ABSTRACT
Aberrant Hedgehog (Hh) signalling has been reported in a number of malignancies, particularly basal cell carcinoma (BCC) of the skin. Clinical trials of Hh inhibitors are underway in many cancers, and these have produced significant clinical benefit in BCC patients, although regrowth of new, or clinically aggressive, variants, as well as development of secondary malignancies, has been reported. αvß6 integrin is expressed in many cancers, where it has been shown to correlate with an aggressive tumour phenotype and poor prognosis. We have previously reported αvß6 up-regulation in aggressive, morphoeic BCC variants, where it modulates the stromal response and induces invasion. To examine a possible link between Hh and αvß6 function, we generated BCC models, overexpressing Gli1 in immortalized keratinocytes (NTert1, HaCaT). Unexpectedly, we found that suppressing Gli1 significantly increased αvß6 expression. This promoted tumour cell motility and also stromal myofibroblast differentiation through integrin-dependent TGF-ß1 activation. Gli1 inhibited αvß6 expression by suppressing TGF-ß1-induced Smad2/3 activation, blocking a positive feedback loop maintaining high αvß6 levels. A similar mechanism was observed in AsPC1 pancreatic cancer cells expressing endogenous Gli1, suggesting a common mechanism across tumour types. In vitro findings were supported using human clinical samples, where we showed an inverse correlation between αvß6 and Gli1 expression in different BCC subtypes and pancreatic cancers. In summary, we show that expression of Gli1 and αvß6 inversely correlates in tumours in vivo, and Hh targeting up-regulates TGF-ß1/Smad2/3-dependent αvß6 expression, promoting pro-tumourigenic cell functions in vitro. These results have potential clinical significance, given the reported recurrence of BCC variants and secondary malignancies in patients treated by Hh targeting.
