Wnt/β-catenin signaling is downregulated but restored by nutrition interventions in the aged heart in mice

Abstract

Aging hallmarks include decreased progenitor cell functions. The Wnt/β-catenin signaling pathway has emerged as a key player in cellular aging in recent years. Wnt activity changes in aged tissues including skin, serum, muscle and artery. In heart, it is hypothesized that Wnt signaling increases with aging and this signaling drives cardiac progenitor cells into fibrogenic lineage. However, experimental evidence supporting this hypothesis has not been established. Here we take a bioinformatics data mining approach, utilizing pre-existing data, to study cardiac aging gene expression data in mice. Contrary to the hypothesis, our study shows that the Wnt/β-catenin signaling is down-regulated in aged heart in mice. Nutrition treatment, with calorie restriction and Resveratrol supplementation, known to retard aging, opposes heart aging by restoring Wnt/β-catenin signaling level in the old heart. In addition, the expression of β-catenin gene, a key regulator of the Wnt/β-catenin signaling pathway, decreases up to 3-fold in aged heart, but is restored to levels found in young heart with methods of nutrition intervention. Combined with database search, our study suggests that some of bioflavonoids may have potential therapeutic benefits to heart aging.

Introduction

Cardiac progenitor cells (CPCs) are small cardiac niches of unspecialized cells that are located predominantly in the atria and apex (Leri, Kajstura, & Anversa, 2005). These cells are capable of dividing and developing into different cell types including cardiac muscle cells and vascular cells (Mummery, 2005). Aging in general is characterized by decrease in pluripotency or multipotency of the undifferentiated cells and their ability for tissue regeneration (Conboy & Rando, 2005). Consequently, the aging heart shows limited ability to adapt to disease or other environmental changes (Mummery, 2005). In mice, the number of functionally competent CPCs reaches its maximum at the age of 20 months, but declines significantly by the age of 28 months (Gonzales et al., 2008). In human, although functionally competent CPCs are present in the aged heart, their ability to sense and respond to signals from the regions of damage reduces (Henning, 2008). The contribution and mechanism of cardiac progenitor cells dysfunction in cardiac aging is not fully understood.

Wnt proteins consist of a large family of cysteine-rich secreted glycoproteins that are highly conserved during evolution. The Wnt family proteins activate a highly conserved intracellular signaling cascade that is most known for their important roles in early embryogenesis and pattern regulations (Cadigan, 2008, Logan and Nusse, 2004, Rao and Kuhl, 2010, Willert and Nusse, 2008). In recent years, however, studies have revealed their roles in later stage of adult life, aging (Brack et al., 2007, Liu et al., 2007, Marchand et al., 2011, Ye et al., 2007). Wnt/β-catenin signaling plays such important roles in animal development and tissue homeostasis that mis-regulation of this signaling pathway has been implicated in several pathological states including cancers, heart disease and Alzheimer's (Katoh and Katoh, 2007, Logan and Nusse, 2004). The canonical Wnt signaling cascade initiates at the cell membrane where Wnt ligand binds to the receptor Frizzled (Fz) and co-receptors LRP5/LRP6.

The β-catenin gene is a key component of the Wnt signaling where it transmits extracellular Wnt signals to the nucleus. The level of β-catenin in both cytoplasm and nucleus is tightly regulated. In the absence of Wnt signal, cytoplasmic β-catenin level is kept low through ubiquitin-mediated proteasomal degradation, which is controlled by the destruction complex consisting GSK-3β/APC/Axin. When cells receive Wnt signals, degradation of β-catenin by the destruction complex and ubiquitin-proteasome complex is inhibited. This causes β-catenin to accumulate in the cytoplasm and translocate into the nucleus. In nucleus, β-catenin heterodimerizes with members of the TCF/LEF transcription factors to activate transcriptions of the target genes (Logan and Nusse, 2004, Rao and Kuhl, 2010). New roles of Wnt signaling in cellular aging have emerged in recent years. Several recent studies underscored the connection between the Wnt/β-catenin signaling pathway and cellular aging (Brack et al., 2007, Liu et al., 2007, Marchand et al., 2011, Ye et al., 2007). Using a mouse model of accelerated aging, Liu et al. observed increases in Wnt signaling in various tissues and organs. These changes were also associated with an increase in cellular senescence and a decrease in population of undifferentiated pluripotent cells (Liu et al., 2007). Brack and his colleagues showed that muscle satellite cells converted from myogenic to fibrogenic lineage during aging and this conversion was attributed to the elevated Wnt signaling. Suppression of Wnt signal preserved muscle satellite cells’ myogenic fate (Brack et al., 2007). A recent study by Marchand and colleagues showed that Wnt/β-catenin signaling pathway was activated in advanced arterial aging between middle aged and old-aged human males (Marchand et al., 2011). Contrary to the above findings, Ye et al. showed that inhibition of Wnt activity induced premature cellular senescence, while its activation promoted cell proliferation and delayed senescence (Ye et al., 2007). Consistent with the finding by Ye et al., several recent studies have demonstrated that increased Wnt signaling may increase tissue regeneration in hair, bone and heart after injury in young animals (Duan et al., 2011, Ito et al., 2007, Kim et al., 2007). Among those, Duan et al. showed that Wnt/β-catenin was required for cardiac function and wound repairs after cardiac injury. Interruption of the Wnt signaling abrogated such healing and repairs (Duan et al., 2011). These results appear to be conflicting. But the opposing effects of the Wnt signal are not uncommon. There are many examples for the pleiotropic even antagonistic effects of Wnt signaling depending on the time or tissue to which it exerts its function (Naito et al., 2010, White et al., 2007). In heart, it is hypothesized that Wnt signaling increases with aging and this signaling drives cardiac progenitor cells into fibrogenic lineage (Deb, 2008).

In this paper, we present a bioinformatics study to investigate molecular mechanism underlying heart aging in mice. Gene expression data from two rodent cardiac aging studies are utilized (Barger et al., 2008). Our study shows that Wnt signaling decreases with aging in heart. Methods of nutritional intervention were also explored. Calorie restriction (CR) and Resveratrol, found in grapes and red wine, both were previously shown to retard aging and increase life-span in animal models although their effects on human yet to be determined (Fernandez and Fraga, 2011, Kemnitz, 2011). Our study shows that both nutritional treatments oppose Wnt signaling changes in the aged heart by restoring the gene expressions to levels found in young heart. Our study suggests a novel role of Wnt/β-catenin signaling in heart aging. Nutritional intervention methods to heart aging are also discussed.