Compromised Spindle Assembly Checkpoint due to Altered Expression of Ubch10 and Cdc20 in Human Papillomavirus Type 16 E6- and E7-Expressing Keratinocytes

Cells expressing human papillomavirus type 16 (HPV-16) E6 and E7 proteins exhibit deregulation of G₂/M genes, allowing bypass of DNA damage arrest signals. Normally, cells with DNA damage that override the G₂ damage checkpoint would precociously enter mitosis and ultimately face mitotic catastrophe and apoptotic cell death. However, E6/E7-expressing cells (E6/E7 cells) have the ability to enter and exit mitosis in the presence of DNA damage and continue with the next round of the cell cycle. Little is known about the mechanism that allows these cells to gain entry into and exit from mitosis. Here, we show that in the presence of DNA damage, E6/E7 cells have elevated levels of cyclin B, which would allow entry into mitosis. Also, as required for exit from mitosis, cyclin B is degraded in these cells, permitting initiation of the next round of DNA synthesis and cell cycle progression. Proteasomal degradation of cyclin B by anaphase-promoting complex/cyclosome (APC/C) is, in part, due to elevated levels of the E2-conjugating enzyme, Ubch10, and the substrate recognition protein, Cdc20, of APC/C. Also, in E6/E7 cells with DNA damage, while Cdc20 is complexed with BubR1, indicating an active checkpoint, it is also present in complexes free of BubR1, presumably allowing APC/C activity and slippage through the checkpoint.Failure to activate cell cycle checkpoints in the presence of any DNA damage leads to genomic instability, polyploidy, and subsequently, aneuploidy, which is a hallmark of many cancers (26). Human papillomaviruses (HPVs) which cause various epithelial cancers, produce two proteins, E6 and E7, whose expression allows bypass or overriding of normal DNA damage and spindle checkpoint signals, primarily through inactivation of p53 and retinoblastoma family members, respectively (11, 16, 17). Our laboratory and others have previously shown that bypass of these arrest signals due to the presence of the viral genes gives rise to a significant population of cells that are polyploid (13, 16, 24, 32). Polyploid and aneuploid cells predominantly arise due to defects in the spindle assembly checkpoint (SAC) during mitosis. While we have some understanding of the mechanisms that lead to bypass of DNA damage arrest signals at the G₂/M stage of the cell cycle, it is not clear how the E6/E7-expressing cells with DNA damage and abnormal chromosomes are allowed to (i) to enter into mitosis and (ii) exit out of mitosis to initiate the next round of replication. Progression through mitosis is regulated by the ubiquitin-dependent degradation machinery, consisting of the anaphase-promoting complex/cyclosome (APC/C), a multisubunit ubiquitin ligase. The activity of APC/C is dependent on the substrate-specifying proteins Cdc20 in metaphase and Cdh1 in telophase (25, 37). In normal cells, spindle checkpoint proteins Mad2 and BubR1 serve to inhibit APC/C until all the chromosomes are aligned correctly on the mitotic spindle by binding Cdc20 and preventing it from activating APC/C (5, 21, 31). In the event of DNA damage and/or unattached kinetochores, the SAC will arrest cells before exit from mitosis by inhibiting activation of APC/C. As a consequence of APC/C inhibition, cyclin B is not degraded, thus preventing cells from mitotic exit (6). Work by Chen''s group (11) has shown that E6- and E7-expressing cells (also referred to here as E6/E7 cells) adapt to an active SAC and are capable of mitotic slippage. So, what is the mechanism that underlies mitotic slippage in E6/E7 cells and allows them to enter the next round of cell cycle? Recent work by van Ree et al. (34) has shown that overexpression of E2 ubiquitin-conjugating enzyme Ubch10 leads to uncontrolled APC/C activity and degradation of cyclin B even in the presence of an active mitotic checkpoint, leading to mitotic slippage. In this report, we show that primary human foreskin keratinocytes (HFKs) expressing E6/E7 have high levels of cyclin B, which allows entry into mitosis in the presence of DNA damage. We show that these cells successfully exit mitosis by, in part, indirect activation of APC/C through upregulation of the E2-conjugating protein, Ubch10, and the substrate-specific component of APC/C, Cdc20, leading to the required degradation of cyclin B. In addition, Cdc20 is detected in different complexes; one includes the protein BubR1, indicating an active checkpoint, while other complexes are free of BubR1 and are thus free to activate APC/C. Upregulation of cyclin B and Ubch10 as well as Cdc20 is primarily through E6 and its ability to target p53 degradation, although inhibition of the pRb family members by E7 may also play a part.