G1-phase cyclin expression in neoplastic B cells

Abstract

By virtue of the role of G1-phase cyclin proteins as regulators of differentiation, proliferation, and apoptosis, their improper expression can lead to altered cellular homeostasis and tumor formation. Cyclins provide the regulatory partner for cyclin-cdk complexes whose enzymatic activity drives cell cycle progression. Overexpression of cyclins can lead to increased cyclin-cdk activity thereby uncoupling normal growth restraints and leading to uncontrolled cellular proliferation - one of the hallmarks of cancer cells. The aim of this thesis was to clarify the expression patterns of the G1 cyclins and define their relevance in neoplastic B cells.

The D-type cyclins (D1, D2, and D3) display variation in their pattern of expression between cell types, suggesting that they may play a role in cellular differentiation as well as proliferation. To further specify the relationship between cellular phenotype and D-cyclin expression, we studied the D-type cyclins in a variety of B cells, including B cells infected with Epstein-Barr virus (EBV). Cyclin D2 was the predominant D-cyclin expressed in lymphoblastoid cell lines (LCLs) and group III Burkitt's lymphoma lines (BLs) and was associated with an activated B-cell phenotype (CD10-, CD39+). In contrast, EBV(-) BL lines and group I and II BL lines expressed only cyclin D3. Although cyclin D2 expression is often associated with the presence of EBV gene products, our results fail to demonstrate this relationship in EBV(-) BLs infected with EBV, and illustrate that host cell phenotype also influences D-cyclin expression.

We explored the expression of the D-cyclins and cyclin E in human adult B-cell acute lymphoblastic leukemias (B-ALL), B-cell chronic lymphocytic leukemias (BCLL), and immunocytoma (IC). Our studies revealed the overexpression of cyclin E protein in blast cells from 6/6 B-ALL patients at relapse compared to blasts from the same patients at diagnosis. This change in protein expression was both quantitative and qualitative as up to four additional smaller molecular weight species of the cyclin E protein were observed at relapse. In addition, we observed in B-ALL patients, that low cyclin E protein levels at diagnosis showed a positive association with improved clinical outcome. Our studies suggest that cyclin E protein levels reflect the malignant status of B-ALL blasts, and implicate the potential use of cyclin E as a prognostic marker in adult B-ALL. We also found cyclin E protein overexpression in B-CLL (4/12) and IC (4/4), however, no correlation with advanced disease or prognosis was observed. In contrast to BALL, these samples were cyclin D3 negative. However, upon stimulation with Staphylococcus aureus and interleukin-2, cyclin D3 expression was induced in BCLL cells in a manner similar to that observed in normal B cells. Our results suggest that the D-cyclin pathway is maintained intact in B-CLL cells but that the expression of cyclin E, as observed in some B-CLL and IC samples, is pathological.

Cyclins have been shown to play a role in the induction of apoptosis in some cell types. We studied cyclins D3 and E in B-cell lines provoked to enter apoptosis by exposure to nitric oxide (NO). The pre-B-cell line NALM-6 was highly susceptible to NO-induced apoptosis and had a concomitant 8-fold increase in expression of the cyclin E protein. No change in protein levels of cyclin D3, cyclin A, or the anti-apoptotic protein bcl-2 were observed. Our results raise the question of an association between cyclin E expression and apoptotic susceptibility.