Smo inhibitors are active against Gorlin syndrome-associated BCC or medulloblastoma where Ptch mutations occur (9
). Smo mutations confer resistance to Smo inhibitors (17
). This study comprehensively interrogated 705 epithelial cancer cell lines for growth response to the Smo inhibitor cyclopamine. Findings were compared with expressed HH pathway-regulated species using a linked genetic database. Ptch and Smo mutations that respectively conferred Smo inhibitor response or resistance were undetected. Rare variant sequences were found, but their functional impact was not established. Because HH pathway activation occurs in lung cancers, findings were validated using different Smo inhibitors in human lung cancer cell lines, transgenic and transplantable murine lung cancer models, and paired normal-malignant lung tissue arrays. Ptch1 or Smo mutations were undetected in examined murine and human immortalized lung epithelial and cancer cell lines.
Smo inhibitor growth response was most significantly associated with high cyclin E (P= 0.000009), low IGFBP6 (P=0.000004), and high Gli1 (P=0.04) levels; high GILZ levels were associated with reduced response (P= 0.002) in . This profile implicated a basal dependence on the HH pathway for the growth of these cancer cells. This possibility was validated in murine and human lung cancer cell lines and transgenic as well as transplantable murine lung cancer models in –. Functional consequences were shown using several pharmacological and genetic Smo inhibitors.
Differential expression of the same species was observed in a malignant versus normal human lung tissue array (). A profile indicative of Smo inhibitor response was also observed in murine lung cancers (). While high IGFBP6 levels were present in some lung tumors, it is notable that lung cancers with both high cyclin E and reduced IGFBP6 expression significantly (P<0.0001) increased Gli1 expression. This pattern was one indicating response to a Smo inhibitor. This points out the potential clinical need to discern profiles of HH pathway-regulated species in human tumors to learn which are likely to be responsive to a Smo inhibitor. Future clinical trials that explore activity of Smo inhibitors should also determine these profiles to uncover possible HH pathway dependence.
Several Smo inhibitors conferred similar effects. Stromal effects are engaged to confer some of these anti-neoplastic effects. This could account for differences between in vitro
and in vivo
effects of Smo inhibition (37
). The findings presented in could explain why trials with Smo inhibitors might underestimate clinical anti-tumor effects of Smo inhibition. Only some cancers express a gene profile indicating possible HH pathway dependence. Notably, autocrine HH pathway signaling occurs in lung cancers (36
). The findings reported here are consistent with this prior study.
These findings have implications for combination cancer therapy. Responses to Smo inhibition occurred whether or not RAS
) mutations were present in cancer cells (). In clinical lung cancers, K-RAS
mutations confer resistance to EGFR-TKIs (32
). Yet, clinical trials revealed activity against lung cancers having K-RAS
mutations when an EGFR-TKI was combined with a rexinoid (33
). This clinical activity was associated with reduced cyclin D1 expression in post-treatment lung cancer biopsies (33
). In the present study, in vivo
responses to Smo inhibition were linked to cyclin D1 repression (). Adding a Smo inhibitor to a regimen that targets cyclin D1 for repression might enhance clinical anti-tumor activity. Smo responses were also associated with cyclin E expression ( and ). Targeting the cyclin E-cdk2 complex exerted anti-tumor responses despite presence of K-RAS
mutations by inducing anaphase catastrophe (19
). Combining a cdk2 antagonist with a Smo inhibitor might augment anti-neoplastic activity.
High Smo inhibitor dosages were associated with these anti-neoplastic effects. A similar dose-response relationship for HH inhibition was observed in different tumor contexts and this might depend on expressed drug transporters (40
). It is notable that the findings displayed in (right panel) using a Smo agonist argue against off-target effects of cyclopamine. The reduction of lung tumors after Smo inhibition in the transplantation model reported here is notable since these lung cancers did not have Ptch1 or Smo mutations. Smo inhibitors might treat or prevent other cancers that lack these mutations.
Taken together, findings presented here indicate a Smo inhibitor should be considered in cancers that lack Smo or Ptch1 mutations. This is especially the case when the tumors express a gene profile indicating basal activation of the HH pathway. This could implicate a dependence on the HH pathway for growth or survival of the same tumors. Future clinical research should explore the translational consequences of these findings for cancer therapy and prevention.