ABSTRACT
The endoplasmic reticulum (ER) is an organelle that houses factors that assist proteins in their folding and
supports the formation of stabilizing covalent modifications. Perturbation of ER functions results in
evolutionarily conserved cell stress, the unfolded protein response (UPR). Recent studies have indicated the
critical role of ER stress in diabetes-induced cardiac cell death. However, a pathway connecting hyperglycemia
to downstream events of UPR has remained elusive. Here, we present preliminary data demonstrating that
Arrestin Domain-Containing protein 4 (Arrdc4) is a glucose-responsive protein that mediates ER stress and
cardiac dysfunction in diabetes mellitus. Arrdc4 is a member of the ancient family of arrestin-fold proteins with
conserved roles in regulating nutrient transporter trafficking as adaptor proteins. We have found that Arrdc4
expression is robustly upregulated by high glucose in cardiomyocytes. Arrdc4 binds to the basal glucose
transporter 1 (GLUT1), induces its endocytosis, and blocks cellular glucose uptake. Arrdc4-mediated glucose
deprivation leads to the activation of UPR in cardiomyocytes, revealing a unique function of Arrdc4 in
cardiomyocyte metabolism and survival. To define the in vivo function of Arrdc4, we generated and
characterized a systemic Arrdc4-knockout (KO) mouse model. In this animal model, Arrdc4-KO mice have
fasting hypoglycemia, and Arrdc4-KO hearts exhibit a robust increase in myocardial glucose uptake without
insulin stimulation. Furthermore, deletion of Arrdc4 reduces ER stress during the development of diabetic
cardiomyopathy and protects the heart against cardiomyocyte apoptosis in type 1 diabetes. These results
define the outlines of an Arrdc4-GLUT1-UPR pathway that provides a strong link between hyperglycemia and
cardiomyocyte survival. This project aims to delineate the molecular nature of this pathway and tests its role in
the pathogenesis of diabetic cardiomyopathy. Aim 1 will test three non-exclusive hypotheses by which high
glucose induces ER stress: (a) Arrdc4 is a target of the glucose-sensing transcriptional mechanism, (b) Arrdc4
sensitizes cardiomyocytes to ER stress by energy insufficiency through GLUT1 inhibition, and (c) Arrdc4 may
even promote ER stress through a mechanism beyond GLUT1 interaction. Aim 2a will characterize the cardiac
metabolic phenotype in the type 2 diabetic model to determine whether Arrdc4-KO mouse hearts are protected
from proteotoxic apoptosis. Moreover, Aim 2b will test the hypothesis that the beneficial effect of Arrdc4
deficiency in diabetes is cardiac autonomous or secondary to the glycemic control by skeletal muscle using our
newly-generated inducible, tissue-specific Arrdc4-KO mice, focusing on the significance of Arrdc4 in exercise
intolerance, an independent predictor of poor prognosis in diabetes. These studies are highly innovative as we
propose a pathway that has never been entertained as a cardiac stress regulator in diabetes. The resulting
knowledge will provide a new mechanistic basis for understanding the defense mechanism to protect
cardiomyocytes against diabetic insults. Based on the novelty of the study, this application is highly significant.
