The trans-differentiation promotion of parietal epithelial cells by magnesium isoglycyrrhizinate to improve podocyte injury induced by high fructose consumption

Background: Podocytes have limited proliferative capacity, which leads to irreversible glomerular injury in diverse kidney diseases. Magnesium isoglycyrrhizinate (MgIG), a hepatoprotective agent in clinic, has been reported to improve glomerular podocyte injury. However, the underlying mechanism of MgIG in ameliorating podocyte injury remains unclear.

Purpose: Glomerular parietal epithelial cells (PECs) are recognized as podocyte progenitors and play a pivotal role in the recovery following glomerular injury. This work aims to investigate the protective mechanisms of MgIG in mitigating glomerular injury by promoting PEC trans-differentiation.

Study design: A rat model of progressive glomerular podocyte injury, and in vitro models using the primary podocytes and primary PECs, were established to further explore the pharmacological mechanism of MgIG.

Methods: Four-week-old male Sprague-Dawley (SD) rats were fed a 10 % fructose solution for 3, 6, 9 and 12 weeks to induce glomerular injury. The effects of MgIG on the progressive changes in podocytes and PECs, and the correlation between PEC density and podocyte loss, were analyzed. The mechanism of MgIG in triggering PEC trans-differentiation was investigated, by examining adenosine secretion in injured podocytes, as well as the expression of cluster of differentiation 44 (CD44), nephrin, adenosine receptor A2B (ARA2B) and glucocorticoid receptor (GR) in PECs both in vivo and in vitro.

Results: Rats fed a high fructose diet exhibited progressive changes in glomerular PECs, including increased cell density and a preference for trans-differentiation. A positive correlation was observed between PEC density and podocyte loss. Co-culture experiments demonstrated that extracellular adenosine accumulation from injured podocytes induced by high fructose exposure promoted PEC trans-differentiation via ARA2B. MgIG significantly improved podocyte injury and exhibited effects similar to dexamethasone on nephrin upregulation and CD44 inhibition. Moreover, the effect of MgIG on PEC ARA2B activation was more effective than that of dexamethasone. The co-expression of paired box 2 (PAX2)⁺-Nephrin⁺ in glomeruli indicated that MgIG induced PEC trans-differentiation and podocyte regeneration in model rats. Accordingly, podocyte loss and increased urine albumin-to-creatinine ratio (UACR) were also alleviated. Moreover, MgIG, which acts as a GR agonist to activate GR, reversed the upregulation of CD44 and decreased ARA2B induced by tumor necrosis factor-α (TNF-α) in primary PECs. The siRNA interference experiment manifested that MgIG exhibited a more pronounced enhancement of GR upregulation, in contrast to ARA2B activation, to promote PEC trans-differentiation.

Conclusion: This work reports for the first time that PECs respond to the accumulation of extracellular adenosine from injured podocytes via activating ARA2B and focuses on the role of adenosine and adenosine receptors in the trans-differentiation of PECs. Furthermore, this study provides the first evidence that MgIG may promote podocyte regeneration by enhancing PEC trans-differentiation through GR activation, providing a research basis for investigating the glucocorticoid-like activity of MgIG in ameliorating glomerular podocyte injury.