An emerging role for the nuclear localization of maspin in the suppression of tumor progression and metastasis.

Maspin, a member of the serpin family of serine protease inhibitors, was originally identified as a tumor suppressor that is expressed in normal mammary epithelial cells but is reduced or absent in breast carcinomas. Early enthusiasm for maspin as a biomarker for disease progression has been tempered by clinical data that associates maspin with favourable outcomes in some studies and poor prognosis in others. Here, we review all of the published clinical studies for maspin in breast and ovarian cancers and propose that the apparent discordance between clinical reports is a consequence of differential cellular distribution of maspin.

Nuclear localization of maspin is essential for its inhibition of tumor growth and metastasis.

Maspin (mammary serine protease inhibitor or SerpinB5) acts as a tumor suppressor when overexpressed in aggressive cancer cell lines. However, its role in human cancer is controversial. Maspin expression has been associated with a poor prognosis in some studies, whereas in others, with favorable outcome.

Proteolytic processing of cut homeobox 1 by neutrophil elastase in the MV4;11 myeloid leukemia cell line.

Proteolytic processing by cathepsin L generates p110 Cut homeobox 1 (CUX1) at the end of the G(1) phase, whereas an alternative transcript encodes p75 CUX1. These short CUX1 isoforms were reported to be overexpressed in cancer cells, and transgenic mice overexpressing the p75 isoform were found to develop myeloproliferative disease-like myeloid leukemias. In the present study, we report that the neutrophil elastase can also generate a short CUX1 isoform in the MV4;11 acute myeloid leukemia cell line.

Increased expression and activity of nuclear cathepsin L in cancer cells suggests a novel mechanism of cell transformation.

It is generally accepted that the role of cathepsin L in cancer involves its activities outside the cells once it has been secreted. However, cathepsin L isoforms that are devoid of a signal peptide were recently shown to be present in the nucleus where they proteolytically process the CCAAT-displacement protein/cut homeobox (CDP/Cux) transcription factor. A role for nuclear cathepsin L in cell proliferation could be inferred from the observation that the CDP/Cux processed isoform can accelerate entry into S phase.

Carboxyl-terminal proteolytic processing of CUX1 by a caspase enables transcriptional activation in proliferating cells.

Proteolytic processing at the end of the G(1) phase generates a CUX1 isoform, p110, which functions either as a transcriptional activator or repressor and can accelerate entry into S phase. Here we describe a second proteolytic event that generates an isoform lacking two active repression domains in the COOH terminus. This processing event was inhibited by treatment of cells with synthetic and natural caspase inhibitors.

A novel proteolytically processed CDP/Cux isoform of 90 kDa is generated by cathepsin L.

The Cut-like genes code for multiple isoforms of the CDP/Cux transcription factor. The full-length protein contains four DNA-binding domains: Cut repeats 1, 2 and 3 (CR1, CR2 and CR3) and the Cut homeodomain (HD). The p75 isoform is expressed from an mRNA that is initiated within intron 20 and contains only CR3 and HD.

The p110 isoform of the CDP/Cux transcription factor accelerates entry into S phase.

The CDP/Cux transcription factor was previously found to acquire distinct DNA binding and transcriptional properties following a proteolytic processing event that takes place at the G1/S transition of the cell cycle. In the present study, we have investigated the role of the CDP/Cux processed isoform, p110, in cell cycle progression. Populations of cells stably expressing p110 CDP/Cux displayed a faster division rate and reached higher saturation density than control cells carrying the empty vector.

Complete and limited proteolysis in cell cycle progression.

An important mechanism of regulation that controls progression through the cell cycle involves the timely degradation of specific regulatory proteins. In parallel to the main degradative pathways, it appears that the function of certain proteins may also be modulated by a process called limited proteolysis. We have recently shown that the CDP/Cux transcription factor is proteolytically processed at the G(1)/S transition by the cathepsin L protease.

A cathepsin L isoform that is devoid of a signal peptide localizes to the nucleus in S phase and processes the CDP/Cux transcription factor.

The subclass of cysteine proteases termed lysosomal cathepsins has long been thought to be primarily involved in end-stage protein breakdown within lysosomal compartments. Furthermore, few specific protein substrates for these proteases have been identified. We show here that cathepsin L functions in the regulation of cell cycle progression through proteolytic processing of the CDP/Cux transcription factor.

CDP/Cux stimulates transcription from the DNA polymerase alpha gene promoter.

CDP/Cux (CCAAT-displacement protein/cut homeobox) contains four DNA binding domains, namely, three Cut repeats (CR1, CR2, and CR3) and a Cut homeodomain. CCAAT-displacement activity involves rapid but transient interaction with DNA. More stable DNA binding activity is up-regulated at the G(1)/S transition and was previously shown to involve an N-terminally truncated isoform, CDP/Cux p110, that is generated by proteolytic processing.

Characterization of a tissue-specific CDP/Cux isoform, p75, activated in breast tumor cells.

Two isoforms of the CCAAT-displacement protein/cut homeobox (CDP/Cux) transcription factor have been characterized thus far. The full length protein, p200, which contains four DNA binding domains, transiently binds to DNA and carries the CCAAT-displacement activity. The p110 isoform is generated by proteolytic processing at the G1-S transition and is capable of stable interaction with DNA.