Novel functional roles of G protein-coupled receptor kinase 2 (GRK2) in Squamous Cell Carcinomas

Resumen

Cancer progression is the result of alterations in many different signaling pathways. G protein-coupled receptor kinase 2 (GRK2) emerges as a key node able to integrate different signaling cascades involved in cancer progression in a tumor and cell-type specific way. However, its role in the tumorigenesis of stratified epithelia had never been addressed. In this thesis, we show that this kinase plays an important role in the progression of Squamous Cell Carcinomas (SCC). GRK2 protein is clearly reduced in the invasive front of skin SCC and in undifferentiated, high-grade SCC tumors of human stratified epithelia, such as cervix or head and neck. Kaplan-Meier analysis generated from different datasets of those tumors indicate that patients with low levels of mRNA expression of the GRK2 gene (Adrbk1) are prone to increased recurrence and metastasis, suggesting that GRK2 down-modulation might favor SCC tumor progression. In contrast, the expression of Snail (Snai1), a transcription factor known to be a key Epithelial-Mesenchymal Transition (EMT) inducer and to be present at the invasive front and the periphery of different types of human SCC, displayed the reverse trend, suggesting a functional antagonism between both proteins. Remarkably, we find that EMT induction via Snail decreases GRK2 levels, whereas restoring GRK2 activity counteracts the mesenchymal phenotype induced by Snail in model epithelial cells, and that GRK2 directly phosphorylates Snail, thus attenuating its functionality. Importantly, depletion of endogenous GRK2 in head and neck carcinoma cells is sufficient to trigger EMT features and to foster a motile and invasive phenotype, suggesting that the downregulation of GRK2 observed in clinical samples might contribute to tumor malignancy. We find that induced systemic or stratified epithelia-specific GRK2 ablation in mice promotes skin hyperplasia, altered differentiation patterns and aberrant hair follicle homeostasis, suggesting an important role in epidermal homeostasis. Moreover, global induced deletion of GRK2 induces spontaneous papillomas in the tongue and makes animals more sensitive to oral carcinogens, putting forward GRK2 as a suppressor of oral tumorigenesis. Interestingly, simultaneous ablation of GRK2 and p53 specifically in the stratified epithelia strongly potentiates epidermal hyperplasia and allows the early formation of spontaneous SCC in the skin, contrary to that observed in animals lacking GRK2 or p53 alone, suggesting synergistic effects of these pathways in SCC tumor progression. Finally, in the search for mechanisms leading to decreased GRK2 expression in these pathological contexts, we find that GRK2 levels markedly decrease in the skin of mice overexpressing the Human Papillomavirus (HPV) E6/E7 oncoproteins in their stratified epithelia, as well as in HPV+ HNSCC cell lines. Moreover, HPV E7 appears to favor GRK2 degradation, while GRK2 directly phosphorylates E7, suggesting a mutual negative feedback loop between these proteins. Overall, our data suggest that GRK2 plays an important role in the control of epidermal cell proliferation and differentiation status in vivo, and that GRK2 downregulation favors the development of tumoral features in stratified epithelia.