A Targeted Apolipoprotein B-38.9-producing Mutation Causes Fatty Livers in Mice Due to the Reduced Ability of Apolipoprotein B-38.9 to Transport Triglycerides
2000; Elsevier BV; Volume: 275; Issue: 42 Linguagem: Inglês
10.1074/jbc.m004913200
ISSN1083-351X
AutoresZhouji Chen, Robin L. Fitzgerald, Maurizio Averna, Gustav Schonfeld,
Tópico(s)Lipid metabolism and disorders
ResumoNonphysiological truncations of apolipoprotein (apo) B-100 cause familial hypobetalipoproteinemia (FHBL) in humans and mice. An elucidation of the mechanisms underlying the FHBL phenotypes may provide valuable information on the metabolism of apo B-containing lipoproteins and the structure-function relationship of apo B. To generate a faithful mouse model of human FHBL, a subtle mutation was introduced into the mouse apo B gene by targeting embryonic stem cells using homologous recombination followed by removal of the selection marker gene by Cre-loxP-mediated site-specific recombination. The engineered mice bear a premature stop codon at residue 1767 and a 42-base pair loxP inserted into intron 24 of the apo B gene, thus closely resembling the apo B-38.9-producing mutation in humans. Apo B-38.9 was the sole apo B protein in homozygote (apob 38.9/38.9 ) plasma. In heterozygotes (apob +/ 38.9 ), apo B-100 and apo B-48 were reduced by 75 and 40%, respectively, and apo B-38.9 represented 20% of total circulating apo B. Hepatic apo B-38.9 mRNA levels were reduced by 40%. In culturedapob +/ 38.9 hepatocytes, apo B-100 was produced in trace quantities, and the synthesis rate of apo B-38.9 relative to apo B-48 was reduced by 40%. However, almost equimolar amounts of apo B-38.9 and apo B-48 were secreted into the media. Pulse-chase studies revealed that apo B-38.9 was secreted at a faster rate and more efficiently than apoB-48. Nevertheless, bothapob +/ 38.9 andapob 38.9/38.9 mice had reduced hepatic triglyceride secretion rates and fatty livers. Thus, low mRNA levels or defective secretion of apo B-38.9 may not be responsible for the FHBL phenotypes caused by the apo B-38.9 mutation. Rather, a reduced capacity of apo B-38.9 for triglyceride transport may account for the fatty livers in these mice. Nonphysiological truncations of apolipoprotein (apo) B-100 cause familial hypobetalipoproteinemia (FHBL) in humans and mice. An elucidation of the mechanisms underlying the FHBL phenotypes may provide valuable information on the metabolism of apo B-containing lipoproteins and the structure-function relationship of apo B. To generate a faithful mouse model of human FHBL, a subtle mutation was introduced into the mouse apo B gene by targeting embryonic stem cells using homologous recombination followed by removal of the selection marker gene by Cre-loxP-mediated site-specific recombination. The engineered mice bear a premature stop codon at residue 1767 and a 42-base pair loxP inserted into intron 24 of the apo B gene, thus closely resembling the apo B-38.9-producing mutation in humans. Apo B-38.9 was the sole apo B protein in homozygote (apob 38.9/38.9 ) plasma. In heterozygotes (apob +/ 38.9 ), apo B-100 and apo B-48 were reduced by 75 and 40%, respectively, and apo B-38.9 represented 20% of total circulating apo B. Hepatic apo B-38.9 mRNA levels were reduced by 40%. In culturedapob +/ 38.9 hepatocytes, apo B-100 was produced in trace quantities, and the synthesis rate of apo B-38.9 relative to apo B-48 was reduced by 40%. However, almost equimolar amounts of apo B-38.9 and apo B-48 were secreted into the media. Pulse-chase studies revealed that apo B-38.9 was secreted at a faster rate and more efficiently than apoB-48. Nevertheless, bothapob +/ 38.9 andapob 38.9/38.9 mice had reduced hepatic triglyceride secretion rates and fatty livers. Thus, low mRNA levels or defective secretion of apo B-38.9 may