Metallothionein and the biology of aging

Abstract

Metallothionein (MT) is a low molecular weight protein with anti-apoptotic properties that has been demonstrated to scavenge free radicals in vitro. MT has not been extensively investigated within the context of aging biology. The purpose of this review, therefore, is to discuss findings on MT that are relevant to basic aging mechanisms and to draw attention to the possible role of MT in pro-longevity interventions. MT is one of just a handful of proteins that, when overexpressed, has been demonstrated to increase mouse lifespan. MT also protects against development of obesity in mice provided a high fat diet as well as diet-induced oxidative stress damage. Abundance of MT is responsive to caloric restriction (CR) and inhibition of the insulin/insulin-like signaling (IIS) pathway, and elevated MT gene expression has been observed in tissues from fasted and CR-fed mice, long-lived dwarf mice, worms maintained under CR conditions, and long-lived daf-2 mutant worms. The dysregulation of MT in these systems is likely to have tissue-specific effects on aging outcomes. Further investigation will therefore be needed to understand how MT contributes to the response of invertebrates and mice to CR and the endocrine mutations studied by aging researchers.

Figure 1. Overview of biological outcomes associated with increased cellular abundance of metallothionein (MT)

Genetic or pharmacological treatments that augment MT abundance have been shown to increase lifespan in model systems (e.g., Mocchegiani et al., 1998; Yang et al., 2006; Bahadorani et al., 2010), and the expression of MT is dysregulated by endocrine mutations and low-calorie diets that promote longevity in worms, flies and mice (Dhahbi et al., 2004; Ebadi et al., 2005; Brown-Borg, 2006; Szewczyk et al., 2006; Swindell, 2007; Leiser and Miller, 2010; Swindell et al., 2010). The figure provides an overview of several key biological consequences associated with increased abundance of MT. In some cases, consequences of increased MT abundance have been primarily investigated or established within a particular mammalian organ or cell type (i.e., astrocytes and adipocytes), and this has been indicated in the figure where appropriate.

Figure 2. Expression of genes encoding metallothionein (MT) and other proteins with anti-oxidant activity across mouse tissues

Microarray data generated as part of the Genomics Institute of the Novartis Research Foundation mouse expression atlas (Gene Expression Omnibus series GSE10246) was used to evaluate expression of the metallothionein gene family (i.e., Mt1, Mt2, Mt3, Mt4, Mtf1) across 91 mouse tissues and cell types (Lattin et al., 2008). For comparison, expression levels of other genes encoding proteins with anti-oxidant activity are shown in the figure as well, including catalase (Cat), superoxide dismutase (Sod1, Sod2 and Sod3), glutathione peroxidase (Gpx1 and Gpx4), thioredoxin (Txn1 and Txn2) and methionine sulfoxide reductase A (Msra) (reviewed by Pérez et al., 2009a; 2009b; Salmon et al., 2010). The mouse tissues (rows) have been clustered hierarchically according to similarity of expression patterns across all transcripts represented on the Affymetrix 430 2.0 Mouse Genome array. Expression signals were generated from a batch of 182 Affymetrix microarrays (normalized using the Robust Multichip Average algorithm), where expression values for each tissue represent the average of two biological replicates. The color scale indicates relative expression levels across tissues and genes, where the “Absent” designation is used to identify cases in which a gene was not significantly expressed above background level (based on MAS 5.0 p-value detection calls). The mouse tissues for which expression of either Mt1 or Mt2 is higher than other genes shown in the figure are indicated by an asterisk symbol (e.g., “*Hippocampus”). The expression signals represented for each gene (column) shown in the figure correspond to a single probe set included on the Affymetrix 430 2.0 Mouse Genome array (i.e., 1422557_s_at (Mt1), 1428942_at (Mt2), 1420575_at (Mt3), 1450645_at (Mt4), 1418333_at (Mtf1), 1416430_at (Cat), 1451124_at (Sod1), 1417193_at (Sod2), 1417633_at (Sod3), 1460671_at (Gpx1), 1451695_a_at (Gpx4), 1416119_at (Txn1), 1452782_a_at (Txn2), 1448856_a_at (Msra)). All tissues were obtained from 8 - 10 week old male C57BL6 mice, with the exception of female organs, which were obtained from female C57BL6 mice of the same age (Lattin et al., 2008).

