In This Issue of Diabetes

The drug roflumilast might be able to reverse diabetes-associated cardiac dysfunction and hyperglycemia, according to Xu et al. (p. 1660). In very early-stage mechanistic studies, the authors explore a cardiac dysfunction mechanism that centers on the expression of phosphodiesterase 4D (PDE4D) and the microRNA miR-1. Focusing initially on mice given a high-fat diet (HFD), the authors used either pharmacological inhibition or cardiac-specific knockdown of PDE4D. They found that the mice developed diabetes as well as cardiac dysfunction and that the PDE4 inhibitor roflumilast could reverse it along with hyperglycemia. On top of that, cardiac-specific expression of the gene PDE4D5 was also raised, and according to the authors, roflumilast resulted in decreased expression of PDE4D and increased miR-1 levels in heart muscle. Further experiments showed that knockdown of PDE4D or overexpression of miR-1 reversed cardiac dysfunction in the mice on HFD, but this was despite hyperglycemia persisting. Moving to cardiomyocytes/cardiac fibroblasts and gain-/loss-of-function studies of PDE4D, they go on to uncover mechanisms that broadly implicate cAMP-CREB-Sirt1 and TGF-β1 signaling that appears to control expression of several key genes. Final experiments demonstrated increased expression of the PDE4D5 gene in human hearts that had failed. While suggesting roflumilast might represent a potential therapeutic option for cardiac dysfunction in diabetes, the authors point out that further studies will be needed because PDE4 is expressed in a much wider range of cell types and signaling pathways away from those of a cardiac nature. Commenting on the study, author Qin Fu said: “Our recent study (PMID 34826603) showed that overexpression of PDE4D in mouse liver is sufficient to trigger nonalcoholic fatty liver disease (NAFLD) and hypertension. HFD-fed mice developed NAFLD and hypertension, with an associated increase in hepatic PDE4D expression, which can be reversed by roflumilast. Our aggregate studies suggest that the inhibitors of PDE4D may be important as potential drugs for treatment of diabetic complications.”

Apolipoprotein M (apoM) may be elevated at very early stages in patients with type 1 diabetes who go on to develop macroalbuminuria, chronic kidney disease (CKD), or both, according to Baker et al. (p. 1795). ApoM was even elevated at the very early stages of disease, even when the albumin excretion rate and estimated glomerular filtration rate were considered normal. According to the authors, this all suggests that the composition and function of HDL (with which apoM is usually associated) is altered at very early stages of disease and may be associated with nephropathy development. The study is based on 386 individuals with type 1 diabetes who were part of the wider Diabetes Control and Complications Trial (DCCT)/Epidemiology of Diabetes Interventions and Complications (EDIC) study that examined the effects of intensive insulin therapy. Plasma apoM levels were determined by enzyme immunoassay in stored plasma samples first at baseline and then also at closeout of the DCCT. The authors then used apoM concentrations to estimate risk of kidney dysfunction in later years after adjusting for a range of factors. They found that apoM levels at baseline were elevated in individuals who went on to develop CKD compared with the majority who remained without any kidney issues. At closeout of the study, individuals who developed macroalbuminuria, CKD, or both also exhibited elevated apoM levels. This translated to a significant overall increase in risk of progression to CKD (risk ratio 1.55; 95% CI 1.17, 2.06) but not for macroalbuminuria or both. The effects were primarily driven by increases in apoM at DCCT closeout, where risk of progression to macroalbuminuria or CKD (but not both) was significantly increased. Based on the findings, the authors suggest that further research should look at whether and how such changes might be treated at early stages of kidney complication progression. Author Nathaniel L. Baker added: “Understanding early alterations in markers of disease progression would present an advance for averting the development of diabetic kidney disease in people living with type 1 diabetes.”

One of the issues with pancreatic islet transplantation is that islet cells sometimes die due to hypoxia caused by insufficient revascularization—basically, they are starved of oxygen. According to Wrublewsky et al. (p. 1706), inhibition of nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) might hold some promise in improving survival and also islet endocrine functions such as insulin release. Combining in vitro and in vivo mouse studies, the authors focus on comparing islets from donor wild-type mice and those of a transgenic Nlrp3–/– version. Further, they also compare wild-type islets exposed to either vehicle or NLRP3 inhibitor CY-09. In initial experiments, they found that the lack of NLRP3 activity does not affect islet viability or their cellular composition. Using a dorsal skinfold chamber mouse model, they showed that transplanted islets had greater rates of graft revascularization when NLPR3 was genetically lost or pharmacologically inhibited. Additional cellular analyses revealed that the loss of NLRP3 activity triggers insulin gene expression, which, in turn, promoted the revascularization of the grafts. Finally, the authors investigated the effect of the loss or inhibition of NLRP3 activity on the endocrine function of transplanted islets. They found that repression of NLRP3 activity leads to normoglycemia in mice with streptozotocin-induced diabetes. Based on the overall outcomes, the authors point out that isolated human islets are commonly cultured to essentially clean them up prior to transplantation. They suggest that using NLRP3 inhibitors at this pretransplant stage might improve clinical outcomes. Commenting further, author Emmanuel Ampofo said: “The identification that NLRP3 represses insulin expression is not only of major interest for islet transplantation but also gives us new insights into the molecular mechanisms of how the inflammasome influences the endocrine function of islets. The next challenge is to understand the role of other inflammasomes and their downstream effector molecules in islet cell function.”

The metabolic benefits of bariatric surgery appear to center on the inhibition of sodium–glucose cotransporter 2 (SGLT2) expression in the kidney, according to the study by Akalestou et al. (p. 1623). Specifically, the authors found that vertical sleeve gastrectomy in lean and obese mice resulted in reduced expression of SGLT2 in the kidney, which in turn accelerated glucose excretion and reduced blood glucose levels. Bariatric surgery can be remarkably effective at reducing weight and in certain circumstances can reverse type 2 diabetes. However, the specific mechanisms behind the effects have remained unclear, prompting the study. They found that surgery resulted in improved glucose control that was independent of weight changes and that SGLT2 expression decreased in kidneys. They also found that pharmacological SGLT2 inhibition with dapagliflozin resulted in no additional changes in SGLT2 expression following surgery and that the mice experienced glucosuria. Meanwhile, mice maintained on a high-fat diet experienced weight increases but also pronounced glucosuria and further changes in the expression of SGLT2. To confirm the role of the SGLT2 protein, the authors then used CRISPR/Cas9 to inactivate the specific SGLT2 gene, which resulted in attenuation of the effects of surgery on glucose levels. While administration of the SGLT2 inhibitor dapagliflozin had no further effects on glucose levels following surgery, that was not case in the mice without the SGLT2 gene. In this case, there were still improvements in glucose tolerance, leading the authors to suggest an apparent off-target effect of the drug. While the authors did investigate a range of potential molecular markers (largely drawing blanks), they say that exploring such pathways might help unlock pharmacological treatments that recapitulate the metabolic effects of bariatric surgery. In their discussion, the authors state: “These findings provide important insights into the mechanisms underlying type 2 diabetes remission following bariatric surgery and point to a potential gut–kidney axis whereby (presently undefined) factors, likely to be released from the intestine, influence gene expression in the kidney.”