Hormetic heat shock and HSF-1 overexpression improve C. elegans survival and proteostasis by inducing autophagy

ABSTRACT

The cellular recycling process of macroautophagy/autophagy is an essential homeostatic system induced by various stresses, but it remains unclear how autophagy contributes to organismal stress resistance. In a recent study, we report that a mild and physiologically beneficial (“hormetic”) heat shock as well as overexpression of the heat-shock responsive transcription factor HSF-1 systemically increases autophagy in C. elegans. Accordingly, we found HSF-1- and heat stress-inducible autophagy to be required for C. elegans thermoresistance and longevity. Moreover, a hormetic heat shock or HSF-1 overexpression alleviated PolyQ protein aggregation in an autophagy-dependent manner. Collectively, we demonstrate a critical role for autophagy in C. elegans stress resistance and hormesis, and reveal a requirement for autophagy in HSF-1 regulated functions in the heat-shock response, proteostasis, and aging.

Stress resistance and longevity are intimately linked in many organisms. In the nematode C. elegans, more than 50 mutations have been identified that not only extend life span but also increase resistance to various stressors, such as heat stress. Heat stress activates the transcription factor HSF-1 to induce the heat-shock response (HSR). Whereas stress exposure can be toxic at high doses, exposure to a mild stress can be beneficial and lead to stress resistance and longevity. These positive effects of mild stress are referred to as ‘hormesis’ and have been observed in many different organisms. Although mild heat stress leads to increased expression of heat-shock proteins in C. elegans, Drosophila and human fibroblasts, it remains unclear whether other homeostatic pathways besides the HSR could also contribute to the beneficial effects of stress-induced hormesis. A candidate mechanism is autophagy, an intracellular recycling process by which cytosolic material, organelles and aggregate-prone proteins are sequestered within double-membrane vesicles called autophagosomes, which subsequently fuse with lysosomes for degradation of the contents. Autophagy has been linked to longevity; e.g., all C. elegans longevity models investigated so far require autophagy genes for their extended life span; however, the contribution of autophagy to stress resistance and hormesis remains unclear.

In a recent study, we demonstrated that a hormetic heat shock in C. elegans is accompanied by induction of autophagy, which is critical for the beneficial effects of hormesis. Specifically, a mild heat shock administered early in the life of C. elegans not only improves the animals' survival, but also systemically increases the number of autophagosomes as well as the mRNA levels of several autophagy-related genes, consistent with an increase in autophagy. This notion was confirmed using bafilomycin A₁ flux assays. We observed similar effects from overexpression of HSF-1, whereas RNAi-mediated reduction of hsf-1 prevents autophagosome formation in several tissues upon heat stress, collectively suggesting that hsf-1 plays a critical role in regulating autophagy at least during thermal stress. Even though the promoters of many autophagy-related genes in C. elegans contain putative HSF-1-binding elements, it remains to be determined if autophagy genes are regulated by direct transcriptional regulation, as observed for heat-shock protein genes in the HSR. The transcriptional regulation of autophagy genes upon heat shock could also occur via other stress-responsive transcription factors. In support of this scenario, we found that the TFEB homolog HLH-30, a conserved regulator of the autophagy response, also plays a role in heat-shock-mediated autophagy. Specifically, heat shock leads to rapid nuclear translocation of HLH-30/TFEB, and hlh-30/Tfeb loss-of-function mutants fail to induce the expression of several autophagy genes upon heat shock. Thus, both HSF-1 and HLH-30/TFEB are critical for autophagy regulation upon heat stress in C. elegans. It will be interesting to investigate how these 2 conserved transcription factors coordinate their physiological effects, and to which extent direct or indirect regulatory mechanisms are involved in increasing autophagy gene expression upon heat shock.

Both hormetic heat shock and overexpression of HSF-1 lead to increased thermoresistance and increased longevity in C. elegans. Consistent with autophagy induction upon heat shock, we found that autophagy genes functioning in different steps of the process, such as unc-51/ATG1/Ulk1, bec-1/VPS30/ATG6/Becn1 and lgg-1/ATG8/Lc3 are required for the increased stress resistance and extended longevity observed in animals after heat shock or HSF-1 overexpression. The transcriptional autophagy regulator hlh-30/Tfeb is similarly required for improved survival upon hormetic heat stress. These observations emphasize that autophagy induction, at least in part via transcriptional regulation, plays a protective role in organismal fitness, a link that we and others have previously reported in several conserved genetic and pharmacological longevity models.

Aging is accompanied by a loss in protein homeostasis, which contributes to several age-related neurodegenerative diseases, such as Huntington disease. This disease can be modeled in C. elegans, in which the expression of polyglutamine (polyQ)-containing proteins leads to aggregate formation in an age-dependent manner. Since hormetic heat shock improves organismal fitness, we tested whether these beneficial effects could be extended to proteostasis in C. elegans expressing polyQ in their intestine or in neurons. Indeed, we found that a hormetic heat shock reduces polyQ-aggregate load later in life, as also observed in HSF-1 overexpressing animals. In animals subjected to heat shock, these changes are accompanied by increased transcription of autophagy-related genes and increased longevity. Notably, the improvement in proteostasis is dependent on autophagy, because RNAi of autophagy genes prevents the beneficial effects of the hormetic heat shock on aggregate formation. Taken together, we demonstrated that autophagy is required for the improved stress resistance, longevity, and proteostasis conferred by hormetic heat stress and HSF-1 overexpression.

Interestingly, animals with reduced autophagy gene expression are still able to mount a HSR after a hormetic heat shock, suggesting that heat-shock proteins alone are not sufficient for the beneficial effects of hormetic heat shock on proteostasis. It will therefore be interesting to further investigate the interplay between the HSR and autophagy and how these homeostatic processes cooperate to increase the organism's ability to cope with thermal and proteotoxic stress and aging. Moreover, future studies should shed light on the exact mechanism by which heat shock-induced autophagy limits polyQ aggregation in C. elegans, and if the effects are also observed in other model systems. If the mechanism is conserved, hormetic heat shocks may prove of therapeutic interest in the treatment or prevention of human diseases associated with polyQ expansions.

Figure 1.

Hormetic heat shock induces autophagy, which is required for stress resistance, longevity and proteostasis in C. elegans. Heat shock leads to the activation of the transcription factor HSF-1, which induces transcription of heat-shock protein (HSP) genes (e.g., hsp-16.2, hsp-70). In our recent study, we found that a mild (or ‘hormetic’) heat shock and HSF-1 overexpression also cause increased expression of many autophagy-related genes, at least in part via HSF-1 as well as the transcription factor HLH-30/TFEB. While it remains to be determined whether these proteins regulate autophagy via direct or indirect mechanisms, autophagy genes were found to be required for the beneficial effects of hormetic heat shock and of HSF-1 overexpression on stress resistance, longevity, and proteostasis. Thus, autophagy plays a protective role in the response to hormetic heat shock in C. elegans. See text for details. HSE, heat shock element.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Funding

C.K. was supported by a postdoctoral fellowship from American Federation for Aging Research (AFAR) (EPD1360) and M.H. was supported by NIH/NIA grants R01 AG038664 and R01 AG039756, and by a Julie Martin Mid-Career Award in Aging Research supported by the Ellison Medical Foundation and AFAR.