Involvement of Rb protein expression and phosphorylation in progression of oral carcinomas

Abstract

Rb protein (pRb) expression and its genetic mutations have been reported in many carcinomas. However, little is known about the relation between pRb phosphorylation and the clinical characteristics of tumors. In this study, we examined Rb protein (pRb) expression and phosphorylation in oral carcinoma specimens. Twenty-six oral carcinoma specimens were immunoprecipitated with anti-Rb antibody and then subjected to Western blotting with antibodies against phosphorylated pRb (Thr373, Ser780, and Ser807/811) . Data were analyzed with a densitometer. Thirteen of 26 specimens (50%) expressed pRb, whereas 13 of 26 (50%) lost pRb. All 6 control specimens expressed pRb. pRb in the control group was unphosphorylated, in contrast to pRb in tumor samples, which was frequently phosphorylated at all sites [Thr373: 7/13 (54%), Ser780: 9/13 (69%), and Ser807/811 12/13 (92%) ] . The extent of phosphorylation tended to increase from T1 to T4 at all phosphorylation sites. The majority of pRb-deficient or hyperphosphorylated samples (inactive group) contained tumors [Thr373: 20/26 (77%), Ser780: 22/26 (85%), and Ser807/811: 25/26 (96%) ] unlike pRb-expressing or hypophosphorylated specimens (active group) . There was a significant difference between controls and the tumor group, but not among the different tumor categories. These results suggest that pRb expression and phosphorylation are important indicators of pRb function. pRb inactivation occurred in the early stages of tumorigenesis, but not during progression of tumor.