Figure 2. Over-activation mechanisms of the Hh pathway in cancer, and gene expression profile in lung cancer cell lines.

(A) Schematic representation of autocrine and paracrine activation of the Hh pathway: i) Ligand-independent: Constitutive activation of the pathway by mutation of the PTCH1 gene, promoting the maintenance of neoplastic cells; ii) Ligand-dependent: Autocrine activation occurs when neoplastic cells secrete their own ligand and achieve self-activation; iii) Paracrine activation: Neoplastic cells produce a ligand capable of activating stromal cells, which in turn secrete growth factors which maintain neoplastic cells; iv) Stromal cells secrete Hh ligand, promoting the activation of the pathway in neoplastic cells. (B) mRNA expression patterns for genes which code for Hh members, gene expression analysis in adenocarcinoma-type lung cancer cells A549, A427, INER51 and INER37(123-125). A549 cells possess higher level of expression of SHH, PTCH1, SMO and GLI-1, confirming previous reports where A549 remains the most adequate cellular model for the study of the SHH pathway in pulmonary, epithelial neoplasms (126). Expression analysis obtained using real-time PCR platform LightCycler 480 (Roche, Mannheim, Germany), with UPL-type specific hydrolysis probes (Roche, Germany) and normalizing expression levels through detection of endogenous gene GAPDH, taking as basal expression level the pooled data of mRNA of the 4 lung cancer cell lines analyzed.