Gadd45a regulates matrix metalloproteinases by suppressing ?Np63a and ß-catenin via p38 MAP kinase and APC complex activation.

The p53-regulated growth arrest and DNA damage-inducible gene product Gadd45a has been recently identified as a key factor protecting the epidermis against ultraviolet radiation (UVR)-induced skin tumors by activating p53 via the stress mitogen-activated protein kinase (MAPK) signaling pathway. Herein we identify Gadd45a as an important negative regulator of two oncogenes commonly over-expressed in epithelial tumors: the p53 homologue ?Np63a and ß-catenin. ?Np63a is one of the several p63 isoforms and is the predominant species expressed in basal epidermal keratinocytes. ?Np63a lacks the N-terminal transactivation domain and behaves as a dominant-negative factor blocking expression of several p53-effector genes. ?Np63a also associates with and blocks activation of the adenomatous polyposis coli (APC) destruction complex that targets free cytoplasmic ß-catenin for degradation. While most ß-catenin protein is localized to the cell membrane and is involved in cell-cell adhesion, accumulation of free cytoplasmic ß-catenin will translocate into the nucleus where it functions in a bipartite transcription factor complex, whose targets include invasion and metastasis promoting endopeptidases, matrix metalloproteinases (MMP). We show that Gadd45a not only directly associates with two components of the APC complex, namely protein phosphatase 2A (PP2A) and glycogen synthase kinase 3ß (GSK3ß) but also promotes GSK3ß dephosphorylation at Ser9, which is essential for GSK3ß activation, and resultant activation of the APC destruction complex. We demonstrate that lack of Gadd45a not only prevents ?Np63a suppression and GSK3ß dephosphorylation but also prevents free cytoplasmic ß-catenin degradation after UV irradiation. The inability of Gadd45a-null keratinocytes to suppress ß-catenin may contribute to the resulting observation of increased MMP expression and activity along with significantly faster keratinocyte migration in Matrigel in vitro and accelerated wound closure in vivo. Furthermore, epidermal keratinocytes treated with p38 MAPK inhibitors, both in vivo and in vitro, behave very similarly to Gadd45a-null keratinocytes after UVR. Similarly, Trp53-null mice are unable to attenuate ?Np63a expression in epidermal keratinocytes after such stress. These findings demonstrate a dependence on Gadd45a-mediated p38 MAPK and p53 activation for proper modulation of ?Np63a, GSK3ß, and ß-catenin after irradiation. Taken together, our results indicate that Gadd45a is able to repress ?Np63a, ß-catenin, and consequently MMP expression by two means: by maintaining UVR-induced p38 MAPK and p53 activation and also by associating with the APC complex. This implicates Gadd45a in the negative regulation of cell migration, and invasion.Oncogene (2004) 23, 1829-1837. doi:10.1038/sj.onc.1207301 Published online 1 December 2003 [ABSTRACT FROM AUTHOR]