The role of GLI2 in human basal cell carcinoma tumourigenesis
Abstract
Abnormal Sonic Hedgehog (SHH) signalling leads to increased transcriptional
activation of its downstream effector, GLI2, which is implicated in the pathogenesis of a
variety of human tumours, including human basal cell carcinoma (BCC). However, little
is known about the molecular mechanisms underlying the tumourigenic role of GLI2 in
human skin keratinocytes. This study examines the effects of inducible and stable
expression of constitutively active GLI2 (GLI2:N) oncogenic transcription factor, on
immortalised human epidermal keratinocytes. It is shown here that GLI2:N
overexpressing N/TERT keratinocytes display gene expression patterns and phenotypic
characteristics reminiscent of those observed in human BCC in vivo. It is also shown for
first time, that expression of GLI2:N in N/TERT keratinocytes is sufficient to induce
accumulation of binucleated/tetraploid cells as evidenced by an increase in G2/M phase
of the cell cycle and binucleate cell counting, and to promote polyploidy and
aneuploidy, in the absence of increased cell death or apoptosis. This cell cycle
deregulation is accompanied by strong activation of anti-apoptotic protein BCL-2 and
simultaneous suppression of important cell-cycle regulators such as 14-3-3σ and CDK
inhibitor p21WAF1/CIP1, with no change in p53 protein levels, indicating uncontrolled
proliferation of cells with ploidy abnormalities and/or DNA damage. Consistently, it is
shown that p21WAF1/CIP1 protein is also absent in human BCC tumours and that forced
overexpression of GLI2:N renders human keratinocytes resistant to apoptosis mediated
by ultraviolet B (UVB, 290-320 nm), one of the most important etiological factors in
BCC formation. Karyotype analysis of GLI2:N N/TERT keratinocytes further
demonstrates that overexpression of GLI2:N induces numerical (tetraploidy,
polyploidy, aneuploidy) and structural instability in N/TERT keratinocytes including
chromosomal translocations and double minute chromosomes. Furthermore, β-catenin
activation is the most common alteration observed during aberrant WNT signalling, and
is often implicated in the development of human carcinogenesis and metastasis. In this
study it is shown that GLI2:N induction induces nuclear accumulation of β-catenin in
keratinocyte cell culture and in the basal layer of organotypic skin rafts, similar to
human BCCs. In addition, several WNT genes were found to be upregulated upon
GLI2:N induction, while β-catenin transcriptional activity is increased upon stable and
conditional expression of GLI2:N. Overall these data give new insights for the possible
mechanisms that mediate the tumourigenic potential of GLI2.
Authors
Pantazi, EleniCollections
- Theses [4223]