Background: The aberrant activation of the transcription factor STAT3 has been identified as a driver of tumour-promoting inflammation and immunity in a variety of cancers. Consequently, STAT3 is a promising target for cancer therapeutics.
Aim: To genetically study the effect of specific STAT3 inhibition, we exploited a novel mouse strain, in which a short hairpin (sh) RNA directed against Stat3, alongside a GFP reporter gene, can be reversibly induced by administration of Doxycycline. As a proof of principle, the resulting shStat3 mice were crossed with, the gp130F/F mice, previously describe to develop gastric tumours in response to excessive STAT3 activation [Jenkins et al., Nature Medicine 2005].
Methods: The CAG-rtTA3;shSTAT3;gp130F/F compound mice were treated with doxycycline and various organs were collected for analysis. Flow cytometric (FACS) analysis was used to map shSTAT3 expression, based on GFP reporter expression. In parallel, tissue samples were subjected to quantitative RT-PCR (qRT-PCR) and Western blot to monitor the expression of Stat3 at the RNA and protein level, respectively. Finally we used FACS analysis to determine the consequence of Stat3 reduction within the gastric tumour microenvironment.
Results: FACS analysis indicated broad expression as well as reversibility of the shStat3. Stat3 silencing was confirmed both at the protein and RNA level, while re-establishment of normal Stat3 expression was only partial in some organs four weeks after doxycycline withdrawal. Stat3 reduction was found to correlate with significant decrease in tumour burden. Interestingly, further FACS analysis showed an increase in tumour infiltrating CD8 T cells in the tumour of both our shStat3-rtTA+;gp130F/F and shLuc-rtTA+;gp130F/F control. This event is however is more prominent in the shStat3-rtTA+;gp130F/F mice, where protein Stat3 is reduced.
Conclusion: Our data provided compelling evidence of the benefits of therapeutically targeting Stat3 in a Stat3-driven gastric cancer model, while also offering some insight to the possible biological effects of STAT3 inhibition in the tumour microenvironment. Our observation of CD8 T cells influx in the tumour may also suggests that our shRNA-rtTA+;gp130F/F compound mouse could possibly provide a model to study therapeutic strategies involving the combination of checkpoint immune blockade.