Abnormal expression of Rb pathway-related proteins in salivary gland acinic cell carcinoma

Salivary gland acinic cell carcinoma (ACC) is a relatively rare neoplasm, and limited information is available regarding its molecular pathogenesis. Because the deregulation of Rb pathway is common to most human tumors, we immunohistochemically investigated the expression of Rb pathway-related proteins, including Rb, Rb proteins phosphorylated at serine 780 and 795 (pRb-S780 and pRb-S795, respectively), cyclin D1, and p16INK4a in 18 cases of ACC. The expression of topoisomerase II-alpha and Ki-67 was also examined to evaluate cell proliferation. All the ACCs exhibited substantial numbers of positive cells against Rb antibody that recognizes both unphosphorylated and phosphorylated Rb proteins. The numbers of positive cells for pRb-S795 and cyclin D1 significantly increased in ACCs as compared with normal salivary glands. Double immunofluorescent staining demonstrated that pRb-S795 was colocalized with cyclin D1 in most tumor cells. However, neither significant change of the expression of Rb protein phosphorylated at serine 780 nor its colocalization with cyclin D1 was observed. The loss of p16INK4a is infrequent, but its expression was correlated with phosphorylated Rb proteins. Our results suggest that serine 795 but not serine 780 is the preferred phosphorylation site induced by cyclin D1. This phosphorylation appeared to be critical for inactivation of Rb-mediated growth suppression and may play an important role in the pathogenesis of ACC.