p27 specifically decreases in squamous carcinoma, and mediates NNK‐induced transformation of human bronchial epithelial cells

Abstract Lung cancer remains the leading cause of cancer‐related deaths, with cigarette smoking being the most critical factor, linked to nearly 90% of lung cancer cases. NNK, a highly carcinogenic nitrosamine found in tobacco, is implicated in the lung cancer‐causing effects of cigarette smoke. Although NNK is known to mutate or activate certain oncogenes, its potential interaction with p27 in modulating these carcinogenic effects is currently unexplored. Recent studies have identified specific downregulation of p27 in human squamous cell carcinoma, in contrast to adenocarcinoma. Additionally, exposure to NNK significantly suppresses p27 expression in human bronchial epithelial cells. Subsequent studies indicates that the downregulation of p27 is pivotal in NNK‐induced cell transformation. Mechanistic investigations have shown that reduced p27 expression leads to increased level of ITCH, which facilitates the degradation of Jun B protein. This degradation in turn, augments miR‐494 expression and its direct regulation of JAK1 mRNA stability and protein expression, ultimately activating STAT3 and driving cell transformation. In summary, our findings reveal that: (1) the downregulation of p27 increases Jun B expression by upregulating Jun B E3 ligase ITCH, which then boosts miR‐494 transcription; (2) Elevated miR‐494 directly binds to 3′‐UTR of JAK1 mRNA, enhancing its stability and protein expression; and (3) The JAK1/STAT3 pathway is a downstream effector of p27, mediating the oncogenic effect of NNK in lung cancer. These findings provide significant insight into understanding the participation of mechanisms underlying p27 inhibition of NNK induced lung squamous cell carcinogenic effect.