Faculty Bibliography

Breast carcinomas < or = 1 cm in size (T1a,b) are being detected more frequently as a result of screening. Because traditional prognostic parameters are either lacking (tumor size) or rare (nodal metastases), a marker(s) is needed to identify the subset of patients who could benefit from adjuvant therapy. A retrospective series of 202 patients with stage T1a,b invasive breast carcinomas was evaluated. The clinicopathological features (age, histological grade, extensive in situ carcinoma, hormone receptor status, and nodal metastasis) as well as microvessel density and the expression of c-erb-B2, p53, MIB-1/Ki-67, and cdc25B were assessed. In addition, expression of the cell cycle inhibitor p27 was evaluated. Nineteen patients (18% of patients who had axillary dissection) had locoregional lymph node metastases. Forty-two % of them died of disease (median survival, 112 months), whereas mortality was 11% in node-negative patients (median survival, 168 months; P = 0.0055). Patients with low p27 expression had a median survival of 139 months (17% mortality) versus 174 months (9% mortality) in the group with high p27 expression (P = 0.0233). Lack of p27 was associated with poor prognosis when node-positive patients were excluded (P = 0.0252). Nodal status and low p27 were found to be the only independent prognostic parameters by both univariate and multivariate analysis, with relative risks of dying of disease of 4.9 (P = 0.001) and 3.4 (P = 0.0306), respectively. Assessment of p27, which yields prognostic information in node-negative patients, could be useful to identify patients with small, invasive breast carcinomas who might benefit from adjuvant therapy.

p27(Kip1) regulates the decision to enter into S-phase or withdraw from the cell cycle by establishing an inhibitory threshold above which G1 cyclin-dependent kinases accumulate before activation. We have used the HL-60 cell line to study regulation of p27 as cells withdraw from the cell cycle following treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA). We found that the amount of p27 is maximal in G0 cells, lower in G1 cells, and undetectable in S-phase cells. In contrast to the protein, the amount of p27 mRNA was the same in these populations, suggesting that accumulation of p27 during the cell cycle and as cells withdraw from the cell cycle is controlled by post-transcriptional mechanisms. In S-phase cells, the degradation of p27 appears to predominate as a regulatory mechanism. In G0 cells, there was an increase in the synthesis rate of p27. Our data demonstrate that, in G0 cells, accumulation of p27 is due to an increase in the amount of p27 mRNA in polyribosomes.

The cell cycle has been the object of extensive studies for the past years. A complex network of molecular interactions has been identified. In particular, a class of cell cycle inhibitory proteins has been cloned and characterized but details of the molecular mechanism of their action have yet to be resolved. These inhibitors regulate the progression through G1 and the G1/S transition via the inhibition of the cyclin-dependent kinase (Cdk) activity. The potential function of these negative regulators as tumor suppressors provides new insights into the link between the cell cycle and oncogenesis. p27 is a potent inhibitor of Cdks. In quiescent cells p27 accumulates without an increase in mRNA or protein synthesis. Cell cycle regulation of p27 levels, both in normal and transformed human cells, occurs via the ubiquitin-proteasome pathway and, compared to proliferating cells, quiescent cells contain a far lower amount of p27 ubiquitinating activity. The specific proteolysis of p27 is probably involved in the pathway of activation of Cdks. p27 is a phosphoprotein and its phosphorylation is cell cycle regulated. Often phosphorylation is a signal for ubiquitination. p27 is phosphorylated exclusively on serine by Erk1 and almost exclusively on threonine by Cdk1 in in vitro experiments. This finding raises the question of whether and how phosphorylation by these kinases is involved in the process of p27 proteolysis.

