STAT3 activation and aberrant ligand-dependent sonic hedgehog signaling in human pulmonary adenocarcinoma

The Sonic hedgehog (SHH) signaling and STAT3 pathways play important roles during carcinogenesis with possible interaction. To determine the association of the activation of SHH signaling pathway and STAT3 pathway in carcinogenesis of human non-small cell lung carcinomas (NSCLC), firstly we examined the expression of SHH signaling molecules including SHH, Gli1(glioma-associated oncogene homolog 1) and HHIP (Hh interacting protein), as well as p-STAT3 (phosphorylation at Tyr705) by immunohistochemistry in 87 cases of NSCLC, 12 atypical adenomatous hyperplasia (AAH) and 20 adjacent normal lung tissues. The expression of SHH, Gli1, HHIP and p-STAT3 was detected respectively in 87/87(100%), 74/87(85.1%), 75/87(86.2%) and 54/87(62.1%) of the NSCLC cases, but not in the adjacent normal lung parenchyma. Ligand-dependent SHH pathway activation and STAT3 signaling activation were observed in most cases of NSCLC, and the high SHH pathway activation rate and STAT3 activation rate were significantly higher in adenocarcinoma compared with squamous cell carcinomas and large cell carcinomas (P<0.05). Both SHH and STAT3 pathway activation level correlated with histological grade in adenocarcinoma, being higher in well-differentiated types (P<0.05). Furthermore, high SHH pathway activation and p-STAT3 expression were also detected in 10/12(83.3%) of AAH cases as well as in most cases of early-stage adenocarcinoma - adenocarcinoma in situ (AIS) and minimally invasive adenocarcinomas (MIA). Correlation analysis showed that p-STAT3 protein expression level was correlated positively with ligand-dependent activation level of SHH signaling in adenocarcinoma (r _s=0.585, P=0.000) and AAH (r _s=0.996, P=0.000). We speculated that activation of STAT3 could up-regulate SHH gene expression directly or indirectly, and thereby activated SHH signaling resulting in lung tumor cell ontogeny. To explore the interactional mechanism, we then performed serial transient co-transfection assay in human pulmonary adenocarcinoma cell line H441 cells, and the results confirmed STAT3 activation can up-regulate SHH gene expression indirectly. In conclusion, aberrant ligand-dependent SHH signaling activation occurs frequently in NSCLC. The signaling plays a more active role in adenocarcinoma, and is an early event in carcinogenesis of lung adenocarcinoma. The involvement of STAT3 pathway activation might function in inducing the process.