Lung cancer is the leading cause of cancer-related mortality in the U.S., and despite focused research pertaining to conventional therapies, the five-year survival rate remains only 17%. The lack of a systematic approach for chemoprevention agent selection and the disregard for individual differences when randomizing to intervention groups have led to clinical prevention trials that have failed or, in some cases, increased the risk for lung cancer. In this regard, understanding the molecular pathogenesis of lung cancer development in a given individual can facilitate identification of targets and/or biomarkers specific to their unique airway molecular profile and the application of targeted chemoprevention. This is expected to yield more effective lung cancer prevention and control. microRNAs (miRNA) have the potential to be robust biomarkers for lung cancer. Our work focuses on preclinical studies to identify mechanisms by which loss of miRNA-125a-3p (miR-125a) expression contributes to the malignant potential of pulmonary epithelial cells harboring an activating point mutation of the K-ras proto-oncogene, one of the most clinically challenging genetic changes commonly found in current and former smokers. Normal human bronchial epithelial cells (HBECs) isolated from large airways of patients were immortalized with hTERT and Cdk4 and subsequently manipulated to express oncogenic K-ras (HBEC K-ras). Compared to their respective vector controls (HBEC Vector), the basal expression of miR-125a is significantly reduced by 3- to 4-fold in premalignant HBEC K-ras lines. Subsequent studies demonstrated that the loss of miR-125a expression in K-ras-mutated HBECs is regulated by the PEA3 transcription factor. Compared with Vector, PEA3 expression is upregulated in HBEC K-ras cells at both the mRNA and protein level, and genetic knockdown of PEA3 with siRNA in HBEC K-ras cells restores miR-125a expression. To assess the impact of miR-125a loss on tumor-promoting factors in K-ras-mutated HBECs, we evaluated a panel of inflammatory and angiogenic proteins and found that the basal protein expression of CXCL1 and VEGF was significantly elevated in HBEC3 K-ras cells compared with Vector. Importantly, overexpression of miR-125a significantly reduced the levels of these tumorigenic factors in HBEC K-ras cells, while no changes were observed in Vector. miR-125a overexpression also reduced the increased proliferation rate of HBEC K-ras cells to the level of Vector and suppressed anchorage-independent growth of K-ras/P53-mutated HBECs by 50%, an inhibition found to be dependent on CXCL1. Importantly, our data also show that pioglitazone, a model chemopreventive agent, increases levels of miR-125a in HBEC K-ras cells through downregulation of PEA3. Moreover, pioglitazone exhibits similar anti-tumor activity as miR-125a overexpression. Treatment of HBEC K-ras cells with pioglitazone downregulates expression of VEGF, suppresses proliferation and induces morphological changes indicative of mesenchymal-to-epithelial transition. Taken together, our novel findings identify the loss of miR-125a expression as a putative biomarker for lung carcinogenesis and possible chemoprevention target. Moreover, our studies illustrate how augmentation of miR-125a by a model chemoprevention agent, in the setting of the K-ras-mutated pulmonary epithelium, can abrogate some of the deleterious downstream events associated with this mutation.
These studies were supported by NHLBI #T32HL072752 (EL) and Lung Cancer SPORE #P50CA70907 (JDM, JEL).
Citation Format: Elvira L. Liclican, Saswati Hazra, Jill E. Larsen, Stacy J. Park, Paul C. Pagano, Tonya C. Walser, Kostyantyn Krysan, John D. Minna, Steven M. Dubinett. The role of miR-125a in the pathogenesis of lung cancer. [abstract]. In: Proceedings of the Twelfth Annual AACR International Conference on Frontiers in Cancer Prevention Research; 2013 Oct 27-30; National Harbor, MD. Philadelphia (PA): AACR; Can Prev Res 2013;6(11 Suppl): Abstract nr A08.