Identification of Alternative HH Regulators in Basal Cell Carcinoma

Basal cell carcinoma is the most prevalent cancer and it has been well-documented to be driven predominantly via overactivation of the Hedgehog signaling pathway. Although vismodegib, a SMO inhibitor, has been proven to be highly effective in treating BCCs, advanced forms of BCCs often possess inherent resistance while many that initially respond to drug treatment develop drug resistance over time. This highlights the importance of identifying targets that are either downstream of SMO or alternative drivers of BCCs in order to bypass SMO-inhibitor resistance. Here, we use a combination of RNA-sequencing and immunofluorescence staining to identify and validate the overexpression of other potential pathways that are upregulated in BCCs. This analysis highlighted the expression of mTOR, PI3K, and SRC. We found that upon pharmacological inhibition of mTOR and PI3K in our Ptch1fl/fl; Gli1-CreERT2 mouse BCC tumor model that although BCC growth is significantly inhibited, HH signaling is not. This implies that both the mTOR and PI3K pathway may work either downstream of or in parallel with HH signaling. Furthermore, our data suggests that mTOR is affecting BCC growth via aPKC independent of HH signaling whereas PI3K is likely driving BCC growth via aPKC- and AKT-driven p21 degradation. However, when SRC was pharmacologically inhibited, it reduced both BCC growth and HH signaling. We were able to demonstrate that SRC promotes BCC growth via aPKC-dependent phosphorylation and activation of GLI1. Together, these findings identify alternative means of being able to target and treat BCCs and, potentially, resistant BCCs as well