The wild-type agouti coat color exhibited by many mammals consists of individual hairs that are black with a sub-terminal band of yellow . The mouse agouti gene product is a secreted paracrine factor that regulates the alternate production of black and yellow pigments produced by hair-bulb melanocytes [2–4]. Binding of αMSH to the Mc1r on the surface of hair-bulb melanocytes results in the production of black pigment that is deposited in the growing hair. The agouti gene is transiently expressed in the skin during the mid-portion of the hair growth cycle. At this time, the agouti protein binds to the Mc1r, thereby excluding αMSH binding and causing a switch from black to yellow pigment production by melanocytes, which results in the appearance of the sub-terminal yellow band in the otherwise black hair [5–10].
Recessive mutations in the agouti gene affect only the coat color of mice, causing either a partial or complete loss of yellow pigment in the hair [11, 12]. The dominant agouti mutations, lethal yellow (A
) and viable yellow (A
), affect coat color by causing an increase in the amount of yellow pigment in the hair. Additionally, these dominant mutations cause mice to develop type II diabetes (peripheral insulin resistance, pancreatic islet hypertrophy and hyperplasia, hyperinsulinemia, and hyperglycemia), obesity (hyperphagia and increased adipose mass), increased somatic growth (increased fat-free dry mass and slightly longer bones), and increased susceptibility to hyperplasia and carcinogenesis in numerous tissues [reviewed in refs. [13–22]]. This syndrome is manifested in lethal yellow and viable yellow mice because they carry regulatory mutations in the agouti gene that cause the normal protein to be produced at abnormally high levels throughout the body [23–26].
In addition to its normal role of regulating pigmentation through Mc1r, agouti can also antagonize αMSH binding to other melanocortin receptor family members [5, 27–31]. The ability of agouti to antagonize binding of αMSH to the Mc4r is of particular relevance, as Mc4r is expressed in the brain and mice lacking functional Mc4r are hyperinsulinemic, hyperphagic, and obese . Mutations in human MC4R were also identified in dominantly inherited forms of human obesity [33, 34]. These results suggest that the hyperinsulinemia, hyperphagia, and obesity in lethal yellow and viable yellow mice is caused primarily by agouti-induced antagonism of Mc4r in the hypothalamus, a center of autonomic regulatory control in the brain . In addition to a role in the central nervous system, agouti expression in adipose tissue also appears to contribute to the obesity syndrome. Transgenic mice with adipocyte-specific agouti expression were shown to have significantly increased fat mass compared to control mice, which was accompanied by an increase in the protein levels of three transcription factors (Pparg, peroxisome proliferator activated receptor gamma; Stat1, signal transducer and activator of transcription 1; and Stat3) in their adipose tissue . These three transcription factors were also upregulated in mature 3T3-L1 adipocytes in culture following treatment with recombinant agouti protein . Additionally, recombinant agouti protein causes an increase in fatty acid synthase expression and activity, and the accumulation of triglycerides in cultured adipocytes . Together, these results suggest that the obesity-related factors of the dominant agouti syndrome are mediated by agouti expression in both the brain and peripheral tissue(s).
Dominant mutations in the agouti gene also cause an increase in the susceptibility to hyperplasia and carcinogenesis in the liver [38–45], skin [46, 47], lung [44, 48–50], mammary gland [38, 39, 51–54], and urinary bladder . It is likely that agouti-mediated antagonism of melanocortin receptors is mainly responsible for the obesity and diabetes of lethal yellow and viable yellow mice, but it is not known if melanocortin receptors are involved in their increased susceptibility to cancer. Whereas the obesity-related factors may contribute to the increased predisposition to carcinogenesis, there is some evidence to support the hypothesis that ectopic expression of the agouti gene per se may promote carcinogenesis in the liver and lung, even in the absence of hyperinsulinemia and obesity .
The liver is a primary site of insulin-mediated glucose disposal and lipogenesis in the mouse. Based on this fact, and on the previous reports of increased susceptibility to hepatic carcinogenesis in dominant agouti mutant mice, we were interested in determining if agouti expression in the liver alone would be sufficient to induce any of the phenotypes observed in lethal yellow or viable yellow mice. For this purpose, we generated lines of transgenic mice in which the wild-type murine agouti cDNA was expressed only in the liver at levels similar to or greater than those observed in lethal yellow or viable yellow mice. Transgenic and control mice were compared with respect to body weights, blood glucose levels, plasma insulin levels, and tumorigenic responses to chemical initiation in the liver.