The vitamin D-binding protein, alpha-fetoprotein, albumin multigene family: detection of transcripts in multiple...

The serum vitamin D-binding protein (DBP), a member of a multigene family that includes alpha-fetoprotein (alpha FP) and albumin (ALB), is expressed at high levels in the adult liver. Although several studies have demonstrated the presence of DBP on the cell surface of immunocytes and cytotrophoblasts, evidence of extra-hepatic DBP synthesis remains inconclusive. Using Northern blot analysis of RNA from multiple rat tissues, the DBP transcripts were present in the expected high levels in the adult liver. In addition, DBP mRNA was found in the adult kidney, testis, abdominal fat, and 18-day fetal yolk sac. The concentrations of DBP mRNA in these tissues is 100-1000-fold less than in the liver. Polymerase chain reaction amplification (PCR) of DBP, alpha FP, and ALB cDNAs, reverse transcribed from a wide variety of rat tissue RNA samples, was performed to examine their respective tissue-specific patterns of gene transcripts. The tissues in which DBP gene transcripts were noted by Northern analysis were paralleled at 20 cycles of PCR with liver much greater than kidney, yolk sac, testis, and abdominal fat. Similarly at 20 cycles, alpha FP-amplified fragments were detected in the yolk sac much greater than brain, testes, uterus, liver, and placenta; and ALB-amplified fragments in liver much greater than testis, uterus, placenta, and yolk sac. Unexpectedly, at 32 cycles of PCR, DBP, alpha FP, and ALB mRNAs could be detected at very low levels in all tissues examined. To examine the nature of the DBP mRNA from these nonhepatic sources, a Sprague-Dawley kidney cDNA library was screened, and a single cDNA recombinant was identified. This clone contained a full length DBP cDNA with 14 nucleotide and 3 predicted amino acid differences from the Fischer liver DBP cDNA reported previously. We conclude that the kidney and liver DBP mRNAs are transcription products of the same DBP locus and that DBP, alpha FP, and ALB genes display different spectra of tissue distribution. The unexpected finding that all three spliced RNA transcripts could be detected in at least trace quantities in all tissues studied suggests a leaky level of transcription by this multigene family or their transcription at low levels by an unidentified stromal cell common to all organs studied.