Doxorubicin (DOX)-induced cardiotoxicity has been a well-known phenomenon to clinicians and scientists for decades; however, molecular mechanisms underlying DOX cardiotoxicity are still being uncovered. Although the majority of prior research have implicated nuclear and mitochondrial events to be an important etiological aspects of DOX cardiomyopathy, recent discoveries in autophagy have highlighted the renewed interest in the role of lysosome in DOX cardiomyopathy. Indeed, dysregulation of lysosomal autophagy is observed in pre-clinical models of DOX cardiotoxicity. In this review, we provide a comprehensive overview on mechanisms describing regulation of the autophagy pathway by DOX and its influence on cardiotoxic outcomes. We have put specific emphasis on experimental models, dosing and treatment duration with DOX, and methods to monitor autophagy, all of which contribute to inconsistencies observed in the literature. We have clarified processes by which DOX dysregulates macroautophagy in the heart by primarily focusing on the contribution of LC3, p62, Beclin, mTOR and AMPK pathways. We have also highlighted the impact of DOX on mitochondrial reactive oxygen species (ROS) and its contribution to the process of mitophagy. We have presented mechanisms by which DOX compromises lysosomal acidification, integrity and chaperone-mediated autophagy through its effect on lysosome-associated and resident proteins such as LAMP, vATPase, Hsp90, Hsc70 and cathepsins. Furthermore, we have discussed novel pathways in DOX cardiotoxicity, the most prominent being DOX-induced loss of TFEB, a member of the MITF family of transcription factors, which governs lysosomal biogenesis and function. This review summarizes that in the myocardium, DOX dysregulates autophagy by impairing transcriptional factors regulating lysosomal function, thereby, precipitating proteotoxicity, mitochondrial dysfunction and cell death, thus rendering the heart susceptible to cardiomyopathic failure.
Impaired cardiac metabolism in the obese and diabetic heart leads to glucolipotoxicity and ensuing cardiomyopathy. Glucolipotoxicity causes cardiomyocyte injury by increasing energy insufficiency, impairing proteasomal-mediated protein degradation and inducing apoptosis. Proteasome-evading proteins are degraded by autophagy in the lysosome, whose metabolism and function are regulated by master regulator transcription factor EB (TFEB). Limited studies have examined the impact of glucolipotoxicity on intra-lysosomal signaling proteins and their regulators. By utilizing a mouse model of diet-induced obesity, type-1 diabetes (Akita) and ex-vivo model of glucolipotoxicity (H9C2 cells and NRCM, neonatal rat cardiomyocyte), we examined whether glucolipotoxicity negatively targets TFEB and lysosomal proteins to dysregulate autophagy and cause cardiac injury. Despite differential effects of obesity and diabetes on LC3B-II, expression of proteins facilitating autophagosomal clearance such as TFEB, LAMP-2A, Hsc70 and Hsp90 were decreased in the obese and diabetic heart. In-vivo data was recapitulated in H9C2 and NRCM cells, which exhibited impaired autophagic flux and reduced TFEB content when exposed to a glucolipotoxic milieu. Notably, overloading myocytes with a saturated fatty acid (palmitate) but not an unsaturated fatty acid (oleate) depleted cellular TFEB and suppressed autophagy, suggesting a fatty acid specific regulation of TFEB and autophagy in the cardiomyocyte. The effect of glucolipotoxicity to reduce TFEB content was also confirmed in heart tissue from patients with Class-I obesity. Therefore, during glucolipotoxicity, suppression of lysosomal autophagy was associated with reduced lysosomal content, decreased cathepsin-B activity and diminished cellular TFEB content likely rendering myocytes susceptible to cardiac injury.
Adipose Triglyceride Lipase (ATGL) performs the first and rate-limiting step in lipolysis by hydrolyzing triacylglycerols stored in lipid droplets to diacylglycerols. By mediating lipolysis in adipose and non-adipose tissues, ATGL is a major regulator of overall energy metabolism and plasma lipid levels. Since chronically high levels of plasma lipids are linked to metabolic disorders including insulin resistance and type 2 diabetes, ATGL is an interesting therapeutic target. In the present study, fourteen closely related analogues of Atglistatin (1), a newly discovered ATGL inhibitor, were synthesized, and their ATGL inhibitory activity was evaluated. The effect of these analogues on lipolysis in 3T3-L1 adipocytes clearly shows that inhibition of the enzyme by Atglistatin (1) is due to the presence of the carbamate and N,N-dimethyl moieties on the biaryl central core at meta and para position, respectively. Mono carbamate-substituted analogue C2, in which the carbamate group was in the meta position as in Atglistatin (1), showed slight inhibition. Low dipole moment of Atglistatin (1) compared to the synthesized analogues possibly explains the lower inhibitory activities.