PNAS：衰老难以抗拒

诸平

PubMed衰老（aging）文献变化

亚利桑拉大学（University of Arizona，UA）2017年10月30日提供的消息，该大学的研究人员已经证实，在数学上是不可能战胜衰老的。衰老是自然生活的一部分,但这并没有阻止人们试图从着手努力阻止这一过程。从古到今，研究长寿的灵丹妙药比比皆是。不幸的是,也许这些尝试都是徒劳的,因为根据美国亚利桑那大学的研究人员的最近研究结果，已经证明在数学上是不可能停止像人类这样的多细胞生物体（multicellular organisms）的老化过程。相关研究成果已经于2017年10月30日在《美国国家科学院院刊》（PNAS）网站发表——Paul Nelson，Joanna Masel. Intercellular competition and the inevitability of multicellular aging. PNAS,  October 30, 2017. www.pnas.org/cgi/doi/10.1073/pnas.1618854114.

UA的生态学与进化生物学系（Department of Ecology and Evolutionary Biology）教授Joanna Masel说：“老龄化是不可避免的，在数学上市严重不可避免的过程。无论是逻辑上、理论上还是数学上都是没有出路的。”发表在PNAS的研究结果是Joanna Masel教授和UA博士后研究人员保罗·纳尔逊（Paul Nelson）合作完成的。他们在文章中介绍了他们在数学上有关衰老的一些发现，因此他们选用了“细胞间竞争和多细胞衰老的必然性（Intercellular competition and the inevitability of multicellular aging）”作为论文标题。更多信息请注意浏览原文或者相关报道：

      Credit: CC0 Public Domain    

Aging is a natural part of life, but that hasn't stopped people from embarking on efforts to stop the process.

Unfortunately, perhaps, those attempts are futile, according to University of Arizona researchers who have proved that it's mathematically impossible to halt aging in multicellular organisms like humans.

"Aging is mathematically inevitable - like, seriously inevitable. There's logically, theoretically, mathematically no way out," said Joanna Masel, professor of ecology and evolutionary biology and at the UA.

Masel and UA postdoctoral researcher Paul Nelson outline their findings on math and aging in a new study titled "Intercellular Competition and Inevitability of Multicellular Aging," published in Proceedings of the National Academy of Sciences.

Current understanding of the evolution of aging leaves open the possibility that aging could be stopped if only science could figure out a way to make selection between organisms perfect. One way to do that might be to use competition between cells to eliminate poorly functioning "sluggish" cells linked to aging, while keeping other cells intact.

However, the solution isn't that simple, Masel and Nelson say.

Two things happen to the body on a cellular level as it ages, Nelson explains. One is that cells slow down and start to lose function, like when your hair cells, for example, stop making pigment. The other thing that happens is that some cells crank up their growth rate, which can cause cancer cells to form. As we get older, we all tend, at some point, to develop cancer cells in the body, even if they're not causing symptoms, the researchers say.

Masel and Nelson found that even if natural selection were perfect, aging would still occur, since cancer cells tend to cheat when cells compete.

"As you age, most of your cells are ratcheting down and losing function, and they stop growing, as well," said Nelson, lead author of the study. "But some of your cells are growing like crazy. What we show is that this forms a double bind - a catch-22. If you get rid of those poorly functioning, sluggish cells, then that allows cancer cells to proliferate, and if you get rid of, or slow down, those cancer cells, then that allows sluggish cells to accumulate. So you're stuck between allowing these sluggish cells to accumulate or allowing cancer cells to proliferate, and if you do one you can't do the other. You can't do them both at the same time."

Although human mortality is an undisputed fact of life, the researchers' work presents a mathematical equation that expresses why aging is an "incontrovertible truth" and "an intrinsic property of being multicellular," Nelson said.

"It's no surprise that we're all going to die; lots of things are obvious because they're so familiar to us, but really, why is it that we age? We start to explain why," said Masel, who also teaches in the UA's Graduate Interdisciplinary Program in Applied Mathematics.

"People have looked at why aging happens, from the perspective of 'why hasn't natural selection stopped aging yet?' That's the question they ask, and implicitly in that is the idea that such a thing as non-aging is possible, so why haven't we evolved it? We're saying it's not just a question of evolution not doing it; it can't be done by natural selection or by anything else," Masel said.

In the end, things just break over time and - according to the math - trying to fix them can make things worse.

"You might be able to slow down aging but you can't stop it," Masel said. "We have a mathematical demonstration of why it's impossible to fix both problems. You can fix one problem but you're stuck with the other one. Things will get worse over time, in one of these two ways or both: Either all of your cells will continue to get more sluggish, or you'll get cancer. And the basic reason is that things break. It doesn't matter how much you try and stop them from breaking, you can't."

As Nelson says: "It's just something you have to deal with if you want to be a multicellular organism."                                                                

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