Resistance to mutant KRASV12-induced senescence in an hTERT/Cdk4-immortalized normal human bronchial epithelial cell line

Mutant KRAS, the most frequently occurring (similar to 30%) driver oncogene in lung adenocarcinoma, induces normal epithelial cells to undergo senescence. This phenomenon, called oncogene-induced senescence (OIS) , prevents mutant KRAS-induced malignant transformation. We have previously reported that mutant KRAS(V12) induces OIS in a subset of normal human bronchial epithelial cell line immortalized with hTERT and Cdk4. Understanding the mechanism and efficacy of this important cancer prevention mechanism is a key knowledge gap. Therefore, this study investigates mutant KRAS(V12)-induced OIS in upregulated telomerase combined with the p16/RB pathway inactivation in normal bronchial epithelial cells. The normal (non-transformed and non-tumorigenic) human bronchial epithelial cell line HBEC3 (also called HBEC3KT ), immortalized with hTERT ( T ) and Cdk4 ( K ), was used in this study. HBEC3 that expressed mutant KRASV12 in a doxycycline-regulated manner was established (designated as HBEC3-RIN2). Controlled induction of mutant KRASV12 expression induced partial epithelial-to-mesenchymal transition in HBEC3-RIN2 cells, which was associated with upregulated expression of ZEB1 and SNAIL. Mutant KRAS(V12) caused the majority of HBEC3-RIN2 to undergo morphological changes; suggestive of senescence, which was associated with enhanced autophagic flux. Upon mutant KRAS(V12) expression, only a small HBEC3-RIN2 cell subset underwent senescence, as assessed by a senescence-associated beta-galactosidase staining (SA-beta G) method. Furthermore, mutant KRAS(V12) enhanced cell growth, evaluated by colorimetric proliferation assay, and liquid and soft agar colony formation assays, partially through increased phosphorylated AKT and ERK expression but did not affect cell division, or cell cycle status. Intriguingly, mutant KRAS(V12) reduced p53 protein expression but increased p21 protein expression by prolonging its half-life. These results indicate that an hTERT/Cdk4-immortalized normal bronchial epithelial cell line is partially resistant to mutant KRAS(V12)-induced senescence. This suggests that OIS does not efficiently suppress KRAS(V12)-induced transformation in the context of the simultaneous occurrence of telomerase upregulation and inactivation of the p16/Rb pathway.

Automated summary

Mutant KRAS induces senescence in normal epithelial cells, preventing malignant transformation. This study investigates the mechanism of mutant KRAS-induced senescence in normal bronchial epithelial cells with upregulated telomerase and inactivation of the p16/RB pathway.