Cell Cycle and Cervical Carcinoma human cancer and in normal tissues, including the uterine cervix, have shown that pRb is expressed in mature and differentiated cells. Both binding and degradation of the Rb proteins by the HPV E7 protein are essential for sustained proliferation of HeLa cervical carcinoma cells, and E7 repres- sion triggers senescence at least in part by acti- vating the Rb pathway in both HeLa and HT-3 cells.42 Salcedo et al. observed that the majority of cells in the proliferating basal layer of the normal epithelium displayed low levels of pRb. pRb immunostaining in invasive cervical lesions is frequently lower than in SIL.43 This low expres- sion of Rb may result from Rb gene mutations or downregulation mechanisms, but may also be related to pRb inactivation resulting from complex formation with high-risk HPV E7 oncoproteins. Rb immunoreactivity was not affected by high- risk HPV infection in most studies of cervical squamous cell carcinoma.46 In contrast, Rb staining was even more frequently found in SIL and in- vasive cancer (93%) compared to normal epithe- lium or reactive atypia (78%).47 Mutations in the Rb gene seem to be rare events in cervical can- cer.46,48 In most studies, Rb gene expression did not strictly correlate with the HPV status,49 al- though Scheffner et al. reported a higher Rb mutation rate in HPV-negative than in HPV-posi- tive cervical cancer cell lines. These mutations re- sulted in aberrant proteins that were not phos- phorylated and unable to complex with the ade- novirus E1A oncoprotein.50 Cyclin D1 is the regu- latory subunit of the holoenzymes that phos- phorylate and, together with sequential phos- phorylation by cyclin E/CDK2, inactivate the cell- cycle inhibiting function of the pRb protein. The hypophosphorylated form of pRb, complexed with E2F, serves as an activator of cyclin D1 tran- scription by binding to its promoter, thus driving cyclin D1 in early and mid G1-phase of the cell cycle. As D-type cyclins and HPV E7 possess similar binding regions for pRb and pRb-related pocket proteins, inactivation of pRb either by the cyclin/ CDK complexes in G1 or by interaction with the high-risk HPV oncoprotein E7 may result in a decreased expression of cyclin D1 (Fig. 1).43,51 The p21WAF1/ CIP1-p27KIP1-cyclin E-CDK2 pathway p21WAF1/CIP1 p21WAF1/CIP1 is a cyclin-dependent kinase inhibi- tor that associates with a class of CDKs and inhibits their kinase activities, leading to cell cycle arrest and the dephosphorylation of pRb. A large body of evidence suggests that p21WAF1/CIP1 plays an important role in cell fate decisions during growth and differentiation. The p21WAF1/CIP1 pro- tein is a p53-inducible protein that inactivates the cyclin/CDK complexes, blocking the cell cycle progression in the G1-S transition. p21WAF1/CIP1 is expressed in cells undergoing either G1 arrest or apoptosis by p53-dependent or -independent mechanisms.52,53 Mutations in the p21WAF1/CIP1 gene were very rarely detected in cervical carci- Fig. 1. Schematic presentation of the HPV viral oncoproteins E6/E7 and cell-cycle regulatory proteins in cer- vical carcinogenesis. Yonsei Med J Vol. 46, No. 5, 2005