D cyclins in lymphocytes

Abstract

D cyclins are essential for the progression of cells through the G1 phase of the cell cycle. There are three distinct D cyclins. Cyclin D1 has been shown to be expressed by many different types of cells but not by lymphocytes. Cyclins D2 and D3 have been found in lymphocytes. We used high-resolution enzymatic amplification staining technology in conjunction with flow cytometry and confocal microscopy and with immunoblotting to reassess the expression of the D cyclins in human lymphocytes. Using high-resolution technology for flow cytometry, we found all three D cyclins in quiescent human peripheral blood lymphocytes. Cyclin D1 was expressed in quiescent and activated cells at levels commensurate with those of actively proliferating tumor cell lines. Cyclin D1 was functional inasmuch as it was complexed with CDK4. In the quiescent cells, cyclin D1 was expressed in the cytoplasm but, after activation, was found in the nucleus. These findings demonstrate that lymphocytes express cyclin D1 and necessitate a reappraisal of the hypothesis that the D cyclins subsume redundant activities with tissue-specific expression.