Failure of T Lymphocytes from Elderly Humans to Enter the Cell Cycle Is Associated with Low Cdk6 Activity and Impaired Phosphorylation of Rb Protein

Cell cycle analyses of activated T lymphocytes from elderly humans generally show that the proportion of noncycling cells increases with age. T cells that are not definitively blocked in G0 usually strain to traverse the G1 phase and may still be arrested at the G1/S boundary. The molecular mechanisms underlying these cell cycle arrests are unknown. Because G0/G1 and G1/S transitions are regulated in part by cyclin-dependent kinase Cdk6, we investigated the possibility that a loss of activity of this kinase is implicated in the age-related dysfunction of the cell cycle in its initial phases. G0/G1 and G1/S blocks were first confirmed by [3H]uridine and [3H]thymidine incorporation studies in anti-CD3 activated T lymphocytes derived from elderly donors. In the same cell preparations, in vitro phosphorylation of recombinant truncated Rb protein by immunoprecipitated Cdk6 was significantly decreased. The reduced Cdk6 activity was not attributable to a low level of the protein since a 24-h activation resulted in a comparable expression of the kinase in T cells from young and old individuals. However, at least two other mechanisms might be incriminated in the loss of Cdk6 activity: (1) a poor induction of the associated cyclin D2 upon anti-CD3 stimulation and (2) a delayed downregulation of the Cdk inhibitor p27 following cell activation. The low Cdk6 activity observed in T lymphocytes from the elderly was associated with a defective phosphorylation of the endogenous Rb protein and an increased sequestration of the E2F-1 transcription factor, possibly resulting in early cell cycle arrest.