Figure 3. Expression of genes encoding metallothionein (MT) and other proteins with anti-oxidant activity across Drosophila larval and adult tissues

Microarray data generated by FlyAtlas (Gene Expression Omnibus series GSE7763) was used to evaluate expression of the Drosophila MT family genes (i.e., MtnA, MtnB, MtnC, MtnD and MTF-1) across 30 cell types or tissues from larvae and adult flies (Chintapalli et al., 2007). For comparison, expression levels of other genes encoding proteins with anti-oxidant activity are shown in the figure as well, including catalase (Cat), superoxide dismutase (Sod1 and Sod2), thioredoxin (Trx-1 and Trx-2) and methionine sulfoxide reductase (MsrA and MsrB). The adult and larval tissues (rows) have been clustered hierarchically according to similarity of expression patterns across all transcripts represented on the Affymetrix Drosophila Genome 2.0 array. Expression signals were generated from a batch of 120 Affymetrix microarrays (normalized using the Robust Multichip Average algorithm), where expression values for each tissue represent averages from four biological replicates. The color scale indicates relative expression levels across tissues and genes, where the “Absent” designation is used to identify cases in which a gene was not significantly expressed above background level (based on MAS 5.0 p-value detection calls). The fly tissues for which expression of MtnA is higher than other genes shown in the figure are indicated by an asterisk symbol (e.g., “*Brain (Adult)”). The expression signals represented for each gene (column) shown in the figure correspond to a single probe set included on the Affymetrix Drosophila Genome 2.0 array (i.e., 1632873_at (MtnA), 1630291_at (MtnB), 1628446_at (MtnC), 1631962_at (MtnD) and 1631619_a_at (MTF-1), 1639480_at (Cat), 1630845_at (Sod1), 1622984_at (Sod2), 1636303_at (Trx-1), 1629426_s_at (Trx-2), 1625276_a_at (MsrA), 1630886_at (MsrB)). All adult tissues were obtained from wild-type Canton S flies (1-week old) that were reared at 22°C and maintained on a standard Drosophila diet.

Figure 4. Potential mechanisms contributing to MT dysregulation in long-lived dwarf mice and CR-fed mice

The expression and abundance of MT is altered in long-lived dwarf mice and is also sensitive to short and long-term reduction of caloric intake (see text). Dwarf mutations and CR/fasting are known to modulate endocrine and intracellular pathways in ways that can have either positive or inhibitory effects on MT abundance. The figure outlines several of these potential mechanisms, including those expected to decrease MT levels as well as those expected to increase MT levels. Potentially, dwarf mutations and/or CR modulate MT levels through more than one mechanism described in the figure, with the cumulative effect on MT levels arising from a shift in the balance between positive and inhibitory mechanisms. Up triangles (▲) are used to denote effects that involve increased abundance of a particular protein or activation of a pathway, while down triangles (▼) denote effects that involve decreased abundance of a protein or inhibitory effects. Both up and down triangles are used, in some cases, to indicate uncertainty in the direction of effect. The dark gray box outlines mechanisms that are IGF-1-dependent in mice, which potentially, are comparable to those influencing MT abundance in daf-2 worms. (Abbreviations: GH = growth hormone; GHR = growth hormone receptor; GHRKO = growth hormone receptor knockout mutation; GRE = glucocorticoid response element; IGF-1 = insulin-like growth factor 1; IL = interleukin; MTF-1 = metal transcription factor 1; PRL = prolactin; STAT = signal transducer and activator of transcription; TF = transcription factor; TSH = thyroid stimulating hormone)