We thus hypothesized that a reduction in the levels of cytochrome C, by deletion of a gene encoding one of its isoforms, might mimic a partial inhibition of cytochrome C function, thereby activating the retrograde response and extending yeast chronological lifespan. While the requirement for the retrograde response, and extension of lifespan by reduced acid accumulation are sufficient to explain the increased longevity of methionine-restricted cells, impairment of cytochrome C is a putative step in this extension, and given the role of Cyc1 in yeast programmed cell death, we sought to determine whether a blunting of PCD might partially underlie the Meth-R phenotype. As the Kobe0065 benefits of Meth-R may be conferred, at least partially, through stress-responsive retrograde signaling, we wondered whether improved stress tolerance might be associated with Meth-R-dependent longevity. These results prompted us to test whether such improved stress tolerance might be associated with extended replicative lifespan in culture. We serially passaged the mock-infected and MTR-KD fibroblasts described above, until these cultures reached their respective replicative lifespan endpoints and were unable to further proliferate. Furthermore, the fact that the efficiency of MTR depletion is directly correlated with the robustness of the resulting stress tolerance and lifespan extension phenotypes confirms that these benefits are not the result of putative technical artifacts associated with strain construction by lenitiviral infection, but rather, that they are specifically engendered by reduced methionine synthase levels. In addition, we note that extension of proliferative lifespan was not caused by slower cell division, because Mtr/MTR knockdown actually increased cell division rates and yielded a CH5132799 greater total number of cell divisions. Regarding the mechanism by which genetic Meth-R confers lifespan extension to mammalian cells, we considered the possibility that, similar to the case in yeast, retrograde signaling might be involved in the Meth-R-dependent extension of replicative lifespan that we observed for mammalian cells.