The AP2β is a transcription factor that plays an important role in regulating apoptosis and control cell division14
however there is limited information about its role in lung cancer. In our study, the AP2β which is located in the nucleoplasm in normal lung tissue was found in either nucleoplasm or nucleoli in NSCLC and the patients with AP2β in the nucleoli had poor survival. Most prognostic markers for NSCLC are identified by assessing the level of RNA or a specific protein in the tumor and then determining whether that level correlates with survival. Immunohistochemistry studies have shown that Ki-67, p53, and Bcl-2 are important in NSCLC and may provide some prognostic information.15
We took a similar approach and investigated the level of AP2β and its relation to survival. Since presence of hTERT has been associated with survival in NSCLC patients,4
we expected that the hTERT promoter AP2β would also be associated with survival. Surprisingly, we found no relationship between the level of AP2β and survival. Although AP2β was expressed in normal lung tissue as well as in NSCLC and that higher AP2β levels were expressed in the NSCLC cell lines than in the normal cells, AP2β level was not a predictor of survival. The difference may be due to how hTERT levels were determined in previous study.4
Wang et al. found the hTERT level by performing in situ
hybridization for hTERT mRNA expression. They looked at the presence of the hTERT mRNA in the cytoplasm of NSCLC cells and found that only 33% of the samples expressed hTERT. In the current study, we found that hTERT was expressed in 76% of the samples studied, which is much closer to 86% of samples identified in other studies.16
Thus, in situ
hybridization method that was used in Wang et al.4
might not reflect the protein levels and protein levels may not provide prognostic information.
However, in both sets of patients, the cellular location of AP2β was a prognostic marker for survival. In normal lung tissue cells, AP2β was diffusely expressed in the nucleoplasm; in contrast, in the NSCLC cells, AP2β was either diffusely expressed in the nucleoplasm or expressed in the nucleoli. In both patient sets, AP2β localization in the nucleoli, which plays an important role in ribosome biogenesis, was found to be an indicator of poor prognosis. These findings suggest that NSCLC tumors are more resistant to cell death when the AP2β is localized in the nucleoli. Other studies have shown that apoptosis-related proteins are present in the nucleoli after the induction of cell death.17
Since AP2β knockout mice die from massive apoptosis,11
the presence of AP2β in the nucleoli may indicate that this protein is present to prevent cell death and allowing the tumor to grow without any controls.
The current study was not without its limitations. One potential limitation was that the current study was retrospective; thus, some data were unavailable for analysis such as type of resection, morbidity of the operation, type of recurrence and cause of death. A larger issue stemming from the retrospective nature of the study was the fact that the patient groups were not balanced. In the first set of patients, more women, never-smokers, and patients with well differentiated tumors were in the diffuse staining group than in the nucleolar staining group, whereas in the second set of patients, more women were in the diffuse staining group than in the nucleolar staining group. To account for the unbalanced group, we used univariate and multivariate analyses, which accounts for these factors, to analyze the relationship between different factors and survival and we found that AP2β nucleolar localization was independent predictor of poor survival. Moreover, in the second patient set, age was also an independent predictor of survival which suggests that patients who undergo lung cancer resection are at a higher risk of dying from other causes such as cardiovascular disease. Thus, later deaths among patients in the validation set may have been due to factors that were independent of tumor aggressiveness but associated with advanced age and its comorbidities. Moreover, our use of two independent groups of patients strengthened the study, with findings in the first set also seen in the second set of patients.
In conclusion, the current study's findings show that AP2β co-localizes with hTERT in either the nucleoplasm or nucleoli of NSCLC cells. Moreover, the AP2β nucleolar localization was associated with very poor survival. The current standard of care for patients with stage I NSCLC is surgical resection. If the AP2β nucleolar pattern is indeed an indicator of poor prognosis, these patients may benefit from adjuvant therapy to improve their survival. Additional studies are needed to confirm our findings, identify the exact mechanism of nucleolar localization of the AP2β and aggressiveness of tumor and determine whether adjuvant therapy prolongs survival in patients predicted to have poor prognosis.