Mechanism of cell cycle control in normal and malignant hemopoiesis: cyclins and cyclin-dependent kinases.

Recent studies into molecolar process that regulates cell proliferation have demonstrated a central role played by a family of proteins, the cyclins, and their catalitic subunits, the cyclin-dependent protein kinases (CDKs). Aim of our study was to investigate in normal and neoplastic human hemopoietic models, as well as in samples from patients with hemopoietic malignancies, the role of two proteins, cyclin A and B, and their catalitic subunits, CDK2 and CDC2. Distribution in the different cell cycle phases, as low proliferative-low RNA (G0) cells, G1, S and G2M was established by DNA/RNA flow cytometry (acridine-orange) and then correlated with protein expression tested by Western blot. Molecular changes occuring in normal human hemopoietic cells during recruitment into the cell cycle were measured at different time points using T-lymphocytes activated by anti CD3 and IL-2. Unstimulated lymphocytes (t=0) after monocyte depletion were 98.4% in G0 and were characterized by absence of cyclin A and B expression. CDK2 and CDC2 were strongly positive after 18 hours, when 25.1% of cells reached G1, although a lower protein expression was also found in unstimulated G0 lymphocytes. At 18-24 hours before cells entering into S phase we start to detect expression of cyclin A and cyclin B wich continue to increase until 48 hours (S=14.1%) Evaluation of protein expression in hematologic cell lines (U937, K562, KG1, U266, Mo7, Daudi) showed always a constant positivity of CDK2, cyclin A, Cyclin B and CDC2. Protein positivity was correlated with absence of cells in G0 but was independent from the origin of cell line analyzed (myeloid, lymphoid leukemia, as well as myeloma cells). Preliminar evaluation of cyclins and their catalitic subunits in leukemic patients showed in ALL higher expression of cyclin A and CDK2 which correlates with high S-phase values. In the other samples cell cycle distribution as well as expression of cyclins, was heterogeneous. Our study demonstrates the role of cyclins in human proliferating cells, by absence of expression of cyclin A and B in resting T-lymphocytes and by their presence in all leukemic cell lines tested. The role of such proteins in hematologic neoplastic disease remaine to be defined.