homeostasis. ΔNp63 isoforms are required for commitment
to stratification, whereas TAp63 isoforms are dispensable
for development. Both classes of isoforms are required for
maintaining stratified epithelial integrity in the adult, and
their individual absence results in cellular senescence and
premature aging. It is interesting that different cellular compartments
use TAp63 or ΔNp63 to maintain homeostasis by
seemingly opposite mechanisms. While TAp63 isoforms
inhibit dermal precursors hyperproliferation that may lead
to senescence, ΔNp63 proteins maintain the proliferative
potential of basal skin stem cells. TAp63 and ΔNp63 maintain
stratified epithelial integrity by working together, but
separated into different compartments, as it would be
expected from the ability of ΔNp63 to inhibit TAp63.
Also interesting is the great difference in expression levels.
ΔNp63α is expressed at high levels in basal epithelial cells,
such as myoepithelial cells and keratinocytes. Thus, the majority
of the studies using these cells as a model system have
attributed the functions of p63 to this particular isoform. We
have demonstrated that at least in mammary epithelial cells,
this is indeed the case, as reintroduction of ΔNp63α, but not
ΔNp63γ or TAp63 isoforms, is able to revert cells back to a
non-motile phenotype after p63 ablation (unpublished). However,
as mentioned earlier, tissue culture and animal genetics
studies have shown that TAp63 is a metastasis suppressor,
despite low expression levels. Clearly, the effects of p63 on
cancer metastasis are cell context-dependent, and more research
is needed in order to understand the molecular mechanisms
underlying isoform-specific effects.
The roles of p63 in tumorigenesis, cancer progression, and
metastasis are still being discovered. As we search for new
combinations of molecular targets for cancer therapy, the need
for understanding the basic molecular mechanisms underlying
cancer progression is as great as ever. The description of both
TAp63 and ΔNp63 as metastasis suppressors opens new
avenues for developing cancer treatments. Thus, a better understanding
of the roles and interactions of the different p63
isoforms and other p53 family members will undoubtedly
prove beneficial to the design of effective cancer therapies.