HO-1 in Control of a Self-Eating Kidney

Autophagy, from Greek meaning self-eating, refers to the degradation of macromolecules and organelles by the lysosomal machinery of cells and is a tightly regulated and normally occurring process that helps maintain a balance among synthesis, degradation, and recycling of cellular components. Recent studies have uncovered fascinating links to human physiology and disease, such as aging and neurodegeneration, cancer, infection, and renal diseases.^1-6 Three types of autophagy, micro, macro, and chaperone-mediated, have been described.^1,2,7

The morphologic hallmark of macroautophagy (hereinafter referred to as autophagy) is the de novo formation of a double or multimembrane-bound structure known as the autophagosome. In mammalian cells, the autophagosome fuses with the lysosome, resulting in the degradation of cellular components and the recycling of nucleotides, amino acids, and free fatty acids that now can be reused for multiple biosynthetic processes. Whereas the mammalian target of rapamycin complex 1 coordinates autophagy with nutrient-sensing signaling pathways, the AuTophaGy (Atg)-related genes encode evolutionarily conserved proteins that are critically important for autophagosome generation.^1,7 For instance, beclin-1, the mammalian homolog of yeast Atg6, contributes to the formation of autophagosomes,⁸ whereas the microtubule-associated protein 1 light chain 3 (LC3; mammalian homolog of yeast Atg8) is essential for autophagosome elongation and expansion in its membrane-bound form LC3-II.⁸

Autophagy is induced under various conditions of cellular stress, including nutrient deprivation, hypoxia, and oxidative stress, but also occurs during normal development and cell growth and is vital for the maintenance of cellular homeostasis in postmitotic cells, such as neurons, myocytes, and podocytes.^2,3 The role of autophagy in apoptosis or programmed cell death after severe stress conditions is unclear and subject to intense investigation.¹ Recent studies demonstrated a protective role for autophagy in experimental models of kidney disease.^3-5 Inhibition of autophagy by pharmacologic or genetic means exacerbates injury, partially by promoting apoptosis in both ischemia-reperfusion and cisplatin-induced renal injury.^4,5 Furthermore, autophagy is essential for podocyte integrity after glomerular injury.³ However, the regulation of autophagy in renal cells is poorly understood.

In this issue of JASN, Bolisetty et al.⁹ identify heme oxygenase 1 (HO-1) as an important modulator of autophagosome formation in proximal renal tubular epithelial cells and propose a novel mechanism by which HO-1 protects kidneys from cisplatin (stress)-induced injury. HO-1 is expressed ubiquitously, induced by oxidative stress, and catalyzes the rate-limiting step in heme degradation, which produces biliverdin, carbon monoxide (CO), and iron. Clearance of excess free heme by HO-1 is critical in preventing heme-induced membrane oxidation and production of reactive oxygen species (ROS). Furthermore, the products of this reaction—CO and biliverdin and its derivative, bilirubin—have antioxidant, antiapoptotic, and anti-inflammatory properties, resulting in cytoprotection.¹⁰ Thus, HO-1 protects from ischemia-reperfusion injury in multiple tissues, including lung, heart, and kidney.¹⁰ HO-1 also protects from cisplatin-induced acute renal failure by attenuating renal tubular cell apoptosis and necrosis by decreasing intracellular ROS, which are thought to be key mediators of nephrotoxicity in this setting.^9,11

Bolisetty et al.⁹ find increased numbers of autophagic vesicles in proximal nephron segments of HO-1–deficient mice under baseline conditions and increased expression of autophagosome markers LC3-II and beclin-1 in HO-1–deficient primary proximal renal epithelial cells, suggesting that loss of renal epithelial HO-1 activity increases autophagy.⁹ Although it is likely that ablation of HO-1 associates with increased macromolecular and organelle degradation, future studies are warranted to investigate the possibility of impaired autophagosome/lysosome fusion or altered lysosomal activity leading to the accumulation of autophagic vacuoles. On the basis of the expression of autophagosome markers Atg5, beclin-1, and LC3-II, Bolisetty et al.⁹ then show that treatment with cisplatin, which normally induces autophagy and apoptosis in renal epithelium, does not lead to additional autophagosome formation in HO-1–deficient cells, whereas apoptosis is enhanced compared with wild-type. This finding emphasizes a functional link between autophagy and apoptosis in renal cells and highlights an important association between lack of inducibility of autophagosome and increased susceptibility to cellular injury and apoptosis. It is also consistent with recent studies in mouse embryonic fibroblasts, which demonstrated functional cross-talk between autophagy and apoptosis: Deletion of BCL-2 family members Bax and Bak result in massive autophagy and beclin-1 and Atg5-dependent cell death after exposure to etoposide.¹² Because the authors show that autophagy is induced before the onset of apoptosis in cisplatin-treated tubular epithelial cells, it is plausible that the extent of autophagy induction generates a signal, which then determines whether cells undergo apoptosis. This hypothesis is supported by in vitro and in vivo studies in which inhibition of autophagy led to apoptosis as a result of failed stress adaptation.¹³ A major question that remains unanswered in the study by Bolisetty et al.⁹ concerns the degree to which regulation of HO-1–dependent autophagy contributes to HO-1–mediated cytoprotection. Pharmacologic or genetic manipulation of autophagic activity in HO-1–competent and – defective backgrounds would be required to dissect antioxidant from autophagy regulatory effects.

Although the effects of HO-1 activity on autophagy may be cell-type specific, as overexpression of HO-1 promotes mitochondrial autophagy in astroglia,¹⁴ the observation of increased renal autophagy in HO-1–deficient mice provokes fundamental questions regarding underlying molecular mechanisms. One possible explanation for this finding is the existence of a pro-oxidant intracellular milieu that has been observed in HO-1–deficient mice.¹⁵ Indeed, Bolisetty et al. provide evidence that increased ROS production associates with increased autophagosome formation in HO-1–deficient renal epithelial cells: HO-1 overexpression reduced ROS levels and delayed autophagy after cisplatin treatment. This is consistent with previous reports that implicated ROS in the regulation of autophagy¹⁶; however, the exact mechanism by which HO-1 controls autophagy—that is, which HO-1 reaction products are involved and to what extent altered heme clearance plays a role in its regulation—remains unclear. CO, for example, which has both pro- and antioxidant properties,¹⁰ functions as a modulator of autophagy in respiratory epithelial cells, where it induces autophagy by increasing mitochondrial-derived ROS.¹⁷

In summary, the findings by Bolisetty et al.⁹ provide a novel link between HO-1 signaling and the regulation of renal autophagy and highlight an important functional association between autophagosome inducibility and sensitivity to nephrotoxic renal injury. This study will certainly stimulate further research into the role of autophagy in renal physiology and disease.