not be responsible for the FHBL phenotypes caused by the apo B-38.9 mutation. Rather, a reduced capacity of apo B-38.9 for triglyceride transport may account for the fatty livers in these mice. apolipoprotein very low density lipoproteins low density lipoproteins high density lipoproteins familial hypobetalipoproteinemia embryonic stem base pair(s) kilobase(s) fast performance liquid chromatography reverse transcription polymerase chain reaction polyacrylamide gel electrophoresis fetal bovine serum Dulbecco's modified Eagle's medium glutathione S-transferase Apolipoprotein B (apo B)1 is the major structural protein component of the triglyceride-rich very low density lipoproteins (VLDLs) secreted from the liver and the chylomicrons secreted from the intestine. The full-length apo B (apo B-100) is composed of 4536 amino acid residues (1Chan L. J. Biol. Chem. 1992; 267: 25621-25624Abstract Full Text PDF PubMed Google Scholar). Due to a posttranscriptional modification of the apo B gene (apob) mRNA that converts codon 2153, CAA, to a stop codon, UAA, the apo B protein also naturally exists in a truncated form corresponding to the NH2-terminal 48% of apo B-100, designated apo B-48 (2Powell L.M. Wallis S.C. Pease R.J. Edwards Y.H. Knott T.J. Scott J. Cell. 1987; 50: 831-840Abstract Full Text PDF PubMed Scopus (710) Google Scholar, 3Chen S.-H. Habib G. Yang C.Y. Gu Z.W. Lee B.R. Weng S.A. Silberman S.R. Cai S.J. Deslypere J.P. Rossened M. Gotto Jr., A.M. Li W.H. Chan L. Science. 1987; 238: 363-366Crossref PubMed Scopus (527) Google Scholar). In humans, apo B-48 is produced only by the intestine, whereas both the intestine and the liver in rodents secrete apo B-48 (4Greeve J. Altkemper I. Diesterich J.H. Greten H. Windler E. J. Lipid Res. 1993; 34: 1367-1383Abstract Full Text PDF PubMed Google Scholar, 5Higuchi K. Kitagawa K. Kogishi K. Takeda T. J. Lipid Res. 1992; 33: 1753-1764Abstract Full Text PDF PubMed Google Scholar). High levels of plasma apo B-containing lipoproteins are a major risk for the development of atherosclerotic diseases. Therefore, mechanisms controlling apo B synthesis and secretion are under active investigation.Nonsense and frameshift mutations in apob that produce nonphysiological COOH-terminal truncations of apo B-100 cause familial hypobetalipoproteinemia (FHBL) in humans, an autosomal codominant disorder characterized by low levels (<5th percentile) of plasma apo B and low density lipoprotein (LDL) cholesterol (6Schonfeld G. Annu. Rev. Nutr. 1995; 15: 23-34Crossref PubMed Scopus (78) Google Scholar, 7Linton M.F. Farese Jr., R.V. Young S.G. J. Lipid Res. 1993; 34: 521-541Abstract Full Text PDF PubMed Google Scholar). Numerous forms of truncated apo B, with sizes ranging from apo B-2 to apo B-89, have been identified in FHBL subjects (6Schonfeld G. Annu. Rev. Nutr. 1995; 15: 23-34Crossref PubMed Scopus (78) Google Scholar, 8Wu J. Kim J. Li Q. Kwok P.-Y. Cole T.G. Cefalu B. Averna M. Schonfeld G. J. Lipid Res. 1999; 40: 955-959Abstract Full Text Full Text PDF PubMed Google Scholar), and they are usually present in plasma at much lower concentrations than apo B-100 due to low production and high fractional catabolic rates (6Schonfeld G. Annu. Rev. Nutr. 1995; 15: 23-34Crossref PubMed Scopus (78) Google Scholar, 9Parhofer K.G. Barrett P.H. Bier D.M. Schonfeld G. J. Clin. Invest. 1992; 89: 1931-1937Crossref PubMed Scopus (61) Google Scholar, 10Krul E.S. Parhofer K.G. Barrett P.H. Wagner R.D. Schonfeld G. J. Lipid Res. 1992; 33: 1037-1050Abstract Full Text PDF PubMed Google Scholar, 11Parhofer K.G. Barrett P.H. Aguilar-Salinas C.A. Schonfeld G. J. Lipid Res. 1996; 37: 844-852Abstract Full Text PDF PubMed Google Scholar). The molecular mechanism(s) underlying these metabolic alterations is yet to be elucidated.In addition to the abnormalities in plasma cholesterol and apo B levels, fatty livers also occur in heterozygous FHBL subjects (12Tarugi P. Lonardo A. Ballarini G. Grisendi A. Pulvirenti M. Bagni A. Calandra S. Gastroenterology. 