Atrophy is decrease in tissue mass due to decrease in size (increased cytoskeleton degradation via ubiquitin-proteasome pathway and autophagy; decreased protein synthesis) and/or number of cells (apoptosis). Causes include disuse, denervation, loss of blood supply, loss of hormonal stimulation, poor nutrition.
Atrophy in myocytes. Atrophy is due to cytoskeleton degradation via ubiquitin-proteasome pathway.
Polyubiquitination: The binding of many ubiquitin molecules to the same target protein. Polyubiquitination of proteins is the triggering signal that leads to degradation of the protein in the proteasome. It is polyubiquitination that constitutes the "kiss of death" for the protein.