Claiming Cellular Senescence for the Hyperfunction Theory of Aging
This mildly infuriating commentary well illustrates just why it is that theories of aging are so very diverse. Even a mechanism as well understood as cellular senescence can be fairly convincingly argued into one camp or another. For those who see aging as damage accumulation, lingering senescent cells are clearly a form of damage, a byproduct of normal metabolism that grows slowly over time and produces tissue dysfunction in proportion to the number of such cells. For those who consider the hyperfunction theory of aging, in which aging is the result of developmental programs failing to shut down in adult life, it is fairly easy to argue that the prevalence of cellular senescence in old people is an embryonic development mechanism or wound healing mechanism run wild. Cellular senescence does indeed play important roles in those circumstances.
Senolytics are drugs that extend lifespan and delay some age-related diseases by killing senescent cells. I want to draw your attention to the paradoxes associated with senolytics, which argue against the dogma that says aging is a functional decline caused by molecular damage. This dogma predicts that senolytics should accelerate aging. If aging is caused by loss of function, then killing senescent cells would be expected to accelerate aging, given that dead cells have no functionality at all. Instead, however, senolytics slow aging, which highlights a contradiction in the prevailing dogma.
The theory of hyperfunctional aging addresses this paradox. Killing senescent cells is beneficial because senescent cells are hyperfunctional. The hypersecretory phenotype or Senescence-Associated Secretory Phenotype (SASP) is the best-known example of universal hyperfunction. Most such hyperfunctions are tissue-specific. For example, senescent beta cells overproduce insulin and thus activate mTOR in hepatocytes, adipocytes, and other cells, causing their hyperfunction, which in turn leads to metabolic syndrome and is also a risk factor for cancer. SASP, hyperinsulinemia and obesity, hypertension, hyperlipidemia and hyperglycemia are all examples of absolute hyperfunction (an increase in functionality).
In comparison, relative hyperfunction is an insufficient decrease of unneeded function. For example, protein synthesis decreases with aging, but that decrease is not sufficient. In analogy, a car moving on the highway at 65 mph is not "hyperfunctional." But if the car were to exit the highway and enter a residential driveway at only 60 mph it would be "hyperfunctional," and stopping that car would likely prevent damage to other objects. Similarly, killing hyperfunctional cells can prevent organismal damage. Senolytics eliminate hyperfunctional cells, which otherwise damage organs.