AIM: The molecular mechanisms by which hepatocyte nuclear factor (HNF)4α 
regulates fetal liver development have not been fully elucidated. We screened 
the downstream molecules of HNF4α during liver development and identified 
sodium-coupled neutral amino acid transporter (SNAT)4. The aim of this study is 
to investigate the regulation of SNAT4 by HNF4α and to clarify its roles in 
differentiating hepatocytes.
METHODS: HNF4α was overexpressed in cultured liver buds using adenovirus, and 
suppression subtractive hybridization screening was performed. Temporal and 
spatial expression of SNAT4 during liver development was investigated. 
Regulation of SNAT4 by HNF4α was examined by promoter analyses and 
electrophoretic mobility shift assays (EMSA). Metabolic labeling and western 
blotting were carried out using primary hepatoblasts with SNAT4 overexpression.
RESULTS: The expression of Slc38a4 encoding SNAT4 showed a marked perinatal 
increase, and was predominant among system A amino acid transporters. It was 
first detected in embryonic day 18.5 liver, and found in most hepatocytes after 
birth. Three alternative first exons were found in the SNAT4 gene. Promoter 
analyses using approximately 3-kb fragments corresponding to each first exon 
(AP1, AP2, AP3) revealed that AP1 and AP2 exhibited strong promoter activity in 
mouse hepatoblasts with endogenous HNF4α. Transactivation of AP2 was upregulated 
by HNF4α in HeLa cells without endogenous HNF4α. EMSA has demonstrated that 
HNF4α directly binds to cis-elements in AP2. Overexpression of SNAT4 facilitated 
amino acid uptake and de novo protein synthesis in primary hepatoblasts.
CONCLUSION: SNAT4 functions downstream of HNF4α and plays significant roles in 
liver development through mechanisms of amino acid uptake and protein synthesis.