NF-κB regulates protein quality control after heat stress through modulation of the BAG3–HspB8 complex

Abstract

SummaryWe previously found that the NF-kB transcription factor is activated during the recovery period after heat shock; moreover, we demonstrated that NF-kB is essential for cell survival after heat shock by activating autophagy, a mechanism that probably helps the cell to cope with hyperthermic stress through clearance of damaged proteins. In this study, we analyze the involvement of NF-kB in basal and heat-stress-induced protein quality control, by comparing the level of multiubiquitylated and/or aggregated proteins, and proteasome and autophagic activity in NF-kB-competent and NF-kB-incompetent cells. We show that NF-kB has only a minor role in basal protein quality control, where it modulates autophagosome maturation. By contrast, NF-kB is shown to be a key player in protein quality control after hyperthermia. Indeed, NF-kB-incompetent cells show highly increased levels of multiubiquitylated and/or aggregated proteins and aggresome clearance defects; a phenotype that disappears when NF-kB activity is restored to normal. We demonstrate that during heat shock recovery NF-kB activates selective removal of misfolded or aggregated proteins -a process also called 'aggrephagy' -by controlling the expression of BAG3 and HSPB8 and by modulating the level of the BAG3-HspB8 complex. Thus NF-kB-mediated increase in the level of the BAG3-HspB8 complex leads to upregulation of aggrephagy and clearance of irreversibly damaged proteins and might increase cell survival in conditions of hyperthermia.

BAG3 induction is required to mitigate proteotoxicity via selective autophagy following inhibition of constitutive protein degradation pathways

“…8,34 In addition, upregulation of BAG3 was recently shown to mediate the removal of damaged proteins after heat shock. 24 Our present study adds a new aspect to this concept of BAG3 as an on-demand, inducible PQC system in situations of acute and chronic stress by showing for the first time that BAG3-mediated selective autophagy is induced upon concomitant pharmacological blockade of the two major constitutive branches for protein degradation in cancer cells and critically required to support the recovery from proteotoxic stress.…”

“…This differential role of BAG3 during the recovery period is consistent with the delayed kinetic of BAG3 upregulation in our experiments, as BAG3 mRNA levels start to increase upon 30-42 h of exposure to ST80/ Bortezomib. Although NF-kB activity has been implicated in transcriptional upregulation of BAG3 after heat shock, 24 additional studies are required to explore the question whether this transcriptional factor is involved in the current model. Constitutive BAG3 expression has been detected in several types of human cancers, such as glioblastoma, pancreatic carcinoma, leukemia and thyroid carcinoma, compared with very low basal levels of BAG3 in non-malignant cells.…”

Loss of malin, but not laforin, results in compromised autophagic flux and proteasomal dysfunction in cells exposed to heat shock

“…These chaperones are induced in an HSF1-independent manner and are regulated by posttranslational modifications. The role of small chaperone BAG3-HspB8 was shown upon heat stress-induced autophagy (Nivon et al 2012). Oxidative stress also induces small heat shock protein beta-1 (HSPB1) which induces autophagy in renal cells (Matsumoto et al 2015).…”

Heat shock response and autophagy—cooperation and control

“…95 Although the exact role and relationship between autophagy and the heat stress response under stressful conditions remains to be determined, they cooperate in maintaining cellular homeostasis by facilitating appropriate folding of partially unfolded proteins or removing irreversibly damaged proteins to help the cell in coping with the cellular stress. 86,88,96 Cooperation between the heat shock response and autophagy during exercise in animals and in humans…”