70-80% of kidney stones (KS) are composed of calcium oxalate, and minor changes in urine oxalate affect the stone risk. Obesity is a risk factor for KS and obese stone formers often have mild hyperoxaluria. A positive correlation between increased body size and elevated urinary oxalate excretion was reported;however, the underlying mechanism(s) remain(s) unknown. Therefore, we initiated studies in the obese ob/ob (ob) mice to define the pathogenesis of the obesity-associated hyperoxaluria (OAH). ob mice were found to have significantly higher urine oxalate (adjusted for creatinine) compared to controls (>2.9-fold), which is not due to overeating using pair-feeding. Significant hyperoxaluria was also seen in the obese db/db mice (>2.3-fold). We observed significantly higher jejunal (>46%) and ileal (>30%) 14C-oxalate and 3H-mannitol (a paracellular marker) absorptive fluxes in ob mice compared to controls ex vivo. Importantly, a significantly higher (>35%) urine 13C-oxalate was observed in ob mice compared to controls following an oral gavage with 13C-oxalate. The proinflammatory cytokines (PCs) IFN-? [IFN] and TNF-? [TNF], which are elevated in obesity, caused >3- fold increase in apical to basolateral 14C-oxalate and 3H-mannitol fluxes in human intestinal Caco2-BBE (C2) cells. The PC-induced increased absorptive fluxes were completed blocked by pretreatment with AMP-18 and GLP-2, peptides known to improve intestinal barrier function. Pretreatment of BALB/c mice with TNF before isolating and mounting jejunal segments in Ussing chambers led to >1.9-fold increase in 14C -oxalate and 3H- mannitol absorptive fluxes, an effect completely blocked by pretreatment with AMP or GLP. IFN, TNF, and IL-6 also significantly inhibited apical 14C-oxalate uptake by C2 cells through mechanisms involving reduced SLC26A6 (A6), an anion exchanger with essential role in intestinal oxalate secretion, mRNA/total protein expression. In addition, ob mice have significantly reduced jejunal A6 mRNA (>65%) and total protein expression. while a small net oxalate secretory flux was observed in control jejunal tissues, a large net absorptive flux was seen in ob tissues, due to significantly reduced secretory flux and an increased absorptive flux . Based on these findings, I will test the hypothesis that PCs play an important role in the OAH, and that anti-TNF, anti-IFN, and/or anti-IL-6 will normalize and/or ameliorate the observed hyperoxaluria. The following specific aims will be pursued: 1a. Evaluate the therapeutic effects of anti-TNF, anti-IFN, and anti-IL-6 on the observed hyperoxaluria.1b. Evaluate the effects of crossing the ob mice with the TNF, IFN, and/or IL-6 null mice on the observed hyperoxaluria to confirm their roles in the OAH. 2a. Assess small and large intestinal paracellular permeability in vivo in ob mice and their controls. 2b. Examine the therapeutic effects of AMP and GLP on the observed hyperoxaluria. 2c. Elucidate the in vivo mechanisms by which AMP, GLP, anti-TNF, anti- IFN, and/or anti-IL-6 normalize and/or ameliorate the observed hyperoxaluria. 3. Evaluate the effects of crossing the ob mice with the A6 null mice on the observed hyperoxaluria to confirm the role of A6 in the OAH.
Obesity is a risk factor for kidney stones (KS), and it is highly expected that the incidence of KS will continue to rapidly increase with the rising prevalence of obesity. Thus, defining the mechanisms underlying the reported positive correlation between increased body size and elevated urinary oxalate excretion is urgently needed.
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|Amin, Ruhul; Asplin, John; Jung, Daniel et al. (2018) Reduced active transcellular intestinal oxalate secretion contributes to the pathogenesis of obesity-associated hyperoxaluria. Kidney Int 93:1098-1107|