II. Endocrine regulation and senescence
II.A. Biomarker aging analysis in dwarf mice
Much has been written about the relationship of endocrine deficiency to retarded aging. Hypophysectomy in rats and mice has been shown to retard aging (Harrison et al, 1982; reviewed in Regelson, 1983). In our lab, we have extensively studied 3 dwarf stocks:
- The Snell dwarf (Pit1dw/Pit1dw) (Pit1 null; growth hormone [GH], thyroid stimulating hormone [TSH], and prolactin deficient)
- The Ames dwarf (Prop1df/Prop1df) (Prop1 null)
- The little mouse (Ghrhrlit/Ghrhrlit) (a missense mutation in the Ghrhr (growth hormone releasing hormone receptor) gene
Ironically, early studies of Snell and Ames dwarfs indicated they might be models of rapid aging. It is now apparent that their short life spans were due to the stresses of conventional colonies. Under optimized colony conditions — specific pathogen free (SPF) protocols; for Snell dwarfs, housing with normal female "caretakers"; for hypothyroid states, near thermoneutral ambient temperature — deficiencies of anabolic hormones seem to retard aging and extend life span (Flurkey et al., 2001; 2002).
Our studies show that
- The dw and lit mutations increase life span
- The dw, df, and lit mutations retard immunologic aging
- The dw and df mutations retard collagen aging
- The dw and lit mutations protect against induced tumors and osteoarthritis
Because reduction in function of the thyroid hormone axis may retard aging in some biological systems, we currently are investigating the effects on aging of the Thrα KO and Thrβ KO mice.
II.B. Molecular pathways associated with longevity in Snell dwarf mice
Snell dwarfs live longer than normal controls. We know that their endocrine environment has similarities to that of diet restricted controls — increased insulin sensitivity and decreased levels of growth hormone (GH) and insulin-like growth factor 1 (IGF-1). We collaborated with Dr. John Papaconstantinou (University of Texas, Galveston), to test whether Snell dwarf mice also resist oxidative stress. Initial findings are that Snell dwarf mice respond to oxidative stress induced by 3-nitropropionic acid differently than their wild-type littermates, suggesting that normal mice are more sensitive to mitochondrial-generated reactive oxygen species than dwarf mice are.
Compared to normal mice, dwarf mice show far less activation of the MEK-ERK kinase cascade, although MEK 1/2 and ERK 1/2 are phosphorylated to a higher degree in non-treated dwarf mice than in their normal littermates. One of the most striking effects of the Snell dwarf mutation is that virtually no phosphorylation of c-June at Ser63 occurs in Snell dwarfs after 3-NPA treatment, despite a robust phosphorylation of Ser63 in wild-type mice. This is especially interesting due to the role that c-Jun plays in many key biological processes such as transcriptional regulation, cell activation, and regulation of apoptosis. These changes may explain how the Snell dwarf mutation reduces cancer incidences. It also suggests that exhaustion of adult stem cells with age may be retarded in Snell dwarf mice.
II.C. Tumorigenesis in Snell dwarf and little mice and in crosses with p53 KO heterozygotes
Our studies have shown that dw and lit mutations diminish spontaneous tumors. Our next step is to determine whether these mutations diminish cancers that are induced rapidly in other mouse models, or whether they retard initial cancer induction, cancer growth, or both. This is why we are crossing them with p53 KO mice.
II.D. Age-related disease in dwarf mice
The effects of endocrine deficiencies — retarded aging and increased life span — suggest that diseases of aging also will be affected. We are crossing growth hormone deficient and hypothyroid mice with mutant mice that develop atherosclerosis — the Apoe KO mice — to test effects of hormone deficiency on atherosclerosis. We are using little mice to examine effects on aging of the kidney and on age-related elevation of insulin and its relationship to pancreatic aging.
II.E. Hypotheses of the relationship of insulin to IGF-1 regulation during aging
Studies of invertebrates indicate an evolutionarily conserved function of the insulin/IGF-1 signal transduction pathway that regulates life span. Our preliminary studies in mutant mice with diminished IGF-1 signaling (Flurkey et al., 2001; 2002) are consistent with this finding. We plan to continue studies with FIRKO, NIRKO, Foxo models.
In collaboration with Drs. Ron Kahn (Harvard) and Domenico Accili (Columbia), we are using newly available mutations that reduce the insulin receptor in fat of the central nervous system or increase the nuclear binding factor FOXO1, to test whether the homologous pathways regulate aging in mammals as well as invertebrates.