Decreased Expression of Transporters Reduces Folate Uptake across Renal Absorptive Surfaces in Experimental Alcoholism Abid Hamid Æ Jyotdeep Kaur Received: 30 April 2007 / Accepted: 26 September 2007 / Published online: 15 November 2007 Ó Springer Science+Business Media, LLC 2007 Abstract In this study, we examined the mechanistic insights of folate reabsorption during alcoholism, consid- ering enhanced renal excretion as one of the major contributing factors to alcohol-induced folate deficiency. Male Wistar rats were fed 1g/kg body weight/day ethanol (20% solution) orally for 3 months. The results on char- acterization of the folate transport system in renal basolateral membrane (BLM) suggested it to be a carrier- mediated, acidic pH-dependent and saturable one. Chronic ethanol feeding decreased the uptake mainly by increasing the K m and decreasing the V max of the transport process at the BLM surface. At the molecular level, reduced folate transport activity in renal tissue during chronic ethanol ingestion was attributable to decreased expression of reduced folate carrier (RFC) and folate binding protein (FBP). Antibodies against RFC protein revealed a parallel change in RFC expression in both brush border and BLM surfaces during chronic alcoholism. Such findings highlight the role of downregulation of RFC and FBP expression and provide mechanistic insight into the observed reduced folate transport efficiency at renal absorptive surfaces in alcoholism, which may result in low blood folate levels commonly observed in alcoholics. Keywords Folate binding protein Á Reduced folate carrier Á Alcoholism Á Basolateral Á Brush border Á Transport Introduction Folate-mediated one-carbon metabolism is of fundamental importance for various cellular processes, including DNA synthesis and methylation. Due to the exogenous require- ment of folate in mammals, there exists a well-developed epithelial folate transport system for regulation of normal folate homeostasis including intestinal uptake, renal tubular reabsorption and tissue distribution. The renal uptake of folate involves glomerular filtration followed by tubular reabsorption of filtered folate, which is essential for the conservation and normal homeostasis of this important vitamin (Sabharanjak & Mayor, 2004). Failure to reabsorb filtered folate would result in urinary losses of approxi- mately 1 mg/day and the subsequent rapid depletion of body stores. A high-affinity folate binding protein (FBP) or folate receptor and low-affinity transporter, the reduced folate carrier (RFC), concentrated respectively in the brush border membrane (BBM) and basolateral membrane (BLM) of proximal tubule cells, are the candidate proteins involved in folate conservation (Elwood, Deutsch & Kol- house, 1991; Wang et al., 2001). Moreover, the exact mechanism by which folates are transported, processed and released by the proximal tubule cells remains largely unresolved. It is a well-established fact that folic acid deficiency is the hallmark of alcoholism worldwide. Excessive alcohol intake exerts a multifaceted impact on the bioavailability and subsequent metabolism of folate and, more broadly, on one-carbon metabolism as a whole (Halsted et al., 2002). Earlier studies had suggested that both acute and chronic ethanol ingestion were associated with high loss of urinary folate (McMartin et al., 1985, 1989) and, hence, contribute to the observed folate deficiency during alcoholism (Hal- sted, 2001). In an attempt to gain mechanistic insight into A. Hamid Á J. Kaur (&) Department of Biochemistry, Postgraduate Institute of Medical Education and Research, Chandigarh 160 012, India e-mail: jyotdeep2001@yahoo.co.in 123 J Membrane Biol (2007) 220:69–77 DOI 10.1007/s00232-007-9075-3