Image obtained from the manuscript
Non-small-cell lung carcinoma (NSCLC) is a devastating illness. Approximately 85% of lung cancers are NSCLCs, and the 5-year mortality rate ranges from 51-99% depending on the stage of diagnosis (for more information, visit the website of the American Cancer Society). Lung adenocarcinomas are common in NSCLC, and genetic analyses have revealed frequent genetic alterations at multiple loci including the oncogene KRAS and tumor suppressors TP53 and CDKN2A. TP53 encodes the important tumor suppressor p53, but the CDKN2A locus contains two genes, INK4a and ARF, each of which has been implicated in suppressing NSCLC to various degrees. However, the direct contribution of ARF to cancer cell growth control has remained enigmatic and controversial. A new study led by Dr. Kemp (Division of Human Biology) suggests that ARF plays a key role in suppressing multiple aspects of NSCLC growth and malignant progression.
The authors used a carcinogen-induced mouse model (urethane exposure) to ascertain the consequence of ARF deletion on cancer progression. Arf-/- mice treated with urethane had shortened life spans and increased lung tumor-associated morbidity compared to control mice (Arf+/+ or Arf+/-), suggesting a critical tumor suppressive function for ARF. The lung tumors in Arf-/- mice were of abnormal size (4-17 mm in diameter) compared to the tumors in control animals (rarely exceeding 3 mm in diameter). The researchers also found that ARF deficiency increased the tumor burden induced by urethane exposure. The tumors in control animals exposed to urethane were mainly pulmonary adenomas, but Arf-/- mice had adenocarcinomas that invaded adjacent tissues and airways (see figure). Strikingly, Arf-/- mice also had tumors that were de-differentiated and often displayed histopathological characteristics common to pulmonary sarcomatoid carcinoma, an aggressive and metastatic tumor type that had not previously been seen in mice.
Busch et al. saw an increase in mitotic cells in Arf-/- tumors, suggesting that ARF normally inhibits cancer cell proliferation. These cells also displayed hallmarks of DNA damage. Moreover, Arf-/- tumors displayed evidence of hyperactive oncogenic KRAS signaling and cyclin D1 upregulation, both of which have been shown to promote cell proliferation.
Perhaps the most controversial aspect regarding ARF’s role in cancer is the question of timing, and at which point does ARF function have the greatest role in inhibiting cancer progression. Busch and colleagues found that ARF was highly expressed in early-stage adenomas but had minimal expression in malignant adenocarcinomas from Arf+/+ urethane-treated animals. This indicated that the early cellular response to urethane treatment induced the expression of ARF to restrain cancer cell growth. The authors also explored a link between ARF and p53 tumor suppression. ARF expression coincided with that of p21, a p53 transcriptional target whose expression halts cell cycle entry. Moreover, the expression of p53 transcriptional targets was decreased in Arf-/- tumors, indicating that ARF functions through canonical p53 signaling pathways to inhibit cancer progression.
The new manuscript published by the Kemp group reveals that Arf loss facilitates the malignant progression of neoplasia originating in the lung. In human cancers, silencing of the entire CDKN2A locus is common, although selective silencing of ARF has also been documented. The sarcomatoid-like tumors found in Arf-/- animals suggest that ARF may suppress cancer cell de-differentiation, although its precise role in the process remains to be elucidated. Moreover, the contribution of Arf to p53 signaling needs to be further investigated. Clearly, the model system set up by the authors will continue to clarify the many roles that ARF plays in tumor suppression.
Busch SE, Moser RD, Gurley KE, Kelly-Spratt KS, Liggitt HD, Kemp CJ. 2013. ARF inhibits the growth and malignant progression of non-small-cell lung carcinoma. Oncogene doi: 10.1038/onc.2013.208.