Oxalobacter formigenes–Derived Bioactive Factors Stimulate Oxalate Transport by Intestinal Epithelial Cells

Abstract

Hyperoxaluria is a major risk factor for kidney stones and has no specific therapy, although <i>Oxalobacter formigenes</i> colonization is associated with reduced stone risk. <i>O. formigenes</i> interacts with colonic epithelium and induces colonic oxalate secretion, thereby reducing urinary oxalate excretion, <i>via</i> an unknown secretagogue. The difficulties in sustaining <i>O. formigenes</i> colonization underscore the need to identify the derived factors inducing colonic oxalate secretion. We therefore evaluated the effects of <i>O. formigenes</i> culture conditioned medium (CM) on apical ¹⁴C-oxalate uptake by human intestinal Caco-2-BBE cells. Compared with control medium, <i>O. formigenes</i> CM significantly stimulated oxalate uptake (>2.4-fold), whereas CM from <i>Lactobacillus acidophilus</i> did not. Treating the <i>O. formigenes</i> CM with heat or pepsin completely abolished this bioactivity, and selective ultrafiltration of the CM revealed that the <i>O. formigenes</i>-derived factors have molecular masses of 10-30 kDa. Treatment with the protein kinase A inhibitor H89 or the anion exchange inhibitor 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid completely blocked the CM-induced oxalate transport. Knockdown of the oxalate transporter SLC26A6 also significantly restricted the induction of oxalate transport by CM. In a mouse model of primary hyperoxaluria type 1, rectal administration of <i>O. formigenes</i> CM significantly reduced (>32.5%) urinary oxalate excretion and stimulated (>42%) distal colonic oxalate secretion. We conclude that <i>O. formigenes</i>-derived bioactive factors stimulate oxalate transport in intestinal cells through mechanisms including PKA activation. The reduction in urinary oxalate excretion in hyperoxaluric mice treated with <i>O. formigenes</i> CM reflects the <i>in vivo</i> retention of biologic activity and the therapeutic potential of these factors.