Cytofluorometric determination of thymidine kinase activity in a mixture of normal and neoplastic cells

Mouse thymidine kinase stability in vivo and after in vitro translation

Using a combination of centrifugal elutriation and recultivation of synchronised cell populations we could show that murine thymidine kinase (TK) is rapidly degraded during mitosis in polyoma virus-transformed mouse fibroblasts, in parallel to the time-course for loss of cyclin A. Transformation is no prerequisite for the instability phenotype since artificial overexpression of TK under the control of a constitutive promoter in normal mouse fibroblasts also resulted in rapid turnover of TK during mitosis. The decay of TK protein could be partially mimicked in vitro with enzymatically active protein translated in a rabbit reticulocyte lysate: full length polypeptide was lost slightly more rapidly in the presence of G2/M cytosolic extracts than with G1/S preparations. In addition, an enzymatically active C-terminal truncation of 37 amino acids at Gln-196 was completely stable under the conditions tested, confining the instability domain between residues 196 to 233. These experiments also indicated the border for intact TK since translation products up to Tyr-189 or less were completely inactive. This was also confirmed by a mutant TK protein from mouse F9tk- teratocarcinoma cells which harboured a similar deletion.

Identification of leukaemic cells in bone marrow and blood samples by a new cytofluorometric assay

The expression of thymidine kinase--an enzyme of the DNA precursor pathway--is strictly regulated during the normal cellular cycle, but is much higher and permanently expressed in malignant growing cells. We used this fact to detect neoplastic cells in samples freshly taken from leukaemia patients and kept frozen in liquid nitrogen until analysis. Using a new cytofluorometric assay for thymidine kinase in single cells, we were able to identify leukaemic cells in a surplus of normal ones. Our results demonstrate the benefits of this assay for leukaemia diagnosis.

Expression of the cyclin-dependent kinase inhibitor p16 during the ongoing cell cycle

It has been demonstrated that protein expression of p16, the inhibitor of cyclin-dependent kinase 4 and 6, increases 4 fold at the G1/S transition when serum-arrested cells are restimulated to logarithmic growth. We examined the cell cycle regulation of this cyclin-dependent kinase inhibitor in cells separated according to their cell cycle phases by centrifugal elutriation. Neither p16 mRNA nor its protein expression are regulated during the cell cycle of normal phytohemagglutinin-stimulated lymphocytes, retinoblastoma protein-negative cells, papilloma virus-transformed cells, and acute promyelocytic leukemia cells. p16 mRNA is constitutively expressed in cells in which we detected the normal E2F-dependent S-phase specific expression of thymidine kinase mRNA. We further observed a G1-phase specific expression of cyclin D1 mRNA in the same cells separated by centrifugal elutriation.