Kazuo Takahashi

PURPOSE: Recent studies have reported that sodium-glucose cotransporter (SGLT) 2 inhibitors ameliorate steatotic liver disease. We investigated the contribution of SGLT2 inhibitor-induced fluid loss due to glycosuria in hepatic ureagenesis for water conservation to the association between improvement of steatotic liver disease and the energy-consuming nature of hepatic ureagenesis. GENERAL METHODS: ob/ob mice fed a high-fat diet without carbohydrate restriction were administered luseogliflozin, an SGLT2 inhibitor, for eight weeks. FINDINGS: Luseogliflozin significantly decreased the liver weight, plasma aspartate aminotransferase and alanine aminotransferase levels, and fat content in mice with enhanced glycosuria (H-GlcV group). The ratio of urinary urea excretion decreased as a substitute for increased glucose excretion in the H-GlcV group. Luseogliflozin significantly increased liver urea content and tended to increase malate dehydrogenase (MDH)-1 activity. Liver MDH-1 activity was significantly positively correlated with liver urea content, suggesting that MDH-1-induced amino acid recruitment from the tricarboxylic acid cycle to the urea cycle may contribute to enhanced ureagenesis. In addition, liver weight was significantly negatively correlated with the liver urea content. CONCLUSIONS: Our data suggest that enhanced hepatic ureagenesis as a compensatory water conservation mechanism for glycosuria-induced fluid loss may be associated with amelioration of steatotic liver disease in SGLT2 inhibitor-treated ob/ob mice.

Immunoglobulin A nephropathy (IgAN) is the most common form of primary glomerulonephritis worldwide with most patients progressing to kidney failure. Although its pathophysiology remains incompletely understood, deposition of IgA-containing immune complexes in the glomerular mesangium induces mesangial cell proliferation and overproduction of extracellular matrix components and cytokines and chemokines, that lead to glomerular injury. The properties of nephritogenic IgA1 include abnormal glycosylation of its polymeric forms and its capacity to bind IgG autoantibodies to form immune complexes. Nephritogenic IgA1 is thought to be secreted by B cells originating from or residing in mucosa-associated lymphoid tissues (MALT), such as gut-associated lymphoid tissues (GALT) and nasopharynx-associated lymphoid tissues (NALT). However, little is known how the immune abnormalities in MALT elevate the circulatory levels of nephritogenic IgA. This review summarizes fundamental insights into IgA production and its regulation in MALT in general, provides an overview of the immune abnormalities in the MALT of patients with IgAN relevant to the production of abnormally glycosylated IgA, and summarizes relevant emerging treatments tested in clinical trials.

BACKGROUND: IgA nephropathy (IgAN) is a mucosal immune-associated disease characterized by the overproduction of galactose-deficient IgA1 (Gd-IgA1) and formation of nephritogenic immune complexes. Epstein-Barr virus (EBV), which preferentially infects IgA+ B cells, has been implicated in IgAN pathogenesis, although its role remains unclear. Given mucosa involvement, we characterized breast milk B cells as available representatives of mucosal tissue to better understand the pathogenesis of IgAN. METHODS: We performed a phenotypic analysis of B cells (CD19+), plasmablasts (CD38+), and CD138+ cells (representing migrating pre-plasma cells, CPC) in the breast milk of IgAN and healthy mothers (HC). EBV infection of cells was detected by using EBV-encoded RNA (EBER). B cell differentiation, IgA/Gd-IgA1 expression, and homing-related markers were characterized using complementary cytometric and microscopic approaches. RESULTS: Breast milk from mothers with IgAN contains a significantly higher proportion of EBER+ B cells compared with HC. Moreover, the proportion of EBER+ CPCs in the breast milk of IgAN mothers is significantly higher than in HC. B cells present in the breast milk of mothers with IgAN exhibit a higher expression of the mucosal homing receptor CCR9 compared to B cells from HC. IgA+ B cells from healthy mothers exhibit a higher overall frequency of surface Gd-IgA1 expression and lack CD138 marker and thus could be classified as memory B cells or plasmablasts considering their class-switched phenotype. In contrast, breast milk from IgAN mothers was enriched for Gd-IgA1+ CD138+ CPC cells, indicating a shift toward terminally differentiated antibody-producing cells. These findings suggest disease-associated alterations in B cell differentiation and compartmentalization rather than increased mucosal Gd-IgA1 production per se. CONCLUSION: Despite the limited number of analyzed samples, we detected interesting differences in B cells in the breast milk of IgAN mothers and HC. The analysis of B cell populations in the breast milk of IgAN mothers indicates EBV-associated B cell dysregulation. The enrichment of EBV+ CPC and Gd-IgA1+ CPC in the breast milk of IgAN mothers agrees with a former model proposing aberrant mucosal B cell differentiation and trafficking involvement in IgAN pathogenesis.

IgA nephropathy (IgAN) is characterized by the deposition of galactose-deficient IgA1 (Gd-IgA1)-containing immune complexes into kidney mesangium leading to glomerular inflammation and injury. In patients with active IgAN non-responding to renin-angiotensin system blocking drugs (RASBs), corticosteroids (CSs) are recommended. Although CSs reduce significantly serum levels of Gd-IgA1, IgA-containing immune complexes, and proteinuria, their clinical effect in IgAN is variable. Because IgAN patients exhibit abnormal serum concentration of several endogenous steroid metabolites, some of which exhibit immuno-protective/regulating functions, we analyzed serum metabolites which could predict clinical response to CS therapy. This prospective study was performed in 18 male IgAN patients to identify potential biomarkers for personalized CS therapy. Using LC-MS set of 85 steroid metabolites was tested in the sera of IgAN patients before CS therapy initiation to identify those with statistically different level in patients clinically responding and not-responding to CS therapy. Responders were those subjects whose proteinuria decrease below 1 g/day after 6-12 months of CS therapy. Statistical analysis revealed significant and clinically relevant differences in the steroid profile between responders and non-responders. The key and consistent finding across the entire analysis is that non-responder status is associated with globally higher level of almost all analyzed steroids. The response of IgAN patients to CS therapy could be predicted by measuring the serum levels of selected steroid metabolites to receive steroid profile. Observation needs to be confirmed in large cohorts of various ethnic origin to be applicable for routine clinical protocols.

IgA nephropathy (IgAN) is characterized by glomerular deposits of IgA-containing immune complexes (IgA-ICs), which are suspected to originate from circulation. However, the composition of these ICs is not fully understood. To address this gap in knowledge, we performed label-free quantitative mass-spectrometry analyses of glomerular and circulatory IgA-ICs with a focus on complement proteins. Glomeruli of patients with IgAN compared to healthy glomeruli had greater amounts of several complement-system proteins associated with classical, alternative, and terminal pathways, including complement factor H-related (CFHR) proteins 1, 2, 3, and 5, C1q chains B and C, and properdin. Circulatory IgA-ICs of patients with IgAN vs. healthy controls had a greater abundance of complement proteins CFHR1, C1q chains A, B, and C, and properdin. Furthermore, levels of several complement proteins in circulatory IgA-ICs of IgAN patients were reduced after immunosuppressive therapy (i.e., tonsillectomy combined with pulse steroid therapy) but not in patients on comprehensive supportive therapy. CFHR1 exhibited the greatest decrease (Fold change = 48.16, P < 0.0001). These data together revealed the complexity of complement proteome in glomerular and circulatory IgA-ICs and suggested an association of complement regulatory proteins, such as CFHR1, with pathogenic IgA-ICs.