transgenic mice, whose epidermal and follicular keratinocytes lack Stat3, are viable and develop normally. However, they have defects in hair cycle processes as well as impaired wound healing, and they develop spontaneous ulcers with age. In vitro, keratinocytes derived from these mice had no defects in proliferation, but growth factor–dependent migration was markedly impaired in contrast to control keratinocytes (17
). In the study described in this issue of the JCI
, Chan et al. used K5Cre.Stat3fl/fl
transgenic mice to investigate the role of Stat3 in chemically induced carcinogenesis of the skin (10
). The role of Stat3 in tumor initiation, the first step of chemically induced carcinogenesis, was addressed both in vitro, in DMBA-treated primary keratinocytes, and in vivo, by topical treatment with this mutagen. Stat3 mutant keratinocytes underwent enhanced apoptosis following DMBA treatment, compared to control keratinocytes. Expression of Ha-ras homolog
) into cultured primary keratinocytes in vitro was used to generate initiated keratinocytes. Upon the introduction of a Stat3 decoy molecule (a high-affinity double-stranded DNA binding site for Stat3), the v-Ha-ras–initiated keratinocytes underwent apoptosis with a concomitant decrease in Bcl-xL levels. In general, inhibiting Stat3 function in cancer-derived cell lines containing abundant phosphorylated Stat3 leads to apoptosis or growth arrest. In contrast, cell lines which contain low or no levels of detectable tyrosine-phosphorylated Stat3 are relatively unaffected by Stat3 inhibitory therapies. It therefore remains unclear how Stat3 protects keratinocytes against DMBA-induced apoptosis, since DMBA does not induce tyrosine phosphorylation of Stat3 in primary keratinocytes, nor is it likely that v-Ha-ras–containing keratinocytes contain abundant levels of phosphorylated Stat3 (18
). Perhaps the low amounts of phosphorylated Stat3 present in these cells are sufficient to drive transcription of antiapoptotic genes such as Bcl-xL
. Alternatively, nonphosphorylated Stat3 may be playing a role as a transcription factor as has been demonstrated for Stat1 (20
). There are a few notable examples where relatively low levels of phosphorylated Stat3 are sufficient to mediate protection from growth arrest or apoptosis (21
). Thus, determination of the relative levels of phosphorylated Stat3 required to impart a phenotype is likely to be cell-type specific and remains an important objective. It has been shown that phosphorylated Stat1 levels are markedly enhanced in Stat3 null hepatocytes (23
). Given that Stat1 activation has been implicated in promoting growth arrest as well as apoptosis, it would be of interest to determine whether the enhanced apoptosis observed in the Stat3 null keratinocytes correlates with increased levels of activated Stat1.
It is hypothesized that keratinocyte stem cells, which are located mostly within the bulge region of the hair follicle, are the target cells for two-stage carcinogenesis (24
). Hair follicle stem cells have been identified within the label-retaining cells (LRCs), a population of cells that, following continuous administration of nucleotide analogs such as BrdU or [3
H]thymidine, retains the label for a sustained period of time, indicating a very slow cycling frequency (25
). Chan et al. (10
) observed that the majority of the Stat3-deficient keratinocytes undergoing apoptosis after exposure to DMBA were located primarily within the bulge region of the hair follicle in an area adjacent to the LRC population. The authors suggest that the DMBA-sensitive cells may be keratinocyte stem cells, given their proximity to the LRCs. However, given the complete lack of overlap between the LRCs and the apoptotic cells, the cell type most sensitive to DMBA-induced apoptosis remains to be identified. The work of Chan et al. generates interesting questions regarding the mechanism(s) by which Stat3 affords protection against apoptosis and the determination of which cell type(s) are most sensitive to the loss of Stat3.