1996; 111: 1125-1133Abstract Full Text PDF PubMed Scopus (86) Google Scholar, 13Ogata H. Akagi K. Baba M. Nagamatsu A. Suzuki N. Nomiyama K. Fujishima M. Am. J. Gastroenterol. 1997; 92: 339-342PubMed Google Scholar, 14Castellano G. Garfia C. Gomez-Coronado D. Arenas J. Manzanares J. Colina F. Solis-Herruzo J.A. J. Clin. Gastroenterol. 1997; 25: 379-382Crossref PubMed Scopus (27) Google Scholar, 15Ahmed A. Keeffe E.B. Am. J. Gastroenterol. 1998; 93: 2598-2599Crossref PubMed Scopus (18) Google Scholar). Truncation-producing mutations in apob might impair the ability of the liver to secrete triglycerides due to low synthesis rates of the truncated apo B, its enhanced intracellular degradation, or its impaired assembly with lipids, resulting in a reduced capacity of the mutant apo B for transporting triglycerides. Gaining insights into apo B and lipid metabolism in FHBL at the molecular and cellular level may provide information critical not only to the elucidation of mechanisms for this disorder but also to defining further the structure-function relationship of apo B-100.However, FHBL patients are usually asymptomatic (6Schonfeld G. Annu. Rev. Nutr. 1995; 15: 23-34Crossref PubMed Scopus (78) Google Scholar, 7Linton M.F. Farese Jr., R.V. Young S.G. J. Lipid Res. 1993; 34: 521-541Abstract Full Text PDF PubMed Google Scholar), posing ethical barriers to obtaining liver and intestinal biopsies for in-depth studies. In recent years, several lines of mice bearing various forms of apo B truncations have been generated by targeting apobin embryonic stem (ES) cells (16Homanics G.E. Smith T.J. Zhang S.H. Lee D. Young S.G. Maeda N. Proc. Natl. Acad. Sci. 1993; 90: 2389-2393Crossref PubMed Scopus (110) Google Scholar, 17Toth L.R. Smith T.J. Jones C. Silva H.V. Smithies O. Maeda N. Gene. 1996; 178: 161-168Crossref PubMed Scopus (13) Google Scholar, 18Kim E. Cham C.M. Veniant M.M. Ambroziak P. Young S.G. J. Clin. Invest. 1998; 101: 1468-1477Crossref PubMed Google Scholar, 19Kim E. Ambroziak P. Veniant M.M. Hamilton R.L. Young S.G. J. Biol. Chem. 1998; 273: 33977-33984Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar). In these mice, the targetedapob alleles were expressed at low levels, leading to very low concentrations of truncated apo Bs in the plasma (16Homanics G.E. Smith T.J. Zhang S.H. Lee D. Young S.G. Maeda N. Proc. Natl. Acad. Sci. 1993; 90: 2389-2393Crossref PubMed Scopus (110) Google Scholar, 17Toth L.R. Smith T.J. Jones C. Silva H.V. Smithies O. Maeda N. Gene. 1996; 178: 161-168Crossref PubMed Scopus (13) Google Scholar, 18Kim E. Cham C.M. Veniant M.M. Ambroziak P. Young S.G. J. Clin. Invest. 1998; 101: 1468-1477Crossref PubMed Google Scholar, 19Kim E. Ambroziak P. Veniant M.M. Hamilton R.L. Young S.G. J. Biol. Chem. 1998; 273: 33977-33984Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar). It is not known whether these low observed levels of mRNA expression faithfully represent the mechanisms of FHBL existing in humans because in the mice, multiple copies of the gene-targeting construct were inserted into apob along with the desired mutations (16Homanics G.E. Smith T.J. Zhang S.H. Lee D. Young S.G. Maeda N. Proc. Natl. Acad. Sci. 1993; 90: 2389-2393Crossref PubMed Scopus (110) Google Scholar, 17Toth L.R. Smith T.J. Jones C. Silva H.V. Smithies O. Maeda N. Gene. 1996; 178: 161-168Crossref PubMed Scopus (13) Google Scholar, 18Kim E. Cham C.M. Veniant M.M. Ambroziak P. Young S.G. J. Clin. Invest. 