Anti-tumor effect of CDK inhibitors on CDKN2A-defective squamous cell lung cancer cells

Squamous cell lung cancer (SqCLC) is a distinct histologic subtype of non-small cell lung cancer (NSCLC). Although the discovery of driver mutations and their targeted drugs has remarkably improved the treatment outcomes for lung adenocarcinoma, currently no such molecular target is clinically available for SqCLC. The CDKN2A locus at 9p21 encodes two alternatively spliced proteins, p16^INK4a (p16) and p14^ARF (p14), which function as cell cycle inhibitors. The Cancer Genome Atlas (TCGA) project revealed that CDKN2A is inactivated in 72% of SqCLC cases. In addition, it was found that CDKN2A mutations are significantly more common in SqCLC than in adenocarcinoma. Down-regulation of p16 and p14 by CDKN2A gene inactivation leads to activation of cyclin-dependent kinases (CDKs), thereby permitting constitutive phosphorylation of Rb and subsequent cell cycle progression. Here, we hypothesized that CDK inhibition may serve as an attractive strategy for the treatment of CDKN2A-defective SqCLC.

We investigated whether the CDK inhibitors flavopiridol and dinaciclib may exhibit antitumor activity in CDKN2A-defective SqCLC cells compared to control cells. The cytotoxic effect of the CDK inhibitors was evaluated using cell viability assays, and the induction of apoptosis was assessed using TUNEL assays and Western blot analyses. Finally, anti-tumor effects of the CDK inhibitors on xenografted cells were investigated in vivo.

We found that flavopiridol and dinaciclib induced cytotoxicity by enhancing apoptosis in CDKN2A-defective SqCLC cells, and that epithelial to mesenchymal transition (EMT) decreased and autophagy increased during this process. In addition, we found that autophagy had a cytoprotective role.

Our data suggest a potential role of CDK inhibitors in managing CDKN2A-defective SqCLC.