Acute Ethanol Exposure Inhibits Renal Folate Transport, but Repeated Exposure Upregulates Folate Transport Proteins in Rats and Human Cells

Abstract

Deficiency of folate in heavy-drinking alcoholic populations can occur partly because of an increased urinary folate excretion. Ethanol directly reduces the reabsorption of folate in the renal proximal tubule (PT) by acting on either of 2 folate transport proteins, the reduced folate carrier (RFC) and the folate receptor (FR). This study was designed to determine the effects of ethanol on the transport of folate by PT cells and to examine the effects of ethanol on RFC and the FR protein expression. Normal human PT (HPT) cells were cultured on membrane inserts to study intracellular transport of 5-methyltetrahydrofolate from the apical or basolateral direction in the presence of ethanol [11-109 mmol/L (50-500 mg/dL)]. The long-term effect of ethanol on the renal folate transport protein content was determined by western blot in treated HPT cells and in vivo in rats pair-fed control diets or ethanol-containing liquid diets. A 1-h treatment of HPT cells with ethanol (> or = 65 mmol/L) reduced the apically directed transport of folate by 20-25% without affecting the basolateral transport. A 5-d exposure of HPT cells to ethanol dose-dependently increased the content of both the FR and RFC proteins, with a greater effect on the RFC. Similarly, a 14-d exposure of rats to ethanol increased the in vivo expression of both the RFC and FR. These studies demonstrate that ethanol decreases the reabsorptive transport of folate by renal PT cells, which would increase urinary folate excretion. In contrast, subchronic exposure of PT cells, both in vivo and in vitro, to folate-depleting concentrations of ethanol leads to an upregulation of the 2 folate transport proteins. The increase in folate transporters partly counteracts the inhibitory effects of ethanol on folate transport activity, which explains the lower magnitude of ethanol's effect on transport with subchronic exposure compared with that with acute exposure.

Increased synthesis of folate transporters regulates folate transport in conditions of ethanol exposure and folate deficiency

“…Similar results have been observed previously in case of dietary FD [19,26,36,41]; however, none of these studies identified the underlying regulatory mechanism responsible for the adaptive alterations in the expression of folate transporters at the mRNA level. In a study carried out by Romanoff et al [42], no change was observed in folate transport after 5 days of ethanol exposure to HPT cells; however, they observed an increase in expression of folate transporters on ethanol exposure which is in agreement to our study. In HEK293 cells, in spite of increased expression of folate transporters, a decrease in the folate levels was seen on ethanol exposure.…”

B-Vitamin dependent methionine metabolism and alcoholic liver disease

“…Whereas these observations can be explained by an effect of acute ethanol on decreasing renal tubular uptake and transport of folate in rats [46] , long-term ethanol exposure in micropigs had no effect on the expression of renal RFC [38] .…”

Folate, alcohol, and liver disease

“…However, long‐term ethanol exposure in a micropig model of ALD had no effect on folic acid transport by isolated renal tubular brush border membranes or on the expression of renal tubular RFC . A more recent in vitro study of human proximal renal tubular cells found decreased transport of 5‐MTHF in response to short‐term ethanol exposure, whereas subchronic ethanol exposure in rats increased the expressions of renal tubular FBP and RFC . These studies suggest that increased urine folate excretion is caused by acute ethanol exposure, with subsequent adaptation to chronic alcoholism.…”

“…In another human experiment, the hematological response to folic acid supplementation was repeatedly interrupted by ingestion of ethanol in an anemic chronic and folate‐deficient patient . The mechanisms for these phenomena remain unexplained though might be related to effects of acute ethanol on redistribution of circulating folate to the enterohepatic cycle or on increasing urine folate excretion . Others showed that acetaldehyde, the initial metabolite of alcohol, causes oxidative destruction of 5‐MTHF in vitro .…”