Liver-specific expression of the agouti gene in transgenic mice promotes liver carcinogenesis in the absence of obesity and diabetes.
BACKGROUND
The agouti protein is a paracrine factor that is normally present in the skin of many species of mammals. Agouti regulates the switch between black and yellow hair pigmentation by signalling through the melanocortin 1 receptor (Mc1r) on melanocytes. Lethal yellow (Ay) and viable yellow (Avy) are dominant regulatory mutations in the mouse agouti gene that cause the wild-type protein to be produced at abnormally high levels throughout the body. Mice harboring these mutations exhibit a pleiotropic syndrome characterized by yellow coat color, obesity, hyperglycemia, hyperinsulinemia, and increased susceptibility to hyperplasia and carcinogenesis in numerous tissues, including the liver. The goal of this research was to determine if ectopic expression of the agouti gene in the liver alone is sufficient to recapitulate any aspect of this syndrome. For this purpose, we generated lines of transgenic mice expressing high levels of agouti in the liver under the regulatory control of the albumin promoter. Expression levels of the agouti transgene in the liver were quantified by Northern blot analysis. Functional agouti protein in the liver of transgenic mice was assayed by its ability to inhibit binding of the alpha-melanocyte stimulating hormone (alphaMSH) to the Mc1r. Body weight, plasma insulin and blood glucose levels were analyzed in control and transgenic mice. Control and transgenic male mice were given a single intraperitoneal injection (10 mg/kg) of the hepatocellular carcinogen, diethylnitrosamine (DEN), at 15 days of age. Mice were euthanized at 36 or 40 weeks after DEN injection and the number of tumors per liver and total liver weights were recorded.


RESULTS
The albumin-agouti transgene was expressed at high levels in the livers of mice and produced a functional agouti protein. Albumin-agouti transgenic mice had normal body weights and normal levels of blood glucose and plasma insulin, but responded to chemical initiation of the liver with an increased number of liver tumors compared to non-transgenic control mice.


CONCLUSIONS
The data demonstrate that liver-specific expression of the agouti gene is not sufficient to induce obesity or diabetes, but, in the absence of these factors, agouti continues to promote hepatocellular carcinogenesis.
Kuklin
Alexander I
AI
Life Sciences Division, Oak Ridge National Laboratory, P,O, Box 2008, Oak Ridge, TN 37831, USA. akuklin@transgenomic.com
Mynatt
Randall L
RL
Klebig
Mitchell L
ML
Kiefer
Laura L
LL
Wilkison
William O
WO
Woychik
Richard P
RP
Michaud
Edward J
EJ
D054366:Agouti Signaling Protein; D000418:Albumins / Q000235:genetics*; D000818:Animals; D001786:Blood Glucose / Q000235:genetics; D001835:Body Weight / Q000502:physiology; D002273:Carcinogens / Q000008:administration & dosage / Q000009:adverse effects; D018076:DNA, Complementary / Q000235:genetics; D003920:Diabetes Mellitus / Q000235:genetics*; D004052:Diethylnitrosamine / Q000008:administration & dosage / Q000009:adverse effects; D007328:Insulin / Q000097:blood; D036341:Intercellular Signaling Peptides and Proteins / Q000096:biosynthesis / Q000235:genetics* / Q000502:physiology; D008099:Liver / Q000737:chemistry* / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D008113:Liver Neoplasms / Q000139:chemically induced / Q000235:genetics*; D008297:Male; D051379:Mice; D008819:Mice, Nude; D008822:Mice, Transgenic; D009765:Obesity / Q000235:genetics*; D009928:Organ Specificity / Q000235:genetics; D011401:Promoter Regions, Genetic / Q000235:genetics; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology
D054366:Agouti Signaling Protein; D000418:Albumins; D001786:Blood Glucose; D002273:Carcinogens; D018076:DNA, Complementary; D007328:Insulin; D036341:Intercellular Signaling Peptides and Proteins; D011993:Recombinant Fusion Proteins; C515183:a protein, mouse; D004052:Diethylnitrosamine