Examination of the pRb-dependent and pRb-independent functions of E7 in vivo

Structure-function analysis of the retinoblastoma tumor suppressor protein - is the whole a sum of its parts?

Biochemical analysis of the retinoblastoma protein's function has received considerable attention since it was cloned just over 20 years ago. During this time pRB has emerged as a key regulator of the cell division cycle and its ability to block proliferation is disrupted in the vast majority of human cancers. Much has been learned about the regulation of E2F transcription factors by pRB in the cell cycle. However, many questions remain unresolved and researchers continue to explore this multifunctional protein. In particular, understanding how its biochemical functions contribute to its role as a tumor suppressor remains to be determined. Since pRB has been shown to function as an adaptor molecule that links different proteins together, or to particular promoters, analyzing pRB by disrupting individual protein interactions holds tremendous promise in unraveling the intricacies of its function. Recently, crystal structures have reported how pRB interacts with some of its molecular partners. This information has created the possibility of rationally separating pRB functions by studying mutants that disrupt individual binding sites. This review will focus on literature that investigates pRB by isolating functions based on binding sites within the pocket domain. This article will also discuss the prospects for using this approach to further explore the unknown functions of pRB.

Human papillomavirus type 16 E7 oncoprotein associates with the cullin 2 ubiquitin ligase complex, which contributes to degradation of the retinoblastoma tumor suppressor

Human papillomavirus type 16 (HPV16) and other high-risk HPVs are etiologically linked to the development of cervical carcinomas and contribute to a number of other tumors of the anogenital tract, as well as oral cancers. The high-risk HPV E6 and E7 oncoproteins are consistently expressed in cervical cancer cells and are necessary for the induction and maintenance of the transformed phenotype. An important aspect of HPV16 E7's oncogenic activities is destabilization of the retinoblastoma tumor suppressor (pRB) through a ubiquitin/proteasome-dependent mechanism, although the exact molecular mechanism is unknown. Here, we report that HPV16 E7 is associated with an enzymatically active cullin 2 ubiquitin ligase complex and that the HPV16 E7/pRB complex contains cullin 2. Depletion of cullin 2 by RNA interference causes increased steady-state levels and stability of pRB in HPV16 E7-expressing cells, and ectopic expression of HPV16 E7 and the cullin 2 complex leads to pRB ubiquitination in vivo. Hence, we propose that the HPV16 E7-associated cullin 2 ubiquitin ligase complex contributes to aberrant degradation of the pRB tumor suppressor in HPV16 E7-expressing cells.

Latent membrane protein 1 suppresses RASSF1A expression, disrupts microtubule structures and induces chromosomal aberrations in human epithelial cells

Epstein-Barr virus (EBV) infection is closely associated with nasopharyngeal carcinoma (NPC) and can be detected in early premalignant lesions of nasopharyngeal epithelium. The latent membrane protein 1 (LMP1) is an oncoprotein encoded by the EBV and is believed to play a role in transforming premalignant nasopharyngeal epithelial cells into cancer cells. RASSF1A is a tumor-suppressor gene commonly inactivated in many types of human cancer including NPC. In this study, we report a novel function of LMP1, in down-regulating RASSF1A expression in human epithelial cells. Downregulation of RASSF1A expression by LMP1 is dependent on the activation of intracellular signaling of NF-kappaB involving the C-terminal activating regions (CTARs) of LMP1. LMP1 expression also suppresses the transcriptional activity of the RASSF1A core promoter. RASSF1A stabilizes microtubules and regulates mitotic events. Aberrant mitotic spindles and chromosome aberrations are reported phenotypes in RASSF1A inactivated cells. In this study, we observed that LMP1 expression in human epithelial cells could induce aberrant mitotic spindles, disorganized interphase microtubules and aneuploidy. LMP1 expression could also suppress microtubule dynamics as exemplified by tracking movements of the growing tips of microtubules in live cells by transfecting EGFP-tagged EB1 into cells. The aberrant mitotic spindles and interphase microtubule organization induced by LMP1 could be rescued by transfecting RASSF1A expression plasmid into cells. Downregulation of RASSF1A expression by LMP1 may facilitate its role in transformation of premalignant nasopharyngeal epithelial cells into cancer cells.

Retinoblastoma tumor suppressor: where cancer meets the cell cycle

The retinoblastoma tumor suppressor gene, Rb, was the first tumor suppressor identified and plays a fundamental role in regulation of progression through the cell cycle. This review details facets of RB protein function in cell cycle control and focuses on specific questions that remain intensive areas of investigation.

Cell Cycle Regulators p105, p107, Rb2/p130, E2F4, p21CIP1/WAF1, Cyclin A in Predicting Cervical Intraepithelial Neoplasia, High-Risk Human Papillomavirus Infections and Their Outcome in Women Screened in Three New Independent States of the Former Soviet Union

BACKGROUND

The growth-controlling functions of the high-risk human papillomaviruses (HPV) depend on their ability to interact with several cellular proteins, including the key regulatory proteins of the cell cycle. We have examined the value of cell cycle regulatory proteins as predictors of the intermediate end point markers in cervical carcinogenesis: (a) grade of cervical intraepithelial neoplasia (CIN), (b) high-risk HPV type, (c) clearance/persistence of high-risk HPV, and (d) disease outcome in women participating in a multicenter follow-up study in three New Independent States countries.

METHODS

Totally, 232 biopsy samples tested high-risk HPV-positive and/or Papanicolaou smear-positive women were immunohistochemically stained for the following cell cycle markers: p105, p107, p130, E2F4, p21(CIP1/WAF1/SDI1), cyclin A, and Ki-67. In addition, apoptotic index (AI) and mitotic index (MI) were determined in H&E-stained sections. Prospective follow-up data were available to disclose the clinical and virological outcome of the lesions.

RESULTS

The expression of Ki-67, p21(CIP1/WAF1/SDI1), and cyclin A and AI and MI values were markedly increased in high-grade lesions, but only MI was an independent predictor of CIN3 in multivariate analysis. Cyclin A was the only independent predictor of high-risk HPV (odds ratio, 1.09; 95% confidence interval, 1.01-1.18; P = 0.021), exceeding the predictive power of CIN grade and high-grade squamous intraepithelial lesion Papanicolaou smears. None of these markers provided any useful predictive information as to the clinical and virological outcomes during the follow-up. Highly significant correlations (P = 0.0001) were found between AI and MI as well as between MI and cyclin A, Ki-67 and p21(CIP1/WAF1/SDI1), Ki-67 and cyclin A, and p21(CIP1/WAF1/SDI1) and cyclin A followed by that between p105 and cyclin A (P = 0.001) and p105 and p130 (P = 0.002).

CONCLUSIONS

All tested factors related to cell cycle were increased, but only MI and cyclin A was an independent predictor of CIN3 and high-risk HPV carriage, respectively.