Cyclins, cyclin-dependent kinases (Cdks), and Cdk inhibitors (CdkIs) are frequently altered in human cancer. p18 INK4C , a member of the INK4 family of CdkIs, is a potential tumorsuppressor gene product. However, the expression of p18 INK4C in hepatocellular carcinoma (HCC) remains unknown. The aim of this study was to examine the expression of p18 INK4C in various liver diseases including HCC and to assess its clinical significance in HCC. To that end, we examined the expression of p18 INK4C by immunohistochemistry in various liver diseases, including 51 HCCs, and also studied the relationship between p18 INK4C expression, the phosphorylation of retinoblastoma protein (pRb), and the activity level of Cdk4 and Cdk6. Immunohistochemical analysis revealed the frequent loss of p18 INK4C expression in HCC, especially in poorly differentiated HCC. The loss of p18 INK4C expression was shown to be associated with a poor prognosis compared with that associated with p18 INK4C -positivity. Further, the kinase activity of Cdk4 was found to be higher in p18 INK4C -negative HCCs than in p18 INK4C -positive HCCs. However, the level of Cdk6 activity was similar in the 2 groups of HCCs. In p18 INK4C -positive HCCs, p18 INK4C dominantly interacted with Cdk4 rather than with Cdk6. pRb phosphorylated at serine(Ser) 780 was detected more frequently in p18 INK4C -negative than in p18 INK4C -positive HCCs. In conclusion, the loss of p18 INK4C expression may play a role in the differentiation and development of HCC through the up-regulation of Cdk4 activity. (HEPATOLOGY 2004;40:677-686.)
R ecently, we have revealed that the aberrant expression of cell cycle-related proteins is one of the major factors contributing to the development of hepatocellular carcinoma (HCC). [1][2][3] The cell cycle is mainly governed by various cyclin-dependent kinases (Cdks); the activity is regulated positively by cyclins, and negatively by Cdk inhibitors (CdkIs). It is also regulated by phosphorylation and dephosphorylation events. 4 -6 In mammalian cells to date, at least 2 distinct families of CdkIs are known. One, the p21 family, consists of p21 CIP1/WAF1 , p27 KIP1 , and p57 KIP2 , which are general inhibitors of G1 to S in the cell cycle. The other known CdkI family, the inhibitor of Cdk4 (INK4) family, consists of p16 INK4A , p15 INK4B , p18 INK4C , and p19 INK4D , which specifically inhibit cyclin D-related kinase activity by binding to Cdk4 or Cdk6. 4 -6 The CdkI proteins, potent negative regulators for the cell cycle, are potential tumor-suppressor gene products, and their loss might play an important role in the development of human cancers. 4 -11 In fact, loss of INK4 family members such as p15 INK4B , p16 INK4A , and p18 INK4C by gene mutation, deletion, and/or methylation has been observed in a variety of human cancers. 6 -11 Clinical findings have revealed that the loss of p16 INK4A expression is associated with poor prognosis for some types of human cancers. 9 -11 These studies have suggested that INK4 family members might play a role in the progression and prognosis of human cancers.