Hsp83 loss suppresses proteasomal activity resulting in an upregulation of caspase-dependent compensatory autophagy

The 2 main degradative pathways that contribute to proteostasis are the ubiquitin-proteasome system and autophagy but how they are molecularly coordinated is not well understood. Here, we demonstrate an essential role for an effector caspase in the activation of compensatory autophagy when proteasomal activity is compromised. Functional loss of Hsp83, the Drosophila ortholog of human HSP90 (heat shock protein 90), resulted in reduced proteasomal activity and elevated levels of the effector caspase Dcp-1. Surprisingly, genetic analyses showed that the caspase was not required for cell death in this context, but instead was essential for the ensuing compensatory autophagy, female fertility, and organism viability. The zymogen pro-Dcp-1 was found to interact with Hsp83 and undergo proteasomal regulation in an Hsp83-dependent manner. Our work not only reveals unappreciated roles for Hsp83 in proteasomal activity and regulation of Dcp-1, but identifies an effector caspase as a key regulatory factor for sustaining adaptation to cell stress in vivo

Regulation and conservation of caspase-activated autophagy

Autophagy is an evolutionarily conserved cellular process that recycles proteins and organelles to maintain cellular homeostasis or provide an alternative source of energy in times of stress. While autophagy promotes cell survival, it can also be regulated by proteins associated traditionally with apoptosis. In an effort to better understand the complex intersections of these disparate cell fates, previous studies in Drosophila identified an apoptotic effector caspase, Dcp-1, as a positive regulator of starvation-induced autophagy. Further, the Drosophila heat-shock protein, Hsp83, was identified as a Dcp-1 interacting protein and a putative negative regulator of autophagy. The aims of my thesis were to investigate the relationship between Dcp-1 and Hsp83 in the context of autophagy, and to determine if caspase-regulated autophagy was functionally conserved in humans. In vivo analyses of Hsp83 loss-of-function mutants in fed conditions showed increases in both autophagic flux and cell death. Hsp83 mutants also had elevated levels of pro-Dcp-1, which was attributed to reduced proteasomal activity. Analyses of an Hsp83/Dcp-1 double mutant revealed that the caspase was not required for cell death in this context but was essential for the ensuing compensatory autophagy, female fertility, and organism viability. These studies not only demonstrated unappreciated roles for Hsp83 in proteasomal activity and new forms of Dcp-1 regulation, but also identified an effector caspase as a key regulatory factor for sustaining adaptation to cell stress in vivo by inducing compensatory autophagy. To address whether effector caspases also regulate starvation-induced autophagy in human cells, caspase-3 (CASP3), a human homolog of Dcp-1, was examined in several human cell lines. These studies showed that CASP3 was required for the upregulation of starvation-induced autophagy in most cell lines examined, but was not required for maintaining basal levels of autophagy. In human cells, another heat-shock family member, HSP60, was identified as a CASP3-interacting protein. HSP60 was shown to negatively regulate autophagy by controlling the subcellular localization of CASP3 in response to nutritional status. Epistasis analyses suggest that the increase in autophagy observed from loss of HSP60 was dependent on the accumulation of cleaved CASP3 in the cytosol. This work highlights a novel function for CASP3 in starvation-induced autophagy in human cells and illustrates how its response is regulated by HSP60-controlled subcellular localization. Altogether, my studies provide novel insights into stress adaptive relationships between heat-shock proteins and caspases in Drosophila and human cells