Tài liệu Báo cáo khoa học: Down-regulation of reduced folate carrier may result in folate

Down-regulation of reduced folate carrier may result in

folate malabsorption across intestinal brush border

membrane during experimental alcoholism

Abid Hamid1

, Nissar Ahmad Wani1

, Satyavati Rana2

, Kim Vaiphei3

, Akhtar Mahmood4 and

Jyotdeep Kaur1

1 Department of Biochemistry, Postgraduate Institute of Medical Education and Research, Chandigarh, India

2 Department of Gastroenterology, Postgraduate Institute of Medical Education and Research, Chandigarh, India

3 Department of Histopathology, Postgraduate Institute of Medical Education and Research, Chandigarh, India

4 Department of Biochemistry, Panjab University, Chandigarh, India

Keywords

alcoholism; brush border membrane; crypt–

villus axis; methylation; reduced folate

carrier

Correspondence

J. Kaur, Department of Biochemistry,

Postgraduate Institute of Medical Education

and Research, Chandigarh 160 012, India

Fax: +91 172 2744401 ⁄ 2745078

Tel: +91 172 2747585 5181

E-mail: [email protected]

(Received 1 August 2007, revised 6 October

2007, accepted 17 October 2007)

doi:10.1111/j.1742-4658.2007.06150.x

Folate plays a critical role in maintaining normal metabolic, energy, differ￾entiation and growth status of all mammalian cells. The intestinal folate

uptake is tightly and diversely regulated, and disturbances in folate homeo￾stasis are observed in alcoholism, attributable, in part, to intestinal mal￾absorption of folate. The aim of this study was to delineate the regulatory

mechanisms of folate transport in intestinal absorptive epithelia in order to

obtain insights into folate malabsorption in a rat model of alcoholism. The

rats were fed 1 gÆkg)1 body weight of ethanol daily for 3 months. A

reduced uptake of [3

H]folic acid in intestinal brush border membrane was

observed over the course of ethanol administration for 3 months. Folate

transport exhibited saturable kinetics and the decreased intestinal brush

border membrane folate transport in chronic alcoholism was associated

with an increased Km value and a low Vmax value. Importantly, the lower

intestinal [3

H]folic acid uptake in ethanol-fed rats was observed in all cell

fractions corresponding to villus tip, mid-villus and crypt base. RT-PCR

analysis for reduced folate carrier, the major folate transporter, revealed

that reduced folate carrier mRNA levels were decreased in jejunal tissue

derived from ethanol-fed rats. Parallel changes were observed in reduced

folate carrier protein levels in brush border membrane along the entire

crypt–villus axis. In addition, immunohistochemical staining for reduced

folate carrier protein showed that, in alcoholic conditions, deranged

reduced folate carrier localization was observed along the entire crypt–vil￾lus axis, with a more prominent effect in differentiating crypt base stem

cells. These changes in functional activity of the membrane transport sys￾tem were not caused by a general loss of intestinal architecture, and hence

can be attributed to the specific effect of ethanol ingestion on the folate

transport system. The low folate uptake activity observed in ethanol-fed

rats was found to be associated with decreased serum and red blood cell

folate levels, which might explain the observed jejunal genomic hypomethy￾lation. These findings offer possible mechanistic insights into folate mal￾absorption during alcoholism.

Abbreviations

BBM, brush border membrane; BBMV, brush border membrane vesicle; LAP, leucine aminopeptidase; RBC, red blood cell;

RFC, reduced folate carrier; SAM, S-adenosyl methionine.

FEBS Journal 274 (2007) 6317–6328 ª 2007 The Authors Journal compilation ª 2007 FEBS 6317