1998; 101: 1468-1477Crossref PubMed Google Scholar, 19Kim E. Ambroziak P. Veniant M.M. Hamilton R.L. Young S.G. J. Biol. Chem. 1998; 273: 33977-33984Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar). The presence of these foreign DNA sequences in the genome might exert unnatural effects on the expression of the targeted gene or on genes nearby (20Hug B.A. Wesselschmidt R.L. Fiering S. Bender M.A. Epner E. Groudine M. Ley T.J. Mol. Cell. Biol. 1996; 16: 2906-2912Crossref PubMed Google Scholar). In the meantime, due to the low intracellular concentrations of the mutant apo Bs in the mouse livers, it has been difficult to study their posttranslational fates. Consequently, little is known about the cellular secretion and the function of the mutant apo Bs. Furthermore, the effect of modifying mouse apob on hepatic triglyceride secretion is yet to be explored.To generate a faithful mouse model of FHBL, we have used ES cell homologous recombination technology to introduce an apo B-38.9-specifying single-nucleotide deletion into the exon 26 of mouseapob. The selection marker gene sequence that was introduced into intron 24 of apob during the homologous recombination step was subsequently deleted using the Cre-loxP system (20Hug B.A. Wesselschmidt R.L. Fiering S. Bender M.A. Epner E. Groudine M. Ley T.J. Mol. Cell. Biol. 1996; 16: 2906-2912Crossref PubMed Google Scholar, 21Gu H. Zou Y-R Rajewsky K. Cell. 1993; 73: 1155-1164Abstract Full Text PDF PubMed Scopus (813) Google Scholar). Thus, the targeted apob allele of the resultant mice contains only a subtle mutation in the coding region plus a 34-base pair (bp) loxP sequence inserted into the middle of intron 24. This model more closely resembles the naturally occurring human apob mutations than the previously reported apob gene-targeted FHBL mouse models, in which large segments of extraneous DNA were retained. The apo B-38.9-bearing mice display fatty livers as well as FHBL phenotypes. We provide evidence suggesting that low mRNA levels or defective secretion of apo B-38.9 may not be responsible for the manifestation of these phenotypes. The apo B-38.9 truncation is secreted by hepatocytes more efficiently than apo B-48, but it has a reduced capacity for cellular triglyceride transport.DISCUSSIONThe FHBL associated with apo B-truncation-producing mutations is the best characterized subset of FHBL (6Schonfeld G. Annu. Rev. Nutr. 1995; 15: 23-34Crossref PubMed Scopus (78) Google Scholar, 7Linton M.F. Farese Jr., R.V. Young S.G. J. Lipid Res. 1993; 34: 521-541Abstract Full Text PDF PubMed Google Scholar). However, although the molecular bases of the genetic defects have been established, the corresponding cellular and molecular mechanisms that produce the FHBL phenotypes are relatively poorly understood. Previous studies usingapob-modified mice suggested that reduced mRNA levels of the mutant apob allele and the presumed resulting low production rates of apo B-containing lipoproteins might be primarily responsible for the FHBL phenotypes (16Homanics G.E. Smith T.J. Zhang S.H. Lee D. Young S.G. Maeda N. Proc. Natl. Acad. Sci. 1993; 90: 2389-2393Crossref PubMed Scopus (110) Google Scholar, 17Toth L.R. Smith T.J. Jones C. Silva H.V. Smithies O. Maeda N. Gene. 1996; 178: 161-168Crossref PubMed Scopus (13) Google Scholar, 18Kim E. Cham C.M. Veniant M.M. Ambroziak P. Young S.G. J. Clin. Invest. 1998; 101: 1468-1477Crossref PubMed Google Scholar, 19Kim E. Ambroziak P. Veniant M.M. Hamilton R.L. Young S.G. J. Biol. Chem. 1998; 273: 33977-33984Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar). In the current study, we have utilized a novel gene-targeting strategy to generate a mouse model that closely resembles the natural apo B-38.9 mutation in humans. In this FHBL model, levels of the apob 38.9 mRNA expression and intrahepatocytic synthetic rates of apo B-38.9 were reduced by approximately 40%. However, we did not note corresponding decreases in the rates of apo B-38.9 