ATF6β is an Adaptive Transcription Factor in Cardiac Myocyte.
Authors: Cheung Tak Ki Dicky , Bilal Alina S. , Blackwood Erik A. , Thuerauf Donna J. , Glembotski Christopher C. 
Published: April 2020

RationaleATF6 is an endoplasmic reticulum (ER) transmembrane protein, which is activated in response to the accumulation of misfolded proteins in the ER, a phenomenon known as ER stress. During ER stress, ATF6 is activated through proteolytic cleavage in the Golgi, which releases the N‐terminal active form that will translocate to the nucleus and act as a transcription factor, regulating a widespan of adaptive genes in an attempt to restore proteostasis in the ER, thus resolving ER stress and promoting cell survival. If ER proteostasis is terminally impaired, a maladaptive response will be activated and cell death will occur. There exist two isoforms of ATF6, ATF6α and ATF6β, which have high sequential and structural homology in their ER transmembrane and DNA binding domains. Our previous studies have shown that acute activation of ATF6α is protective in cardiac myocytes under different simulated pathologies in the heart, such as ischemic injury and pressure overload. On the other hand, the precise role of ATF6β in cardiac myocyte gene regulation and viability has yet to be determined.HypothesisATF6β is an adaptive transcription factor that is critical to cardiac myocyte viability.MethodWe used cultured primary neonatal rat ventricular myocytes (NRVM) as a model to study the role of ATF6β, in a loss‐of‐function or gain‐of‐function approach. To study the impact of ATF6β loss‐of‐function, we used small interfering RNA targeted to ATF6β to knockdown its expression. To study the impact of ATF6β gain‐of‐function, we overexpressed ATF6β by infection of NRVMs with ATF6β‐encoding adenovirus. Thereafter, in both models, we assessed cell viability by MTT assay over a period of four days after treatment with or without the chemical ER‐stress inducer tunicamycin (TM). We also determined the adaptive and maladaptive gene profiles of NRVMs in both loss‐of‐function and gain‐of‐function models using real‐time quantitative PCR.ResultsOur results showed that ATF6β knockdown significantly decreased cardiac myocyte viability over time without TM, and TM‐induced ER‐stress exacerbated this loss in viability. ATF6β overexpression maintained cell viability throughout the time course compared to cells treated with control adenovirus, both with and without TM. Gene expression analysis in the ATF6β loss‐of‐function model showed a downregulation of adaptive genes, such as the chaperone Hsp90b1 and anti‐apoptotic gene Bcl2, and an upregulation of the maladaptive, pro‐apoptotic gene Bbc3. Consistent with the loss‐of‐function model, in the gain‐of‐function model, gene expression analysis showed an upregulation of chaperone gene Hspa5 and a downregulation of Bbc3.ConclusionThe effect of ATF6β loss‐ and gain‐of‐function on cell viability and Bbc3 gene expression demonstrate that ATF6β has an adaptive role in cardiac myocyte viability and suggest a regulatory role of ATF6β on Bbc3 gene expression to achieve its effect on viability.Support or Funding InformationAmerican Heart Association, National Institute of Health