Altered methionine metabolism in long living Ames dwarf mice

Abstract

Ames dwarf mice (df/df) are deficient in growth hormone, prolactin, and thyroid-stimulating hormone and live significantly longer than their normal siblings. In the current study, we found that the hormone deficiencies affect methionine metabolism. We previously reported that the dwarf mice exhibit enzyme activities and levels that combat oxidative stress more efficiently than those of normal mice. Moreover, methionine or metabolites of methionine are involved in antioxidative processes. Thus, we performed an experiment that compared various parameters of methionine metabolism between 18-month old male dwarf (N=6) and wild type (N=5) mice. The specific activity of liver methionine adenosyltransferase (MAT) was significantly elevated (205%, p<0.0001) in the dwarf mice, as were cystathionine synthase (50%, p<0.01), cystathionase (83%, p<0.001), and glycine N-methyltransferase (GNMT, 91%, p<0.001) activities. Even though the activities of MAT and GNMT were elevated, the concentration of liver S-adenosylmethionine was decreased (24%, p<0.001) and S-adenosylhomocysteine increased (113%, p<0.001) in the dwarf mice. These data indicate that dwarf mice, compared to wild type mice, have a markedly different metabolism of methionine. Altered methionine metabolism may partially explain earlier reports indicating less oxidative damage to proteins in dwarf mice. Taken together, the data suggest that methionine metabolism may play a role in oxidative defense in the dwarf mouse and should be studied as a potential mechanism of extended lifespan.

Introduction

Long-lived Ames dwarf mice have a homozygous recessive mutation at the Prop-1 locus, which leads to a lack of differentiation of somatotrophic, lactotrophic, and thyrotrophic pituitary cells. As a result, these mice lack growth hormone (GH), prolactin, and thyroid stimulating hormone (Hauck et al., 2001). Recent studies by Brown-Borg (Brown-Borg et al., 1999, Brown-Borg and Rakoczy, 2000, Brown-Borg et al., 2001) have focused on the relevance of reduced GH signaling to longevity and lower oxidative stress in the dwarf mice. The significantly longer life span exhibited by these GH-deficient mice is contrasted with reduced life span and increased free radical processes in transgenic mice that overexpress GH (Rollo et al., 1996, Brown-Borg and Rakoczy, 2000). In further support of a role for GH in longevity determination, two additional mouse lines, the GH-receptor/binding protein knockout (Laron dwarf) mouse and a line of transgenic mice that overexpress a GH antagonist also live significantly longer than normal animals from the same strain (Bartke, 1998, Coschigano et al., 2000).

Brown-Borg reported that concentrations of hepatic inorganic peroxides and mitochondrial H₂O₂ were decreased and liver and kidney catalase were markedly increased in Ames dwarf mice (Brown-Borg et al., 1999, Brown-Borg and Rakoczy, 2000, Brown-Borg et al., 2001). Catalase activity and protein were significantly elevated in livers from dwarf mice at 3, 6, 13–15, and 24 months of age when compared to age-matched wild type mice. Kidneys from old dwarf mice exhibited significantly increased catalase activity (22%), protein (16%) and mRNA expression (59%) compared to wild type mice (Brown-Borg and Rakoczy, 2000). Hypothalamic catalase has also been found to be elevated in these mice (Hauck and Bartke, 2000). The results of these studies suggested that GH status modulates antioxidative mechanisms and that catalase is important in overall defense capacity with respect to lifespan in the Ames dwarf mouse.

Growth hormone can also affect other metabolic pathways including methionine metabolism (Fig. 1). For example, glycine N-methyltransferase (GNMT), which plays a crucial role in the regulation of tissue concentrations of S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH), is regulated by GH (Aida et al., 1997).

Methionine metabolism also changes with aging. With age, the activities of Methionine adenosyltransferase (MAT), betaine homocysteine methyltransferase (BHMT), and methionine synthase decrease with age while cystathionine synthase (CS) and cystathionase (CTH) increase (Finkelstein, 1962, Finkelstein et al., 1971). MAT also decreases in male rats after puberty (Oscarsson et al., 2001). The concentration of SAM in the lens decreases with increasing age (Geller et al., 1988). The concentration of SAH in rat liver and cerebral cortex increases while brain SAM decreases with age (Hoffman et al., 1979, Trolin et al., 1994). Therefore, the expression of metabolites in the methionine metabolic pathway is tissue and age dependent.

As mentioned, the Ames dwarf mouse is lacking in GH, prolactin and thyroid stimulating hormone. These mice also have decreased concentrations of glucose, insulin, and thyroid hormones, and a reduced core body temperature (Borg et al., 1995, Hunter et al., 1999). Hypothyroid rats have decreased total plasma homocysteine and increased hepatic activities of CS and CTH; thyroid hormone administration to thyroidectomized rats normalized the plasma homocysteine and the activities of the enzymes (Jacobs et al., 2000). Pan and Tarver (Pan and Tarver, 1967) showed that thyroidectomized rats have increased activities of liver MAT and that administration of bovine GH to hypophysectomized rats resulted in a decrease in the activity of the enzyme. They also reported that administration of T₃ to normal rats resulted in a decreased activity of MAT. Keating et al. (Keating et al., 1988) also reported that the activity of MAT is elevated in hypothyroidism and that this resulted in an increased concentration of SAM.

Thus, the hormonal changes found in the Ames dwarf mouse may affect methionine metabolism. The objective of our studies was to determine whether or not methionine metabolism is indeed altered and if so, whether this may help explain the extended survival of these mice.