Silencing of Integrated Human Papillomavirus Type 18 Oncogene Transcription in Cells Expressing SerpinB2

Abstract

SerpinB2, originally described as plasminogen activator inhibitor type 2 (PAI-2), is expressed by a range of cell types including activated macrophages and many tumors and is a major product of differentiating squamous epithelial cells (33, 43). PAI-2 was one of the first identified members of a unique and growing subclass of serine protease inhibitors (serpins) called ovalbumin-like serpins (ov-serpins) (49). Ov-serpin family members often appear to have nucleocytoplasmic distributions (8, 15), and many have intracellular activities: for instance, CrmA and PI9 are involved in apoptosis inhibition, MENT is involved in DNA binding, and Maspin and Headapin are involved in tumor suppression (8, 49). Although extracellular PAI-2 is well documented as an inhibitor of the extracellular protease urokinase-type plasminogen activator (31), PAI-2 was recently shown to have an additional intracellular activity as a retinoblastoma protein (Rb) binding protein (15). PAI-2 was found to bind the C pocket of Rb via a novel binding motif called the PENF homology motif, which is present in the large C-D interhelical loop region of PAI-2. PAI-2 expression resulted in decreased Rb turnover, with the subsequent increase in Rb levels causing an increase in Rb-mediated activities. The PAI-2-mediated increase in Rb protein levels required both Rb binding via the C-D interhelical region of PAI-2 and an intact reactive site loop (RSL), which plays a pivotal role in the known protease inhibitory activity of PAI-2 (15). The new Rb-associated role for intracellular PAI-2 may explain why PAI-2 expression is often able to confer a series of Rb-related phenotypes such as resistance to apoptosis (19,23,61), regulation of gene transcription (1, 37, 48), promotion of differentiation (29,34,57), and tumor suppression (20,23,31,34,38,41,56).A dramatic phenotype resulting from stable PAI-2 expression in HeLa cells was recovery of Rb and loss of E7 protein levels in these human papillomavirus type 18 (HPV-18)-transformed cells (15). High-risk HPVs such as HPV-18 are often associated with cervical cancer (16), and cells from such cancers usually constitutively express the HPV oncoproteins E6 and E7 from HPV-derived DNA integrated into the host cell genome (36). E6 targets p53 and c-Myc, and E7

SerpinB2 Is an Inducible Host Factor Involved in Enhancing HIV-1 Transcription and Replication

“…3B). Taken together with the known influence of SerpinB2 on Rb-mediated transcription (15,16,19), these data suggest that SerpinB2 may be involved in enhancing HIV-1 transcription via its effect on Rb levels (15).…”

“…The SerpinB2-mediated increase in Rb protein levels was found to enhance Rb-mediated activities, such as repression of E2F-1-dependent gene expression and promotion of G 1 cell cycle arrest (15). SerpinB2 expression has also been shown to influence transcription of mammalian (15,19) and viral genes (16), with the latter two studies illustrating that this activity was mediated via the elevation of Rb protein levels. Cells of the monocyte/macrophage lineage are important targets for infection by the human immunodeficiency virus type-1 (HIV-1).…”

“…SerpinB2 is a member of the Clade B or ovalbumin-like serine protease inhibitor (ov-serpin) subgroup of serpins, which lack a classical secretory signal peptide and are frequently found to reside intracellularly with a cytoplasmic or nucleocytoplasmic distribution (14). SerpinB2 has recently been shown to have an intranuclear activity as a retinoblastoma protein (Rb)-binding protein (15)(16)(17)(18). SerpinB2 expression inhibited Rb degradation, which resulted in increased Rb protein levels.…”

“…Certain inflammatory conditions (22), TNF␣ (23) and bacterial infections (21, 24) enhance the replication of HIV-1 in monocytes/ macrophages and potently induce SerpinB2 expression in these cells (2). These considerations and the reported influence of SerpinB2 expression on transcription (15,16,19), prompted an investigation of the role of SerpinB2 expression in HIV-1 infection. …”

A Transcriptome- and Marker-Based Systemic Analysis of Cervical Cancer

“…The gene product is an inhibitor of the serine-type proteases like the plasminogen activator (also known as PLAU). On one hand, SERPINB2 suppression has been determined using both microarrays as well as genomic studies in CC (Table 3); but on the other, its expression in HeLa cells can stabilize the expression levels of the Rb protein and suppress the oncoproteins E6 and E7 of HPV18 (Darnell et al, 2005). This suggests that low levels of SERPINB2 promote CC development, being this a potentially good molecular marker.…”