Evidence, Mechanism, and Clinical Relevance of the Transdifferentiation from Lung Adenocarcinoma to Squamous Cell Carcinoma

Lung adenocarcinoma (ADC) and squamous cell carcinoma (SCC) are two distinct subtypes of non-small-cell lung carcinoma. Interestingly, approximately 4% to 9% of human non-small-cell lung carcinoma tumors contain mixed adenomatous and squamous pathologies in a single lesion, clinically termed adenosquamous cell carcinoma. More important, these two different pathological components frequently share identical oncogenic mutations, indicative of a potential transition. Indeed, recent data have provided convincing evidence in supporting the ADC to SCC transdifferentiation in lungs. In the liver kinase B1 (official name STK11)-deficient mouse model, lung ADC can progressively transdifferentiate to SCC through pathologically mixed adenosquamous cell carcinoma as the intermediate status. Mechanistic studies further identify essential roles of extracellular matrix remodeling and metabolic reprogramming during this phenotypic transition. Small molecular compounds, including lysyl oxidase inhibitors and reactive oxygen species-inducing reagents such as phenformin, significantly accelerate the transition from lung ADC to SCC and thus confer lung tumors with drug resistance. Consistent with these findings, recent clinical studies have shown that epidermal growth factor receptor-mutant lung ADC can transdifferentiate to SCC in relapsed cancer patients. Together, these data support that this phenotypic transition from lung ADC to SCC might represent a novel mechanism for drug resistance. This review will summarize our current understanding of the transdifferentiation from lung ADC to SCC.