Human placental tissue expresses a novel 22.7 kDa apolipoprotein A-I-like protein.
Since apolipoprotein A-I (apo A-I) and HDL stimulate the expression of the placental hormone human placental lactogen (hPL), experiments were performed to determine whether the human placenta synthesizes apo A-I. Western blot analysis of a partially purified extract of human term placenta with an antiserum to human apo A-I yielded an immunoreactive band with an apparent mass of approximately 23.5 kDa, which is smaller than human plasma apo A-I (28 kDa). HPLC chromatography of the partially purified placental extract on a preparative reverse-phase C-18 column yielded two fractions that reacted to the apo A-I antiserum. The mass of both fractions by mass spectral analysis was 22 721 daltons, and N-terminal amino acid sequences were identical to the first four amino acids of apo A-I (Asp, Glu, Pro, Pro). The apo A-I-like protein was not a proteolytic product of apo A-I since Northern analysis of placental RNA with a 641 bp apo A-I cDNA fragment encoding most of the 5' region of the apo A-I mRNA detected a single band of 850 nt, which is smaller than the size of apo A-I mRNA (1100 nt). Placental mRNA, however, did not hybridize with a 3' apo A-I riboprobe, indicating that the 3' region of the apo A-I-like mRNA is different from that of apo A-I mRNA. Differences in the mRNAs were confirmed by S1 nuclease analysis of placental RNA with a cDNA probe that included the 3' end of the apo A-I cDNA and by RT-PCR analysis with a series of oligonucleotide primers that span the entire cDNA for apo A-I. Since there is only a single apo A-I gene in the human genome, these findings strongly suggest that human placental tissue expresses a novel 22.7 kDa apo A-I-like protein (ALP) that results from alternative splicing of the apo A-I primary transcript.
Richardson
B
B
Division of Endocrinology, Children's Hospital Medical Center, Cincinnati, Ohio, USA.
Palgunachari
M N
MN
Anantharamaiah
G M
GM
Richards
R G
RG
Azrolan
N
N
Wiginton
D
D
Handwerger
S
S
D016632:Apolipoprotein A-I / Q000096:biosynthesis* / Q000737:chemistry / Q000302:isolation & purification; D001483:Base Sequence; D015152:Blotting, Northern; D006528:Carcinoma, Hepatocellular; D002460:Cell Line; D002851:Chromatography, High Pressure Liquid; D017931:DNA Primers; D015342:DNA Probes; D005091:Exons; D005260:Female; D006801:Humans; D007438:Introns; D008099:Liver / Q000378:metabolism*; D008113:Liver Neoplasms; D008969:Molecular Sequence Data; D008970:Molecular Weight; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D012333:RNA, Messenger / Q000032:analysis / Q000096:biosynthesis; D014407:Tumor Cells, Cultured
D016632:Apolipoprotein A-I; D017931:DNA Primers; D015342:DNA Probes; D012333:RNA, Messenger