Studies from the last two decades support a critical role for inflammation in the etiology of DN39. Herein, we investigated the role of REDD1 in diabetes-induced renal inflammation. Diabetes increased renal NF-κB activation and enhanced pro-inflammatory cytokine expression, with augmented renal infiltration of M1 pro-inflammatory macrophages in a manner that was dependent on REDD1. REDD1 was necessary and sufficient to promote NF-κB activation in podocytes exposed to hyperglycemic conditions. Importantly, the deletion of REDD1 specifically in podocytes attenuated macrophage infiltration in the kidneys of diabetic mice. Overall, the studies support a model wherein REDD1 expression in podocytes promotes NF-κB- and NLRP3-mediated inflammatory responses in the kidney including podocyte pyroptosis and the recruitment and polarization of macrophages in DN (Fig. 7).
Hyperglycemia is a determining factor in the development and progression of DN40,41. Herein, hypo-insulinemia was induced in mice by administration of STZ, resulting in secondary hyperglycemia. While STZ is a valuable tool for modeling diabetic complications in genetically manipulated mice, it is important to note that strain-dependent variability in renal responses to STZ have been reported. With STZ administration, the C57BL/6 and B6;129 strains used in this study develop mild pathological changes in the kidney with mild-to-moderate albuminuria, as compared to other inbred mouse lines (e.g., DBA/2, KK-H1J)42–44. Importantly, the STZ-diabetic C57BL/6 mouse strain continues to be a helpful experimental paradigm to investigate diabetes-associated renal inflammation45–47. REDD1 levels are elevated in the kidneys of diabetic patients and in preclinical murine models of type 1 and type 2 diabetes16,17. Upregulation of REDD1 occurs in multiple cell types exposed to diabetogenic conditions16,17,27,48,49. Normalization of blood glucose concentrations by SGLT2 inhibition attenuated REDD1 protein abundance in the kidney of diabetic mice, which was concomitant with a reduction in immune cell infiltration. The observation builds on prior studies demonstrating increased REDD1 in renal cell cultures exposed to hyperglycemic conditions16,17,49.
A growing body of research demonstrates that REDD1 controls critical cellular and metabolic functions50, and is vital in the pathogenesis of metabolic disorders including diabetic retinopathy19,20,27,48 and nephropathy16,17,49. In the past decade, increasing evidence supports a pro-inflammatory role for REDD119,20,22,23,49. Chronic low-grade inflammation and activation of the innate immune response are integral to the pathogenesis of diabetes and its complications51. Inflammatory mediators like IL-1β, IL6 and CCL2 are upregulated in the kidneys of diabetic patients and act as pathogenic mediators in DN9,52. Our data agree with these works and advance the understanding of mechanisms whereby REDD1 drives immune signaling in the context of DN. Specifically, REDD1 was necessary for activation of NF-κB, increased expression of cytokines and chemokines, and immune cell infiltration in the kidneys of diabetic mice. Lee et al. previously reported that REDD1 plays a role in the recruitment of immune cells into adipose tissue in murine model an obesity22. The data here support that REDD1 has a similar role in the recruitment of M1 pro-inflammatory macrophages into the kidney in the context of diabetes.
Podocyte dysfunction and loss is an early event in DN pathogenesis and predicts diabetic kidney injury53. Damaged podocytes produce inflammatory cytokines and chemokines that drive immune cell recruitment and glomerular inflammation11,49. Activation of inflammatory pathways in non-hematopoietic kidney resident cells including podocytes promote inflammatory processes that aggravate renal injury in DN14. Indeed, podocyte-specific suppression of the NLRP3 inflammasome prevents diabetes-induced proteinuria11. Herein, REDD1 was required for NF-κB activation and the production of inflammatory cytokines and chemokines by podocytes. This advances findings from Wang et al. showing that REDD1 knockdown attenuates expression of TNFα, IL6, and IL-1β in podocyte cultures exposed to hyperglycemic conditions49. Additionally, in vitro transmigration assays demonstrated that hyperglycemia-induced REDD1 in podocytes was required for macrophage chemotaxis to the site of inflammatory injury. Importantly, in mice with podocyte-specific REDD1 deletion, diabetes failed to increase immune cell infiltration and renal recruitment of M1 pro-inflammatory macrophages. Notably, the attenuated inflammatory response within glomeruli observed with podocyte-specific REDD1 deletion correlated with preserved glomerular architecture and filtration function, as well as reduced podocyte loss37.
Independent investigations have demonstrated that canonical and non-canonical activation of the inflammasome is a characteristic event in diabetic complications38. In the context of diabetic kidney disease, studies have shown that excessive cell pyroptosis mediated by caspase-1-associated canonical54 cleavage of GSDMD (as well as caspase-11/4 non-canonical GSDMD cleavage10) promotes podocyte damage and renal immune cell infiltration. Moreover, in preclinical models of DN, podocyte-specific activation of the NLRP3 inflammasome is both necessary and sufficient to promote glomerular dysfunction and kidney damage11. In recent years, investigations delineating the regulation of NLRP3 inflammasome activation have implicated a role for REDD1 in both priming and activation of the inflammasome complex21,23,55. Prior work from our laboratory demonstrated a role for REDD1-dependent NLRP3 inflammasome activity in the context of diabetic retinopathy21. The findings presented herein extend these prior studies and demonstrate that REDD1 expression in podocytes is required for NLRP3 inflammasome activation and subsequent induction of pyroptosis in experimental models of diabetes.
A major limitation of the current standards of care for DN is that they predominantly focus on controlling blood glucose levels and fail to address the specific underlying cause of DN. The studies here delineate specific molecular events that contribute to renal inflammation caused by diabetes. Podocytes perform immune-surveillance functions and initiate immune responses that make the glomerular filtration barrier vulnerable to inflammatory disorders like DN11. The proof-of-concept studies here are consistent with a mechanism of action whereby REDD1 drives renal injury by promoting NF-κB activation in podocytes, thereby enhancing the renal pro-inflammatory immune response to diabetes. Thus, interventions targeting REDD1 in the context of nephropathies linked to metabolic illnesses such as diabetes could improve current DN treatment.