Tom Mahoney wrote:
> I would define proliferative lifespan, in vitro, as the replicative
> capacity of the cells up to the point of reaching crisis.
OK, thank you for elaborating. The definition of mouse cell senescence
that I was using, namely the entry into the slow growth phase, is well
established (see, for example, Sedivy PNAS 95:9078 which you have often
cited). As you go on to say, your non-standard definition explains much
of what we were disagreeing about (though not all -- read on). Please
explain your terminology more clearly when it is non-standard.
> cells are not anaplastic beyond the slow growth stage at 10 population
> doublings and appear to function normally until they reach crisis. This
> would suggest that telomeric shortening is not a cause of this phenomenon
Quite. So, do you conclude that introduction of telomerase would NOT stop
cells from entering senescence (the slow growth phase)? If you do, then
you're no longer contending that "the replicative cells of mice respond in
the same manner as human cells when the catalytic component for telomerase
is introduced".
> In vivo the situation is a little murkier in that having by-passed the
> slow growth stage, the cells rapidly grow ... however there is no loss
> of differentiation or reversion to a more primitive form.
Sure -- mouse tumours are somewhat different from human tumours, as we
might expect given that they arise without going through a crisis stage.
> >> (Biochemistry (Mosc) 1997 Nov;62(11):1296-305
> This paper, however is right on point. The "transformation" they are
> referring to is the escape from the initial senescence block at 10
> population doublings. There are no anaplastic or malignant transformations
> noted in either the cell lines with active telomerase nor those lacking
> telomerase
Sure, but that doesn't mean that they are normal. You have just noted
that, in vivo, "At this stage the tumor is somewhat anaplastic in that it
does generate additional capillaries through angiogenesis and the cellular
matrix and orientation is disrupted". Something causes the slow growth,
then something causes escape from it. Cells that have escaped it are thus
not normal, period.
> The introduction of telomerase or its catalytic subunit into cells not
> normally expressing telomerase, apparently, does have the effect of
> immortalizing these cells, in a phenotypically youthful manner.
OK, I have to presume that you are again using "immortalizing" to mean
allowing indefinite growth of a culture that has either already been in
and out of the slow growth phase, or else been transformed so that with
no telomerase it progresses to crisis without a slow growth phase. Am
I correct? If so, I agree that that is no big deal, but I repeat that
such cells are not normal (however phenotypically youthful they may
appear in vitro), and hence that this is NOT an example of mouse cells
responding "in the same manner as human cells". If, on the other hand,
you are saying that introduction of telomerase allowed the indefinite
growth of a culture that has NOT become abnormal in either of the above
ways, then it is a very big deal.
> Having assumed that this particular question had been answered, I had
> moved on
Indeed. Please wait and see whether there is agreement that a question
has been answered before assuming that it has been. Sorry for the
pompous tone, but seriously, this is vital for productive discussions.
Aubrey de Grey
