Down Regulation of Thiamine Transporter Expression in Human Tubular Epithelial Cells In Vitro and Increased Thiamine Clearance in Diabetes
James R Larkin, Naila Rabbani, Daniel Zehnder and Paul J Thornalley
Talk at American Diabetes Association 71st Scientific Sessions, San Diego, California, USA (2011)
Diabetes 60 Supp 1 A1. DOI: 10.2337/db11-1-378
Increased renal clearance of thiamine (vitamin B1) is common in diabetic patients linked to decreased renal reuptake of thiamine. Two recent clinical studies evaluating thiamine supplements to correct thiamine loss in patients with type 2 diabetes and microalbuminuria showed decreased urinary albumin excretion and reversal of microalbuminuria. The aim of our investigation is to elucidate the mechanism of decreased renal reuptake of thiamine in diabetes. Thiamine is actively scavenged from glomerular ﬁltrate by two thiamine transporter proteins, THTR-1 and THTR-2, encoded by genes SLC19A2 and SLC19A3. The location of these transporters in human kidneys and the effect of high glucose concentration on transporter expression in human tubular epithelial cells in primary culture were investigated.
Renal location of THTR-1 and THTR-2 was investigated by immunohistochemical staining of parafﬁn-embedded human kidneys. Freshly extracted human proximal tubular epithelial cells were grown in primary culture in medium containing low and high glucose concentrations (5 or 26 mmol/L, respectively) with 4 nmol/L thiamine and the expression of SLC19A2 and SLC19A3 investigated.
Immunohistochemical staining of human kidneys showed particularly intense staining of THTR-1 and THTR-2 in the proximal tubule. When human proximal tubular epithelial cells were incubated in high glucose concentration, SLC19A2 and SLC19A3 mRNA was decreased (-76% and -56% respectively; p<0.001). Thiamine transporter protein levels were also decreased in high glucose concentration (THTR-1 -77%; THTR-2 -83%; both p<0.05). Concomitantly, forward:reverse apparent permeability of monolayers of proximal tubule epithelial cells to [3H]thiamine was decreased 23% in high glucose concentration (p<0.001).
We conclude that the proximal tubule is the likely major site in the kidney of reuptake of thiamine from glomerular ﬁltrate. The decreased renal reuptake of thiamine in diabetes is likely due to hyperglycemia-induced decreased expression of thiamine transporters in the renal tubular epithelium.
Supported by: Diabetes UK