secretion. In fact, apo B-38.9 was secreted more efficiently than apo B-48. Thus, we demonstrated a dissociation between low apo B-38.9 mRNA levels and low apo B-38.9 synthesis rates on the one hand and the high secretion efficiency of apo B-38.9 on the other. Despite efficient apo B-38.9 secretion, rates of hepatic triglyceride secretion were reduced. This indicates that apo B-38.9 is defective for triglyceride transport, resulting in development of fatty livers in the mice. These are new insights into the structure-function relationship of apo B, as well as the FHBL syndrome.The apo B-38.9 mouse model reported here is unique in that only a single base pair deletion (nucleotide 5449) was introduced into the mouse apob coding region and one copy of Lox-P sequence (43 bp) was inserted into the middle of intron 24. Therefore, unlike the complex alterations introduced into apob in the previously reported FHBL mice (16Homanics G.E. Smith T.J. Zhang S.H. Lee D. Young S.G. Maeda N. Proc. Natl. Acad. Sci. 1993; 90: 2389-2393Crossref PubMed Scopus (110) Google Scholar, 17Toth L.R. Smith T.J. Jones C. Silva H.V. Smithies O. Maeda N. Gene. 1996; 178: 161-168Crossref PubMed Scopus (13) Google Scholar, 18Kim E. Cham C.M. Veniant M.M. Ambroziak P. Young S.G. J. Clin. Invest. 1998; 101: 1468-1477Crossref PubMed Google Scholar, 19Kim E. Ambroziak P. Veniant M.M. Hamilton R.L. Young S.G. J. Biol. Chem. 1998; 273: 33977-33984Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar), the apob modifications occurring in our mouse model are relatively minor and are very close to replicating a natural apo B-truncation-producing mutation in humans. The apo B-38.9-bearing mice displayed characteristics typical of FHBL in humans, including low plasma levels of apo B-100 and apo B-38.9 and LDL cholesterol. Plasma levels of HDL cholesterol were dramatically reduced in the homozygous mice. Similar observations have been made in prenatal mice homozygous for apob null-knockout (29Huang L.-S. Voyiaziakis E. Markenson D.F. Sokol K.A. Hayek T. Breslow J.L. J. Clin. Invest. 1995; 96: 2152-2161Crossref PubMed Scopus (100) Google Scholar), adult mice homozygous for the apo B-70-truncation (16Homanics G.E. Smith T.J. Zhang S.H. Lee D. Young S.G. Maeda N. Proc. Natl. Acad. Sci. 1993; 90: 2389-2393Crossref PubMed Scopus (110) Google Scholar), and mice bearing a liver-specific knockout of the microsomal triglyceride transport protein large subunit gene (30Chang B.H.-J. Liao W. Li L. Nakamuta M. Mack D. Chan L. J. Biol. Chem. 1999; 274: 6051-6055Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar, 31Raabe M. Veniant M.M. Sullivan M.A. Zlot C.H. Bjorkegren J. Nielsen L.B. Wong J.S. Hamilton R.L. Young S.G. J. Clin. Invest. 1999; 103: 1287-1298Crossref PubMed Scopus (357) Google Scholar). The decreased HDL levels in these mice appeared to be mainly due to enhanced HDL catabolism because apo AI synthesis was not affected (16Homanics G.E. Smith T.J. Zhang S.H. Lee D. Young S.G. Maeda N. Proc. Natl. Acad. Sci. 1993; 90: 2389-2393Crossref PubMed Scopus (110) Google Scholar, 29Huang L.-S. Voyiaziakis E. Markenson D.F. Sokol K.A. Hayek T. Breslow J.L. J. Clin. Invest. 1995; 96: 2152-2161Crossref PubMed Scopus (100) Google Scholar). We also observed no significant changes in the liver contents of apo AI protein and apo AI secretion by the hepatocytes due to the apo B-38.9 mutation (data not shown). Viableapob 38.9/38.9 mice were born and reached adulthood in significant numbers, albeit fewer were born than expected. Apo B plays an essential role in yolk sac nutrient transport for normal mouse embryonic development (29Huang L.-S. Voyiaziakis E. Markenson D.F. Sokol K.A. Hayek T. Breslow J.L. J. Clin. Invest. 1995; 96: 2152-2161Crossref PubMed Scopus (100) Google Scholar, 32Farese Jr., R.V. Ruland S.L. Flynn L.M. Stokowski R.P. Young S.G. