18 April 2009. To fight against diseases of aging, scientists must understand the aging process itself. A paper published online in Science April 16 provides the first evidence that the pathway that battles the stress of low-oxygen conditions also prolongs life. In the tiny worm Caenorhabditis elegans, ramping up the hypoxic response, even if oxygen levels are normal, leads to long-lived worms that resist the effects of polyglutamine and amyloid-β, researchers at the University of Washington in Seattle have found. They also report that the hypoxic response’s effect on longevity was distinct from the dietary restriction and insulin signaling pathways that have the same effect.
Scientists in the laboratory of Matt Kaeberlein had previously found that dietary restriction not only extended lifespan in C. elegans, but also staved off the paralysis that nematode models of neurodegenerative disease exhibit (Steinkraus et al., 2008). The scientists suspected that members of the protein degradation pathway could prevent proteotoxicity, so joint first authors Ranjana Mehta, Katherine Steinkraus, and colleagues looked to E3 ubiquitin ligases, which tag proteins for destruction. They used RNA interference to knock down E3 ligases in worms expressing a YFP-labeled 35-residue polyglutamine repeat related to the toxic protein in Huntington disease. Aggregated protein ultimately paralyzes these animals. The researchers screened for genes that, when dampened, would allow the worms to keep wriggling for longer.
They found what they were looking for in VHL-1, a homolog of the human von Hippel-Landau tumor suppressor. While a normal nematode lives approximately 24 days, VHL-1 deletion mutants survive to a ripe old thirtysomething, Kaeberlein said. VHL-1 ubiquitinates the hypoxic response transcription factor HIF-1, sending it to the proteasome for destruction. The VHL-1 deletion only lengthened life in the presence of HIF-1, suggesting that HIF-1, which would be more active in the absence of VHL-1, is the downstream mediator of the ligase’s longevity effects.
Next, the researchers looked for interactions between HIF-1 and other pathways already known to be involved in longevity. Science has found many ways to bestow long life on worms, flies, mice, and rats. Cutting caloric intake is one method that has garnered attention and even been tried in people (see ARF related news story and Witte et al., 2009). The insulin/insulin growth factors signaling pathway also modulates lifespan, and loss-of-function mutations in the IGF receptor are over-represented in centenarians (see ARF related news story and Suh et al., 2008).
In this study, knockdown of HIF-1 did not affect the lifespan of eat-2 mutants, which live longer because of a pharyngeal pumping defect that prevents them from eating a normal diet. Similarly, HIF-1 levels did not affect the longevity of animals treated with RNAi for daf-2, an insulin/IGF-1-like receptor that increases lifespan when its activity is decreased. These experiments “argue that VHL-1 and HIF-1 are acting in a pathway that is genetically distinct at least from the insulin pathway and dietary restriction,” Kaeberlein said.
Worms with mutant VHL-1 also accumulated less of the age marker lipofuscin. This substance is “autofluorescent junk” that builds up in older animals. Kaeberlin said. The dimmer autofluorescence of VHL-1 mutants suggested to him that the animals were truly better able to clear damaged proteins.
Previous work by others has linked proteasomal degradation to longevity (Ghazi et al., 2007; Li et al., 2007), but scientists had not discovered the downstream effectors of this system. “Finally, there is a substrate,” said Malene Hansen, who studies C. elegans aging at the Burnham Institute in La Jolla, California, and was not involved in this study. The further down scientists can define a pathway, the more specific drug targets they are likely to find, Kaeberlein said: “From a therapeutic perspective, what you really want is to target the bottom of the pathway.”
It is possible that further research may link the HIF-1-based hypoxic response to other known longevity effectors. HIF-1 is affected by insulin signaling and the TOR pathway (Dekanty et al., 2005), both implicated in lifespan extension (Vellai et al., 2003). It may be that several signaling pathways converge on the same mechanisms for longevity. “It is kind of a big orchestra…that could have multiple conductors,” Hansen said. Hansen also suggested that the HIF-1 lifespan pathway could involve heat shock proteins, which have previously been associated with long life (see ARF related news story and Hsu et al., 2003). Kaeberlein has not looked into heat shock proteins and HIF-1 yet, but it is on his list of planned experiments. “I think it is possible that both pathways converge on shared downstream targets, and HSPs are good candidates,” he wrote in an e-mail to ARF.
VHL-1 or HIF-1 could be therapeutic targets for diseases such as Alzheimer’s and Huntington’s—neurodegenerative diseases where toxic proteins aggregate, the scientists said. It’s already known that metal chelators that induce HIF-1 expression extended survival of a mouse model of amyotrophic lateral sclerosis (Petri et al., 2007). However, altering the VHL-1 system comes at a price. In worms, Kaeberlein and his coauthors noted that VHL-1 deletion mutants laid fewer eggs, a common phenotype in long-lived mutants. “If you are going to live a long time, the resources needed to repair that accumulated damage has to come from somewhere,” Kaeberlein said. “It often comes from the reproductive capacity of the organism.”
In people, the cost could be even steeper, as interfering with VHL-1, a tumor suppressor, could conceivably lead to cancer. Mutations in VHL-1 have long been linked to hereditary cancers (for review, see Kaelin, 2002). To minimize risk, any therapeutic would have to be targeted to specific tissues, Kaeberlein said. VHL-1’s diverse effects present an interesting contrast, said Pankaj Kapahi of the Buck Institute in Novato, California, who was not involved in the current study. Normally one would expect a gene that fights cancer also to fight neurodegenerative disease, and vice versa, Kapahi said, but VHL-1 plays both sides. “I think that’s an interesting question for the field,” he said.
Aging research has “turned the corner,” Kaeberlein said, in that it has begun to seriously consider targeting aging pathways in order to treat diseases of aging. With research like this, “you have the potential to influence multiple age-associated diseases at the same time,” he said. He predicts that more and more future drug therapies will arise from research on aging in model organisms.—Amber Dance.
Mehta R, Steinkraus KA, Sutphin GL, Ramos FJ, Shamieh LS, Huh A, Davis C, Chandler-Brown D, Kaeberlein M. Proteasomal regulation of the hypoxic response modulates aging in C. elegans. Science. Epub 2009 Apr 16. Abstract