HOXB7 IN LUNG CANCER: A NOVEL ROLE IN STEM CELL AND IPS BIOLOGY

Current diagnostic tools do not allow prognostic evaluation of patients with early stage lung cancer or selection of patients that might benefit from adjuvant chemotherapy. Therefore, the identification of novel prognostic markers in early-stage lung cancer is paramount. In this scenario, the transcription factor HOXB7, belonging to the homeobox family, has been shown to correlate with poor prognosis in different types of cancer and recently also in stage I lung adenocarcinoma. To better understand the prognostic implication of alterations in HOXB7 expression in lung cancer, we performed a bioinformatics analysis of multiple lung cancer expression datasets in order to identify gene sets representing cancer-relevant biological functions enriched in high-HOXB7 expressing tumors. We found several gene sets enriched in high-HOXB7 expressing tumors representing molecular mechanisms involved in epithelial to mesenchymal transition, in cancer progression, and, interestingly, in stemness and cellular reprogramming. Based on these results, we hypothesized that HOXB7 may have a role in the expansion of the stem cell compartment in cancer, a mechanism that has been shown to be a hallmark of enhanced tumorigenicity and of increased metastatic potential. Analysis of the stem-related surface marker CD90 revealed that overexpression of HOXB7 in lung cells increases the CD90high sub population. CD90high, but not CD90low cells, are able to form spheroids, which is an hallmark of stemness. Indeed, the sphere forming efficiency of normal lung BEAS-2B cells was 22% and 1.64% in CD90high and CD90low populations, respectively. In addition, we found that silencing of LIN28B counteracts the expansion of the CD90high population. LIN28B was recently described as an oncogene that regulates the cancer stem cell compartment. We found that LIN28B is under the direct transcriptional control of HOXB7. Therefore, we propose a novel molecular mechanism driven by HOXB7 and can increase stem-like properties in lung cells. We further demonstrated that the HOXB7-LIN28B axis plays an important role in reprogramming of adult cells into induced pluripotent stem cells (iPS). Indeed, HOXB7 may enhance the reprogramming efficiency achieved by the three genes OCT4, KLF4, SOX2 in both mouse embryonic fibroblast and human epithelial BEAS-2B cells by substituting MYC in the transcription factor cocktail of reprogramming factors used by Yamanaka. Of note, LIN28B silencing strongly decreases the number of reprogrammed colonies in high-HOXB7 expressing cells. These findings suggest that HOXB7, through transcriptional induction of the LIN28B gene, activates a program relevant for stem/iPS cell biology and for tumor progression, possibly opening a new line of research for the development of more effective therapies for metastatic lung cancer patients.