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 1774-1778Crossref PubMed Scopus (213) Google Scholar). The ability ofapob 38.9/38.9 mice to survive indicates that apo B-38.9 may be capable of performing this transporting role, at least in part.One of the major issues we wanted to clarify was whether and to what extent the apo B-38.9-specifying mutation affects the steady-state level of apoB mRNA, given the previously reported low apoB mRNA levels (∼ 10–30% of normal) in the apoB truncation-bearing mice (16Homanics G.E. Smith T.J. Zhang S.H. Lee D. Young S.G. Maeda N. Proc. Natl. Acad. Sci. 1993; 90: 2389-2393Crossref PubMed Scopus (110) Google Scholar, 18Kim E. Cham C.M. Veniant M.M. Ambroziak P. Young S.G. J. Clin. Invest. 1998; 101: 1468-1477Crossref PubMed Google Scholar, 19Kim E. Ambroziak P. Veniant M.M. Hamilton R.L. Young S.G. J. Biol. Chem. 1998; 273: 33977-33984Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar). On allele-specific mRNA analysis and Northern blotting, the apo B-38.9 mutation did not affect the levels of mRNA expression of the wild type allele in heterozygotes, but it decreased the levels of mRNA expression of the apob 38.9 allele by 40–50%. The magnitude of this decrease is much smaller than those reported in other genetically engineered FHBL mice (16Homanics G.E. Smith T.J. Zhang S.H. Lee D. Young S.G. Maeda N. Proc. Natl. Acad. Sci. 1993; 90: 2389-2393Crossref PubMed Scopus (110) Google Scholar, 18Kim E. Cham C.M. Veniant M.M. Ambroziak P. Young S.G. J. Clin. Invest. 1998; 101: 1468-1477Crossref PubMed Google Scholar, 19Kim E. Ambroziak P. Veniant M.M. Hamilton R.L. Young S.G. J. Biol. Chem. 1998; 273: 33977-33984Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar), including the apo B-39-producing mice, which bear an apob premature stop codon only 18 residues downstream from the apo B-38.9-COOH terminus (19Kim E. Ambroziak P. Veniant M.M. Hamilton R.L. Young S.G. J. Biol. Chem. 1998; 273: 33977-33984Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar). The mutantapob 81 allele was expressed at an even lower level than the apob 39 allele (19Kim E. Ambroziak P. Veniant M.M. Hamilton R.L. Young S.G. J. Biol. Chem. 1998; 273: 33977-33984Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar) although the apo B-81- and apo B-39-bearing mice were produced using an identical gene-targeting strategy (18Kim E. Cham C.M. Veniant M.M. Ambroziak P. Young S.G. J. Clin. Invest. 1998; 101: 1468-1477Crossref PubMed Google Scholar, 19Kim E. Ambroziak P. Veniant M.M. Hamilton R.L. Young S.G. J. Biol. Chem. 1998; 273: 33977-33984Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar). In contrast with premature stop codons in the nonphysiological apo B-truncation sites, placing a nonsense mutation by in/out gene targeting at the apo B-editing site (apo B-48) had no effect on apo B mRNA (23Farese Jr., R.V. Veniant M.M. Cham C.M. Flynn L.M. Pierotti V. Loring J.F. Traber M. Ruland S. Stokowski R.S. Huszar D. Young S.G. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 6393-6398Crossref PubMed Scopus (114) Google Scholar). In fact, premature stop codons can be associated with either low (33Hamosh A. Trapnell B.C. Zeitlin P.L. Montrose-Rafizadeh C. Rosenstein B.J. Crystal R.G. Cutting G.R. J. Clin. Invest. 1991; 88: 1880-1885Crossref PubMed Scopus (118) Google Scholar, 34Longo N. Langley S.D. Griffin D. Elaas L.J. Am. J. Hum. Genet. 1992; 50: 998-1007PubMed Google Scholar, 35Frangi D.M. Cicardi M. Sica A. Colotta F. Agostoni A. Davus A.E. J. Clin. Invest. 1991; 88: 755-759Crossref PubMed Scopus (23) Google Scholar) or unaltered (36Liebhaber S.A. Coleman M.B. Adams J.G. Cash F.E. Steinberg M.H. J. Clin. Invest. 1987; 80: 154-159Crossref PubMed Scopus (32) Google Scholar, 37Winslow R.M. Swenberg M.