Expression of the Y-box protein YB-1 is increased in proliferating normal and cancer cells, but its role in cell proliferation and cell cycle progression is unclear. We have identified a cell cycle-dependent relocalization of YB-1 from the cytoplasm to the nucleus at the G 1 /S phase transition and demonstrate that both the charged zipper and the cold shock domain are involved in regulating this process. Using cell lines that constitutively overexpress YB-1, we show that nuclear accumulation of YB-1 is associated with increased cyclin A and cyclin B1 mRNA and protein expression. We provide evidence that deregulated YB-1 expression is linked to adhesion-independent cell proliferation through the induction of cyclin A. Thus, we have identified YB-1 as a cell cycle stage-specific transcription factor important for cell proliferation.

We have studied the expression of the apoptosis-regulating genes bcl-2, bcl-x, bax and APO-1/fas (CD95) in human breast cancer. The expression pattern of these genes in human breast-cancer tissues and breast-cancer-derived cell lines was compared to that seen in normal breast epithelium and breast epithelial cell lines. No difference with regard to bcl-2 and bcl-xL expression was observed between normal breast epithelium and tumor tissue or breast cancer and non-malignant epithelial cell lines. In contrast, bax-alpha, a splice variant of bax, which promotes apoptosis, is expressed in high amounts in normal cell lines and breast tissue, whereas only weak or no expression could be detected in cancer-cell lines and malignant tissue. In contrast to malignant cell lines, which express low levels of bax-alpha, non-malignant epithelial cell lines displaying high amounts of bax-alpha were highly sensitive to induction of programmed cell death by both serum starvation and APO-1/fas (CD95) triggering. We therefore propose that dysregulation of apoptosis contributes to the pathogenesis of breast cancer, at least in part, due to an imbalance between anti-apoptosis genes (such as bcl-2/bcl-x) and apoptosis-promoting genes (bax).

The human papillomavirus type 18 (HPV-18) promoter contains a TPA responsive element TRE) which confers TPA responsiveness on a heterologous promoter. In the context of the HPV-18 promoter, however, this AP-1 site is inactive. We have identified a negative regulatory domain in the HPV-18 promoter which represses the constitutive and TPA-induced AP-1 activity. This negative regulatory sequence has been mapped to 44 nucleotides (OL13). We identified this element as a transcriptional silencer based on its ability to interfere with transcriptional initiation. This HPV-18 silencer domain was narrowed down further to 23 nucleotides, the OL13B element, which bears similarity to three other silencer sequences, present in the mouse N-ras gene upstream regulatory region, the mouse albumin gene enhancer and the adeno-associated virus P5 promoter. The transcriptional repressor protein YY1, which negatively regulates the P5 promoter, binds to the HPV-18 silencer with high affmiity. Mutation of the YY1 binding site leads to an enhanced activity of the HPV-18 promoter, strongly suggesting that YY1 plays an important role in controlling HPV-18 early gene expression.