Human papillomaviruses (HPVs) with tropism for mucosal epithelia are the major aetiological factors in cervical cancer. Most cancers are associated with so-called high-risk HPV types, in particular HPV16, and constitutive expression of the HPV16 E6 and E7 oncoproteins is critical for malignant transformation in infected keratinocytes. E6 and E7 bind to and inactivate the cellular tumour suppressors p53 and Rb, respectively, thus delaying differentiation and inducing proliferation in suprabasal keratinocytes to enable HPV replication. One member of the Rb family, p130, appears to be a particularly important target for E7 in promoting S-phase entry. Recent evidence indicates that p130 regulates cell-cycle progression as part of a large protein complex termed DREAM. The composition of DREAM is cell cycle-regulated, associating with E2F4 and p130 in G0/G1 and with the B-myb transcription factor in S/G2. In this study, we addressed whether p130-DREAM is disrupted in HPV16-transformed cervical cancer cells and whether this is a critical function for E6/E7. We found that p130-DREAM was greatly diminished in HPV16-transformed cervical carcinoma cells (CaSki and SiHa) compared with control cell lines; however, when E6/E7 expression was targeted by specific small hairpin RNAs, p130-DREAM was reformed and the cell cycle was arrested. We further demonstrated that the profound G1 arrest in E7-depleted CaSki cells was dependent on p130-DREAM reformation by also targeting the expression of the DREAM component Lin-54 and p130. The results show that continued HPV16 E6/E7 expression is necessary in cervical cancer cells to prevent cell-cycle arrest by a repressive p130-DREAM complex.
Matched MeSH terms: Human papillomavirus 16/pathogenicity*
Cervical cancer is the fourth most common cancer among women. Infection by high-risk human papillomavirus (HPV) is the main aetiology for the development of cervical cancer. Infection by high-risk human papillomavirus (HPV) and the integration of the HPV genome into the host chromosome of cervical epithelial cells are key early events in the neoplastic progression of cervical lesions. The viral oncoproteins, mainly E6 and E7, are responsible for the initial changes in epithelial cells. The viral proteins inactivate two main tumour suppressor proteins, p53, and retinoblastoma (pRb). Inactivation of these host proteins disrupts both the DNA repair mechanisms and apoptosis, leading to rapid cell proliferation. Multiple genes involved in DNA repair, cell proliferation, growth factor activity, angiogenesis, as well as mitogenesis genes become highly expressed in cervical intraepithelial neoplasia (CIN) and cancer. This genomic instability encourages HPV-infected cells to progress towards invasive carcinoma. The key molecular events involved in cervical carcinogenesis will be discussed in this review.
Matched MeSH terms: Human papillomavirus 16/pathogenicity
The High-Risk Human Papillomaviruses (HR-HPVs) 16 and 18 are known to cause cervical cancer, which is primarily attributed to E6 and E7 oncoproteins. In addition, recent studies have focused on the vital role of the p130 pocket protein as an oncosuppressor to limit the expression of E2F transcription factors required for cell cycle progression. In view of this, the current study was conducted to investigate the mechanism by which transfection with HPV16/18 E7 leads to the deregulation of the host cell cycle, altering the localisation of p130, and expression of differentiation genes in Human Keratinocytes (HaCaT) cells. Co-immunoprecipitation, Western blot analysis, immunofluorescence microscopy, flow cytometry, quantitative-Polymerase Chain Reaction (qPCR), and the inhibition of p130 by MG132 inhibitor were employed to investigate the loss of p130 and its disruption in HPV 16/18 E7-transfected HaCaT cells. The HPV16- and HPV18-transformed cells, known as CaSki and HeLa, respectively, were also used to complement the ectopic expressions of E7 in HaCaT cells. Normal keratinocytes displayed higher level of p130 expression than HPV-transformed cells. In addition, the immunofluorescence analysis revealed that both HPV 16/18 E7-transfected HaCaT and HPV-transformed cells exhibited higher level of cytoplasmic p130 compared to nuclear p130. A significant increase in the number of S/G2 phase cells in HPV-transformed cells was also recorded since E7 has been shown to stimulate proliferation through the deactivation of Retinoblastoma Protein (pRB)-dependent G1/S checkpoint. Furthermore, the findings recorded the down-regulation of keratinocyte differentiation markers, namely p130, keratin10, and involucrin. The proteasomal degradation of the exported p130 confirmed the cellular localisation pattern of p130, which was commonly observed in cancerous cells. The findings provide strong evidence that the localisation of nuclear p130 nuclear was disrupted by HPV16/18 E7 led to the deregulation of the cell cycle and the impairment of cellular differentiation ultimately lead to cellular transformation.
Matched MeSH terms: Human papillomavirus 16/pathogenicity
It is well-established that HPV E7 proteins, encoded by human papillomavirus (HPV) genes, frequently associated with cervical cancers bind avidly to the retinoblastoma (RB) family of pocket proteins and disrupt their association with members of the E2F transcription factor family. Our previous study showed that the repressive p130-dimerization partner, RB-like, E2F and multi-vulval class (DREAM) complex was disrupted by HPV16 E7 proteins in order to maintain the viral replication in CaSki cells. However, we would like to address whether the activator B-myb-DREAM complex is critical in regulating the replication and mitosis phase since our previous study showed increased B-myb-DREAM expression in HPV-transformed cell lines when compared to control cells.
Matched MeSH terms: Human papillomavirus 16/pathogenicity