Killing cells is easy. Killing only the cells that you want to kill, while leaving all other cells untouched, is very much more challenging. The ability to do this is fundamental to much of the future of medicine, however. The aging body contains many cell populations that cause significant harm and should be removed, including misconfigured T cells, age-associated B cells, precancerous cells, and of course senescent cells of many different types. Great benefits to health and longevity might be obtained via efficient means of targeting that enable therapies to only destroy unwanted, harmful cells.
This point is well illustrated by present efforts to selectively destroy senescent cells. Today’s open access paper is one of a number of recent publications that focus on using galactose conjugation to produce prodrugs that are highly selective to senescent cells. Senescent cells produce a lot of β-galactosidase, a protein that acts to strip galactose from other molecules. It is thus possible to combine any one of a range of toxic cell-killing compounds with galactose to produce molecules that are entirely innocuous until they encounter β-galactosidase, making the therapy very specific to senescent cells.
Researchers have tried this approach with the overly toxic senolytic drug navitoclax, with some success, but one really doesn’t have to be clever about the drug used. In principle any of the cytotoxic compounds employed widely in the cancer research community will work. Thus other groups have used duocarmycins, while the researchers noted here instead chose gemcitabine, and a long list of alternative options exist beyond these.
Previous studies have shown that compounds termed ‘senolytics‘ could kill senescent cells. Reported senolytics target anti-apoptotic pathways, which are up-regulated to inhibit apoptosis in senescent cells. These senolytics have been reported to eliminate certain types of senescent cells and have shown the potential to improve physiological function in several tissues. However, senolytic drugs have significant limitations in killing senescent cells in terms of specificity and broad-spectrum activity because of the dynamic and highly heterogeneous nature of the senescence program, which leads to the varying sensitivity of different types of senescent cells to current senolytic drugs. To overcome these challenges, it is highly demanded to develop a new strategy that permits selectively deleting senescent cells in a wide spectrum of cell types or tissues for anti-aging interventions.
To specifically target senescent cells, we focused on one primary characteristic of senescent cells – the increased activity of lysosomal β-galactosidase, exploited as senescence-associated β-galactosidase (SA-β-gal). Notably, SA-β-gal in diverse types of senescent cells is one widely used marker for identifying senescence in vitro and in vivo, which is linked to the increased content of lysosomes. Therefore, we hypothesized that lysosomal β-gal could be utilized for the design of a galactose-modified prodrug to target senescent cells in a broader spectrum. This prodrug could be processed into a cytotoxic compound by β-gal and subsequently delete senescent cells in a specific manner, a strategy that could overcome the limitations of current senolytic drugs.
Here, we designed a new prodrug, SSK1, that was specifically cleaved by lysosomal β-gal into cytotoxic gemcitabine and induced apoptosis in senescent cells. This prodrug eliminated both mouse and human senescent cells independent of the senescence inducers and cell types. In aged mice, our compound reduced SA-β-gal-positive senescent cells in different tissues, decreased senescence- and age-associated gene signatures, attenuated low-grade chronic inflammation, and improved physical function.
While SA-β-gal is widely used as a marker of cellular senescence, its elevated activity can be found in some other cells such as activated macrophages. These SA-β-gal-positive macrophages can be harmful and have been found to accumulate in injured and aged tissues contributing to chronic inflammation. Importantly, we have shown that SSK1 decreases the number of SA-β-gal-positive macrophages in injured lungs and aged livers, which is consistent with our observation of reduced secretion of chronic inflammation-related cytokines. Therefore, eliminating macrophage accumulation by SSK1 might reduce chronic inflammation and benefit aged organisms.