SKP2 and head and neck squamous cell carcinoma: Treatment with Metformin inhibited the growth of HNSCC caused by G1 arrest leading to a decrease in Cdks (2, 4 and 6), cyclins (D1 and E) and Cdk inhibitors (p15, p16, p18 and p27) and decreased the levels of oncogenic proteins Skp2 and β-Trcp