The cell-cycle inhibitor p27 is a potential tumor suppressor, but its gene has never been found inactivated in human tumors. Because cell-cycle regulation of p27 cellular abundance occurs at the post-transcriptional level, we analyzed p27 protein expression and degradation in human colorectal carcinomas. Proteasome-mediated degradation activity of p27 was compared with its protein levels in a subset of tumor samples. We found that carcinomas with low or absent p27 protein displayed enhanced proteolytic activity specific for p27, suggesting that low p27 expression can result from increased proteasome-mediated degradation rather than altered gene expression. Patients whose tumors expressed p27 had a median survival of 151 months, whereas patients who lacked p27 (10%) had a median survival of 69 months. By multivariate analysis, p27 was found to be an independent prognostic marker. Lack of p27 was associated with poor prognosis (2.9 risk ratio for death; P = 0.003). The absence of p27 protein expression is thus a powerful negative prognostic marker in colorectal carcinomas, particularly in stage II tumors, and thereby may help in the selection of patients who will benefit from adjuvant therapy. These data suggest that aggressive tumors may result from the selection of a clone or clones that lack p27 due to increased proteasome-mediated degradation

Endomitosis mWohcs DNA replicalion in the absence ol propei mitosis and cytokinesis. We found that during the endomilolic cell cWclc ol tWo Jilicrcnl mcgakaiyocylic cell lines, the lcW'cls of CWclm B l protein and the actiWitW ol Cdc2, the CWclm Bl dcpcndcnl kinasc. were reduced as compared to mcgakaryocytcs undergoing a mitolic cell cycle. In contrast, the levels ol CWclm A. another Cdc2-associatcd cyclin, Wcrc compaiahle dunng both cell cycles. The expression of cyclin Bl nrRNA was c-qnnalent m piohlcralmg and pohploidi/mg cells, bul the rate ol Cyclin Bl protein degradation ""'as enhanced in polWploidiJ.mg mcgakaryocWtes. These Undings lead us lo lurthcr mWcstigatc whether the ubiquilinprotcasomc palhuay icsponsiblc lor CWchn B degradation is accelerated in polyploid mcgakaiyocylcs Our data indicated that polWploidi/mg megakaiyoc-ytic cell lines and pnmary bone manoW mcgakaiWocWlcs display an increased activity o! the ubiquitin-prolcasomc pathvWaW Which dcgiadcs CWclm BI, as compared It) proh I crating mcgakaiyocWlic cell lines or to bone manou cells, icspcclncly. This dcgiadation had all the hallmarks ol a ubiquitm palhwaj, including the dependent) on ATI', inhibition by apWrasc. and the appealance of high molecular ucighl conjupalcd lorms ol Cyclin Bl. The increased potential ol polyploid nrcgakaty ocW tes to degrade CAchn Bl maW be part ol cellular progiamming u Inch leads li > aborted mitosis

In eukaryotic cells, each phase of the cell division cycle is controlled by the sequential activation of various cyclin-dependent kinases (Cdks). These kinases are known to phosphorylate various substrates whose activity is critical for cell cycle progression. As key regulators of the cell cycle, Cdks must be strictly controlled by both extracellular and intracellular signals for adequate responses to occur. There are several distinct molecular mechanisms for controlling the activity of the different Cdks: regulated synthesis and destruction of the activating subunit (cyclin), regulated synthesis and destruction of the inhibitory subunit (Cki), and posttranslational modification of the kinase subunit by highly specific kinases and phosphatases. During the G1 phase of the cell cycle, cells sense, integrate positive and negative signals, and transmit them to the cell cycle machinery. Because of this pivotal role, a vast majority of oncogenic events selectively target elements controlling the G1. In this review we discuss the elements controlling the G1 phase in relationship to the genesis of cancer.

The p27 mammalian cell cycle protein is an inhibitor of cyclin-dependent kinases. Both in vivo and in vitro, p27 was found to be degraded by the ubiquitin-proteasome pathway. The human ubiquitin-conjugating enzymes Ubc2 and Ubc3 were specifically involved in the ubiquitination of p27. Compared with proliferating cells, quiescent cells exhibited a smaller amount of p27 ubiquitinating activity, which accounted for the marked increase of p27 half-life measured in these cells. Thus, the abundance of p27 in cells is regulated by degradation. The specific proteolysis of p27 may represent a mechanism for regulating the activity of cyclin-dependent kinases