In this study, we applied global gene expression profiling on samples of a well-established mouse model of tumor recurrence, and identified differentially expressed candidate genes some of which have been described previously in the context of HNSCC development or tumor recurrence, such as mucins, kallikreins, tryptase alpha/beta 1, claudin-10 and lactotransferrin [15
]. We also provided experimental evidence for the crucial role of MYBBP1A during malignant progression of HNSCCs.
Originally, MYBBP1A was identified as a c-Myb proto-oncogene product interacting protein [9
]. Several findings support the assumption that MYBBP1A is a key regulator of tumor cell physiology: (I) in the preclinical mouse model of tumor relapse and in matched pairs of HNSCC patients MYBBP1A showed high expression in primary tumors which was severely reduced in recurrent tumor samples, and (II) MYBBP1A expression in murine SCC-7 cells as well as human HNSCC cell lines support proliferation but was inversely correlated with tumor cell migration.
Hitherto, the molecular mechanism resulting in reduced MYBBP1A protein levels in recurrent tumors remains elusive and will be a major challenge for the future. So far, our knowledge on regulation and function of MYBBP1A protein with regard to physiological and pathophysiological conditions is limited. Concerning post-translational modification proteolytic processing of MYBBP1A has been reported in some cells types [21
]. Moreover, MYBBP1A was identified as a novel aurora B kinase substrate, suggesting an important role of MYBBP1A phosphorylation in the regulation of its function [22
]. Accordingly, human HNSCC cell lines used in our study exhibited comparable transcript levels while expressing different amounts of MYBBP1A protein, suggesting that MYBBP1A protein levels are at least in part regulated by posttranslational mechanisms. In line with this assumption, a pilot study demonstrates regulation of MYBBP1A protein stability by ubiquitination, since treatment of SCC-25 cells with the proteasome inhibitor MG-132 resulted in an enrichment of the protein when compared to untreated cells (data not shown). Hence, a major challenge for the future will be to unravel the molecular components involved in this process.
Intriguingly, high levels of MYBBP1A protein were associated with increased proliferation but reduced migration and invasion of murine SCC-7 and human HNSCC cell lines, a phenotype that was reverted by silencing of MYBBP1A expression. Recent publications established a critical function for MYBBP1A as an important regulator of distinct transcription factors, such as the proto-oncogenes MYB and NFκB as well as the tumor suppressor p53, implicated in cell cycle control and carcinogenesis [10
]. It is worth to note that aberrant regulation of p53 and NFκB is a frequent event in human HNSCC and critically involved in tumor cell proliferation and the malignant phenotype [25
]. More recently, the molecular mechanism has been elucidated, how mitotic stress and nucleolar disruption activates MYBBP1A resulting in acetylation and accumulation of p53 in a p300-dependent manner [23
]. The fact that all HNSCC cell lines used in this study express no or only mutated p53 suggests that the physical interaction between MYBBP1A and p53 is not required for its opposing functions on tumor cell proliferation and migration, and it will be interesting to investigate the consequence of MYBBP1A over expression and silencing in primary keratinocytes as well as tumor cells with functional p53.
Although the underlying molecular mechanism, how MYBBP1A regulates the tumor cell physiology remains to be elucidated our data suggest that tumor cells with reduced MYBBP1A protein expression belong to a subpopulation of slow-cycling cells with high mobility. Concerning the later phenotype, improved migration and invasion capacity of tumor cells with low or absent MYBBP1A expression might be, at least in part, due to a reduction in membrane proteins that are critically implicated in cell-cell adhesion, such as E-Cadherin. However, more detailed studies are required to highlight the molecular function of MYBBP1A under physiological and pathological conditions, including cancer. The existence of a slow-cycling subpopulation of MYBBP1A negative tumor cells in HNSCC patients could be of high clinical importance since most current therapeutic regimens target the rapidly proliferating tumor bulk.