-L. Gross E. Chervenick P.A. Buchman R.R. Anderson W.F. J. Clin. 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A surprising finding is that although the apo B-38.9 mutation decreased apob 38.9 mRNA levels and the rates of intrahepatocytic apo B-38.9 synthesis by 40–50%, it did not cause a corresponding decrease in the cellular secretion of apo B-38.9 because apo B-38.9 was secreted more efficiently than apoB-48 by hepatocytes. In fact, nearly equal molar amounts of total apo B were secreted by theapob +/+,apob +/ 38.9 , andapob 38.9/38.9 hepatocytes. Previous studies (39McLeod R.S. Zhao Y. Selby S.L. Westerlund J. Yao Z. J. Biol. Chem. 1994; 269: 2852-2862Abstract Full Text PDF PubMed Google Scholar, 40Cavallo D. McLeod R.S. Rudy D. Aiton A. Yao Z. Adeli K. J. Biol. Chem. 1998; 273: 33397-33405Abstract Full Text Full Text PDF PubMed Scopus (30) Google Scholar) using rat hepatoma cells overexpressing truncated apo B proteins have suggested that COOH-terminal truncation of apo B-100 does not impair the secretion of the resultant truncated apo B. The present study, for the first time, provides data verifying these observations under physiological settings. Unlike the situation in human livers, in which the apo B-100 is the sole apob product, apo B-48 accounted for more than 95% of the wild type apoB allele product from hepatocytes of the apob +/+ andapob +/ 38.9 mice, consistent with previous observations (5Higuchi K. Kitagawa K. Kogishi K. Takeda T. J. Lipid Res. 1992; 33: 1753-1764Abstract Full Text PDF PubMed Google Scholar, 30Chang B.H.-J. Liao W. Li L. Nakamuta M. Mack D. Chan L. J. Biol. Chem. 1999; 274: 6051-6055Abstract Full Text Full Text PDF PubMed Scopus (116) Google Scholar, 41Twisk J. Gillian-Daniel D.L. Tebon A. Wang L. Barrett P.H.R. Attie A.D. J. Clin. Invest. 2000; 105: 521-532Crossref PubMed Google Scholar). The relative rates of secretion of apoB-100 and apoB 38.9 remain to be determined inapob 100/38.9 mice.Triglyceride secretion was decreased in the hepatocytes ofapob +/ 38.9 mice and severely impaired inapob 38.9/38.9 mice, indicating that apo B-38.9, although secreted in expected molar quantities, may not be as capable as apo B-48, the physiological form of truncated apo B, for transporting triglycerides. This finding is consistent with a recent study of apo B-39-bearing mice that showed that the particle volume of plasma apo B-39-VLDL was only about half that of the apo B-48-VLDL (19Kim E. Ambroziak P. Veniant M.M. Hamilton R.L. Young S.G. J. Biol. 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Thus, selected mutations in the apo B gene could cause a subset of fatty livers.Although apo B-48 and apo B-38.9 were secreted at similar rates inapob +/ 38.9 mice, the plasma concentration of apo B-38.9 was only about half of that of apo B-48. Likewise, the apo B-38.9 levels in plasmas of heterozygous FHBL human subjects were much lower than those of apo B100 (22Groenewegen W.A. Averna M.R. Pulai J. Krul E.S. Schonfeld G. J. Lipid Res. 1994; 35: 1012-1025Abstract Full Text PDF PubMed Google Scholar). Most of the human (22Groenewegen W.A. Averna M.R. Pulai J. Krul E.S. Schonfeld G. J. Lipid Res. 1994; 35: 1012-1025Abstract Full Text PDF PubMed Google Scholar) and mouse apo B-38.9 particles are of HDL particle sizes, whereas the apo B-48 particles are much larger. Due to their smaller sizes, the Lp B-38.9 particles may be distributed in the interstitial fluid, as well as in plasma, thereby diluting the plasma pool of apo B-38.9. Furthermore, apo B-38.9 may be cleared from plasma at a faster rate than apo B-48. We have previously shown that human lipoproteins containing short apo B truncations, including apo B-38.9, were cleared from the plasma much more rapidly than the apo B-100-
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