Biotin-dependent expression of the asialoglycoprotein receptor in HepG2

The asialoglycoprotein receptor (AsGR) is characteristic of fully differentiated hepatocytes. AsGR expression in confluent cultures of HepG2 cells grown in minimal essential medium (MEM) requires a 300-350-dalton dialyzable fraction of fetal bovine serum (FBS). Addition to dialyzed FBS (dFBS) of 10(-7) M biotin or biocytin (Mr 372) permitted full expression of AsGR by HepG2. Affinity chromatography of FBS on streptavidin-Sepharose abolished its ability to support AsGR production. The bound material, when released by heat denaturation and resolved by thin layer chromatography, yielded three cinnamaldehyde-positive components, of which the major detectable one migrates with authentic biocytin and reconstitutes dFBS. Sera from several species, which do not support AsGR production by HepG2, contain less than 10% biotin found in FBS as determined by direct enzyme-linked immunosorbent assay. These results indicate that biotin or a derivative is the low molecular weight serum factor of FBS required for expression of AsGR. Isolation of messenger RNA from HepG2 revealed no difference in AsGR transcripts when cells were grown in MEM-10% FBS or MEM-10% dFBS. Thus a biotin-dependent post-transcriptional event permits the ultimate expression of the AsGR by HepG2 cells.