SMPDL3b modulates insulin receptor signaling in diabetic kidney disease

Abstract

Sphingomyelin phosphodiesterase acid-like 3b (SMPDL3b) is a lipid raft enzyme that regulates plasma membrane (PM) fluidity. Here we report that SMPDL3b excess, as observed in podocytes in diabetic kidney disease (DKD), impairs insulin receptor isoform B-dependent pro-survival insulin signaling by interfering with insulin receptor isoforms binding to caveolin-1 in the PM. SMPDL3b excess affects the production of active sphingolipids resulting in decreased ceramide-1-phosphate (C1P) content as observed in human podocytes in vitro and in kidney cortexes of diabetic db/db mice in vivo. Podocyte-specific Smpdl3b deficiency in db/db mice is sufficient to restore kidney cortex C1P content and to protect from DKD. Exogenous administration of C1P restores IR signaling in vitro and prevents established DKD progression in vivo. Taken together, we identify SMPDL3b as a modulator of insulin signaling and demonstrate that supplementation with exogenous C1P may represent a lipid therapeutic strategy to treat diabetic complications such as DKD.

Modulation of radiation‐induced damage of human glomerular endothelial cells by SMPDL3B

“…Recent work established SMPDL3b as a C1P‐interacting protein which can also bind to CERK 13 . In addition, overexpression of SMPDL3b was reported to increase the conversion of C1P into ceramide 18 . These results are corroborated by our finding that radiation increases levels of SMPDL3b and phosphatase activity along with drop in C1P levels.…”

“…Western blotting analysis revealed a gradual and time‐dependent increase in the protein levels of SMPLD3b starting at 12 hours post‐irradiation, with the increase going beyond 2‐fold at 24 hours (Figure 2E). Recent work suggested a role for SMPDL3b in C1P dephosphorylation, 18 which prompted us to conduct a phosphatase assay. Our results show a significant elevation in phosphatase activity at 12 and 24 hours post‐IR (Figure 2F).…”

“…C1P phosphatase assay was conducted as described by Mitrofanova et al 18 About 50 μM of C6‐NBD‐C1P complexed to 10 μM of BSA was incubated with homogenates (8 μg of protein) in 200 mM of NaCl, 2 mM of EDTA, 100 mM of HEPES, pH 7.4, and a protease inhibitor cocktail (1:200) for 10 minutes at 37°C. The reaction was then terminated by 3.75 volumes of chloroform:methanol (1:2), 1.25 volume of 100% chloroform and 1.25 volumes of 2 M KCl/0.2 M H3PO4.…”

“…The sphingomyelin phosphodiesterase acid‐like 3b (SMPDL3b) enzyme is of particular interest to our work. Although the enzymatic activity of SMPDL3b is not well‐established yet, evidence pinpoints that SMPDL3b is located on the lipid rafts of podocytes and is involved in regulating ceramide and ceramide‐1‐phosphate (C1P) levels 17‐18 . Podocyte‐specific expression of SMPDL3b has been shown to play a role in focal segmental glomerulosclerosis (FSGS) and to facilitate cytoskeletal remodeling and cell migration 17,19 .…”

Lipotoxicity and Diabetic Nephropathy: Novel Mechanistic Insights and Therapeutic Opportunities

“…Mitrofanova et al described the involvement of the lipid raft enzyme sphingomyelinase-like phosphodiesterase 3b (SMPDL3b) in podocyte injury by altering active sphingolipid production and reducing ceramide-1-phosphate (C1P) [104,105]. Furthermore, restoration of C1P levels was sufficient to normalize the proteinuria levels observed in a murine model of DN [106].…”