Cell cycle regulatory proteins and apoptosis in B-cell chronic lymphocytic leukemia

Abstract

Background and objectives: The pathogeny of B-cell chronic lymphocytic leukemia (B-CLL) involves both deregulated proliferation and inhibition of cell death. A particular role in the regulation of these phenomena is played by proteins involved in early G1 phase regulation: pRb kinases: cyclin-dependent kinases (cdk): cdk4 and cdk6 activated by cyclins D, and universal cdk inhibitor p27(Kip1).

Design and methods: We determined by flow cytometry the expression of p27(Kip1) and cyclins D (D2 and D3) in populations of peripheral blood lymphocytes obtained from 59 (for p27(Kip1)) and 31 (for cyclins D) previously untreated patients with B-CLL, and compared them with cell cycle parameters, cell viability and apoptosis in 72-hour cultures in medium only. As a control we determined the expression of p27(Kip1), cyclin D2 and D3 in peripheral blood CD5+/CD19+ lymphocytes from 15 healthy donors.

Results: p27(Kip1) was present in nearly 100% of lymphocytes in all B-CLL populations tested. Its cellular content estimated semiquantitatively by specific mean fluorescence intensity was higher than in normal CD5+/CD19+ lymphocytes, p27(Kip1) was inversely correlated with patients' age and not correlated with other clinical variables, cell cycle or apoptosis rate. Cyclin D2 was detectable in 25 out of 31, and cyclin D3 in all B-CLL lymphocytes populations studied. In contrast to p27Kip1 present in all CD5+/CD19+ lymphocytes, both cyclins were detected only in a subset of neoplastic cells: 27.5 to 87% (mean 51.2) for cyclin D2 and 20.3 to 98% (mean 76.5) for cyclin D3. In cyclin D2- and D3-positive normal CD5+/CD19+ lymphocytes and B-CLL cell populations, cyclin D3 was expressed in a higher percentage of cells than cyclin D2. Both cyclin D2-and cyclin D3-positive fractions of B-CLL cells were, on average, larger than corresponding fractions of normal CD5+/CD19+ peripheral blood lymphocytes.

Interpretation and conclusions: Our results indicate that cyclin D3 plays an important role in the regulation of normal and neoplastic CD5+/CD19+ cells, and point to the possibility of the exit of a number of CLL lymphocytes from quiescence.