ATF6 Regulates Cardiac Hypertrophy by Transcriptional Induction of the mTORC1 Activator, Rheb

Abstract

Rationale: ER stress dysregulates ER proteostasis, which activates the transcription factor, ATF6, an inducer of genes that enhance protein folding and restore proteostasis. Due to increased protein synthesis, it is possible that protein folding and, thus, ER proteostasis are challenged during cardiac myocyte growth. However, it is not known whether ATF6 is activated, and if so, what its function is during hypertrophic growth of cardiac myocytes. Objective: To examine the activity and function of ATF6 during cardiac hypertrophy. Methods and Results: We found that ATF6 was activated and ATF6-target genes were induced in mice subjected to an acute model of trans-aortic constriction (TAC), or to free-wheel exercise, which promote adaptive cardiac myocyte hypertrophy with preserved cardiac function. Cardiac myocyte-specific deletion of Atf6 (ATF6 cKO) blunted TAC- and exercise-induced cardiac myocyte hypertrophy and impaired cardiac function, demonstrating a role for ATF6 in compensatory myocyte growth. Transcript profiling and chromatin immunoprecipitation identified RHEB as an ATF6-target gene in the heart. RHEB is an activator of mTORC1, a major inducer of protein synthesis and subsequent cell growth. Both TAC and exercise upregulated RHEB, activated mTORC1, and induced cardiac hypertrophy in WT mouse hearts, but not in ATF6 cKO hearts. Mechanistically, knockdown of ATF6 in neonatal rat ventricular myocytes blocked phenylephrine (PE)-, and insulin-like growth factor 1 (IGF1)-mediated Rheb induction, mTORC1 activation, and myocyte growth, all of which were restored by ectopic RHEB expression. Moreover, AAV9-RHEB restored cardiac growth to ATF6 cKO mice subjected to TAC. Finally, ATF6 induced RHEB in response to growth factors, but not in response to other activators of ATF6 that do not induce growth, indicating that ATF6 target gene induction is stress-specific. Conclusions: Compensatory cardiac hypertrophy activates ATF6, which induces Rheb and activates mTORC1. Thus, ATF6 is a previously unrecognized link between growth stimuli and mTORC1-mediated cardiac growth.

Designing Novel Therapies to Mend Broken Hearts: ATF6 and Cardiac Proteostasis

“…Upon sensing an increase in protein folding demand, ATF6 acts as the primary adaptive responder that is liberated from the ER to act as an active transcription factor regulating a gene program that fosters the maintenance of proteostasis in cardiac myocytes [95][96][97]. In response to CVD and various hypertrophic growth stimuli in the heart, ATF6 is activated, which allows for homeostatic growth and regulation of proteostasis via transcriptionally inducing gene targets that function to either temper protein synthesis, fold nascent proteins, or degrade misfolded and surplus proteins [50].…”

“…For the most part, ATF6 is known for regulating canonical gene targets as part of an adaptive panel destined for the ER and designed to regulate ER protein folding (Figure 2, Steps 4 and 5) comprised of ER-resident chaperones (e.g., GRP78), PDIs, and ERAD components (e.g., HRD1) [11,14,16,18]. However, recent studies have illuminated a remarkable ability of ATF6 to induce non-canonical gene targets that were not previously known to be genes related to the UPR nor reside in the ER, but instead are induced by ATF6 in a stimulus-specific manner and localize to specific regions of cardiac myocytes including the lysosome [50], peroxisome [28] and sarcolemma (Figure 2 ① In its inactive state, ATF6 is a 90kD ER transmembrane protein that is anchored in the membrane in oligomers via GRP78 and intermolecular disulfide bonding. ② Upon stressors, like cardiac hypertrophy that increases protein synthesis and the protein folding demand or AMI that elevates cellular levels of reactive oxygen species (ROS), GRP78 dissociates from the ER luminal domain of ATF6 and the disulfide bonds are reduced allowing monomerization of ATF6, which allows the 90 kD form of ATF6 to translocate to the Golgi, where is it cleaved by S1P and S2P to liberate the N-terminal approximately 400 amino acids (50 kD) of ATF6 from the ER membrane.…”

“…In considering other possible non-canonical roles for ATF6, a recent study set out to test whether the increase in protein synthesis and protein folding demand that occur during cardiac hypertrophy poses a challenge to the protein folding machinery, resulting in ATF6 activation [50]. Indeed, ATF6 was activated in mouse hearts subjected to conditions that mimic not only pathological (pressure-overload), but also physiological (exercise) cardiac myocyte growth.…”

“…These diverse ATF6 activators include oxidative stress and growth stimuli, each of which impact global proteome integrity, not just the ER proteome. Remarkably, during these various pathophysiological maneuvers, ATF6 activates unique gene programs, depending on the stimulus, and these programs serve stress-specific adaptive effects [50]. For example, oxidative stress results in ATF6-dependent induction of antioxidants, e.g., catalase, but growth regulators, e.g., Rheb.…”

“…While adaptive, the increase in protein folding associated with physiological hypertrophy would be expected to strain the proteostasis network. However, studies have shown that not only is ATF6 activated to regulate an adaptive gene panel allowing for continued growth [50], but also, exercise can revert many of the age-related symptoms leading to CVD, such as attenuated accumulation of misfolded protein aggregates within cardiac myocytes [51].…”