas the activity of other lysosomal pathways increased. These changes correlated with electron microscopic observations and morphometric measurements of lysosomes. With proteasomal inhibitors, it was found that, in exponentially growing cells in the absence of serum, activity of the ubiquitin-proteasome pathway increases, whereas under confluent conditions the contribution (in percentage) of proteasomes to degradation decreases, especially in cells deprived of amino acids. Interestingly, in confluent cells, the levels of two components of the 19 S regulatory complex and those of an interchangeable beta-subunit decreased. This was associated with a marked increase in the levels of components of PA28-immunoproteasomes. Thus confluent conditions affect proteasomes in a way that resembles treatment with interferon-gamma. Altogether, these results show that the activity of the various proteolytic pathways depends on the growth conditions of cells and will be useful for investigation of the specific signals that control their activity.
authors.affiliation: Instituto de Investigaciones Citológicas, Fundación Valenciana de Investigaciones Biomédicas, Amadeo de Saboya 4, 46010-Valencia, Spain.
authors.forename: Graciela
authors.initials: G
authors.lastname: Fuertes
authors.forename: José Javier
authors.initials: JJ
authors.lastname: Martín De Llano
authors.forename: Adoración
authors.initials: A
authors.lastname: Villarroya
authors.forename: A Jennifer
authors.initials: AJ
authors.lastname: Rivett
authors.forename: Erwin
authors.initials: E
authors.lastname: Knecht
mesh_terms: D000596:Amino Acids / Q000378:metabolism; D018929:Cell Culture Techniques; D002455:Cell Division; D002478:Cells, Cultured; D016895:Culture Media, Serum-Free; D003546:Cysteine Endopeptidases / Q000187:drug effects / Q000378:metabolism*; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D005347:Fibroblasts / Q000187:drug effects / Q000201:enzymology* / Q000648:ultrastructure; D006801:Humans; D008247:Lysosomes / Q000187:drug effects / Q000201:enzymology* / Q000648:ultrastructure; D009097:Multienzyme Complexes / Q000187:drug effects / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D011506:Proteins / Q000378:metabolism
chemical_list: D000596:Amino Acids; D016895:Culture Media, Serum-Free; D015853:Cysteine Proteinase Inhibitors; D009097:Multienzyme Complexes; D011506:Proteins; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex