Phosphorylation of the retinoblastoma protein by cdk2

Identification of G1 kinase activity for cdk6, a novel cyclin D partner

A family of vertebrate cdc2-related kinases has been identified, and these kinases are candidates for roles in cell cycle regulation. Here, we show that the human PLSTIRE gene product is a novel cyclin-dependent kinase, cdk6. The cdk6 kinase is associated with cyclins D1, D2, and D3 in lysates of human cells and is activated by coexpression with D-type cyclins in Sf9 insect cells. Furthermore, we demonstrate that endogenous cdk6 from human cell extracts is an active kinase which can phosphorylate pRB, the product of the retinoblastoma tumor suppressor gene. The activation of cdk6 kinase occurs during mid-G1 in phytohemagglutinin-stimulated T cells, well prior to the activation of cdk2 kinase. This timing suggests that cdk6, and by analogy its homolog cdk4, links growth factor stimulation with the onset of cell cycle progression.

Analysis of p107-associated proteins: p107 associates with a form of E2F that differs from pRB-associated E2F-1

The binding of viral oncogenes to cellular proteins is thought to modulate the activities of these cellular targets. The p107 protein is targeted by many viral proteins, including adenovirus E1A, simian virus 40 large T antigen, and human papillomavirus type 16 E7 protein. A panel of monoclonal antibodies against p107 was raised and used to identify cellular proteins that interact with the p107 protein in vivo. p107-associated proteins included cyclin A, cyclin E, and cdk2. In addition, p107 was found to associate with 62- to 65- and 50-kDa phosphoproteins in ML-1 cells, a human myeloid leukemia cell line. The 62- to 65-kDa proteins have many of the properties of the transcription factor E2F but were distinguished from pRB-associated E2F-1 by both immunologic and biochemical properties.

Overproduction of Rb protein after the G1/S boundary causes G2 arrest

The Rb protein is known to exert its activity at decision points in the G1 phase of the cell cycle. To investigate whether it may also play some role(s) at later points in the cell cycle, we used a system of rapid inducible gene amplification to conditionally overexpress Rb protein during G2 phase. A cell line expressing a temperature-sensitive simian virus 40 large T antigen (T-Ag) was stably transfected with plasmids containing the Rb cDNA linked to the simian virus 40 origin of replication: pRB-wt, pRB-fs, and pRB-Dra, carrying wild-type murine Rb cDNA, a frameshift mutation close to the beginning of the Rb coding region, and a single-amino-acid deletion in the E1A/T-Ag binding pocket, respectively. Numerous independent cell lines were isolated at the nonpermissive temperature; cell lines displaying a high level of episomal amplification of an intact Rb expression cassette following shiftdown to the permissive temperature were chosen for further analysis. Plasmid pRB-fs did not express detectable Rb antigen, while pRB-Dra expressed full-length Rb protein. The Dra mutation has previously been shown to abrogate phosphorylation as well as T-Ag binding. Fluorescence-activated cell sorting (FACS) analysis revealed that cultures induced to overexpress either wild-type or Dra mutant Rb proteins were significantly enriched for cells with a G2 DNA content. Cultures that amplified pRB-fs or rearranged pRB-wt and did not express Rb protein had normal cell cycle profiles. Double-label FACS analysis showed that cells overexpressing Rb or Rb-Dra proteins were uniformly accumulating in G2, whereas cells expressing endogenous levels of Rb were found throughout the cell cycle. These results indicate that Rb protein is interacting with some component(s) of the cell cycle-regulatory machinery during G2 phase.

Overproduction of Rb protein after the G1/S boundary causes G2 arrest

The Rb protein is known to exert its activity at decision points in the G1 phase of the cell cycle. To investigate whether it may also play some role(s) at later points in the cell cycle, we used a system of rapid inducible gene amplification to conditionally overexpress Rb protein during G2 phase. A cell line expressing a temperature-sensitive simian virus 40 large T antigen (T-Ag) was stably transfected with plasmids containing the Rb cDNA linked to the simian virus 40 origin of replication: pRB-wt, pRB-fs, and pRB-Dra, carrying wild-type murine Rb cDNA, a frameshift mutation close to the beginning of the Rb coding region, and a single-amino-acid deletion in the E1A/T-Ag binding pocket, respectively. Numerous independent cell lines were isolated at the nonpermissive temperature; cell lines displaying a high level of episomal amplification of an intact Rb expression cassette following shiftdown to the permissive temperature were chosen for further analysis. Plasmid pRB-fs did not express detectable Rb antigen, while pRB-Dra expressed full-length Rb protein. The Dra mutation has previously been shown to abrogate phosphorylation as well as T-Ag binding. Fluorescence-activated cell sorting (FACS) analysis revealed that cultures induced to overexpress either wild-type or Dra mutant Rb proteins were significantly enriched for cells with a G2 DNA content. Cultures that amplified pRB-fs or rearranged pRB-wt and did not express Rb protein had normal cell cycle profiles. Double-label FACS analysis showed that cells overexpressing Rb or Rb-Dra proteins were uniformly accumulating in G2, whereas cells expressing endogenous levels of Rb were found throughout the cell cycle. These results indicate that Rb protein is interacting with some component(s) of the cell cycle-regulatory machinery during G2 phase.

Hexamethylenebisacetamide-induced erythroleukemia cell differentiation involves modulation of events required for cell cycle progression through G1

Hexamethylenebisacetamide (HMBA), a potent inducer of differentiation of transformed cells such as murine erythroleukemia cells, causes a prolongation of the G1 phase of the cell cycle during which commitment to terminal differentiation is first detected. Removal of HMBA prior to the G1 phase aborts commitment. To further define the relationship between the G1 phase and commitment to differentiation, we used two inhibitors of cell cycle progression: aphidicolin, which blocks cells at the G1/S interphase, and deferoxamine, which blocks cells at an earlier stage during G1. HMBA-induced prolongation of G1 is associated with the accumulation of underphosphorylated retinoblastoma protein, decrease in cyclin A protein levels, and commitment to differentiation. G1 arrest of murine erythroleukemia cells induced by aphidicolin or deferoxamine is not associated with accumulation of under-phosphorylated retinoblastoma protein, suppression of cyclin A protein, or commitment of cells to terminal differentiation. Neither of the cell cycle inhibitors alters the effect of HMBA in inducing the G1-associated changes or commitment to differentiation. Taken together, the present findings indicate that the site of action of HMBA which leads to commitment is in a stage of the G1 phase prior to the point of cell cycle block caused by deferoxamine or aphidicolin. HMBA appears to cause cell differentiation with suppression of cell cycle progression by an action that affects events required for cell progression through G1, including accumulation of underphosphorylated retinoblastoma protein and changes in regulation of cyclin levels.

Okadaic acid regulation of the retinoblastoma gene product is correlated with the inhibition of growth factor-induced cell proliferation in mouse fibroblasts

Okadaic acid, a specific inhibitor of protein phosphatases 1 and 2A, was used to study the mechanism of action of transforming growth factor beta (TGF-beta) on cell cycle progression in C3H/10T1/2 mouse embryonic fibroblasts, where TGF-beta exerts a growth-stimulatory effect. Concentrations of okadaic acid as low as 5 nM inhibited TGF-beta (5 ng/ml)- or 10% serum-induced [3H]thymidine incorporation into postconfluent, quiescent cells. Further, these inhibitory effects were observed when okadaic acid was added as late as 10 hr after TGF-beta or serum stimulation. Since C3H/10T1/2 fibroblasts undergo the G1/S transition at 10-14 hr after TGF-beta and 8-12 hr after serum stimulation, these observations indicate that a phosphatase activity may be required for S-phase entry. In a parallel experiment, okadaic acid partially inhibited TGF-beta-induced [14C]leucine incorporation by 20-65%, depending upon the okadaic acid concentration. In conjunction with the effect of okadaic acid on DNA and protein synthesis, Western blot analysis indicated that okadaic acid inhibited phosphorylation of the retinoblastoma gene product and decreased its protein level, even when added 10 hr after TGF-beta or 8 hr after serum stimulation. These findings strongly suggest that protein phosphatases play a pivotal role for S-phase entry in mouse fibroblasts. Moreover, protein phosphatases may be required in the intermediate steps of TGF-beta or serum growth factor signal-transduction pathways for the stimulation of phosphorylation of the retinoblastoma protein, especially in late G1.

Negative regulation of G1 in mammalian cells: inhibition of cyclin E-dependent kinase by TGF-beta

Transforming growth factor-beta (TGF-beta) is a naturally occurring growth inhibitory polypeptide that arrests the cell cycle in middle to late G1 phase. Cells treated with TGF-beta contained normal amounts of cyclin E and cyclin-dependent protein kinase 2 (Cdk2) but failed to stably assemble cyclin E-Cdk2 complexes or accumulate cyclin E-associated kinase activity. Moreover, G1 phase extracts from TGF-beta-treated cells did not support activation of endogenous cyclin-dependent protein kinases by exogenous cyclins. These effects of TGF-beta, which correlated with the inhibition of retinoblastoma protein phosphorylation, suggest that mammalian G1 cyclin-dependent kinases, like their counterparts in yeast, are targets for negative regulators of the cell cycle.

Regulation of the cell cycle by the cdk2 protein kinase in cultured human fibroblasts

In mammalian cells inhibition of the cdc2 function results in arrest in the G2-phase of the cell cycle. Several cdc2-related gene products have been identified recently and it has been hypothesized that they control earlier cell cycle events. Here we have studied the relationship between activation of one of these cdc2 homologs, the cdk2 protein kinase, and the progression through the cell cycle in cultured human fibroblasts. We found that cdk2 was activated and specifically localized to the nucleus during S phase and G2. Microinjection of affinity-purified anti-cdk2 antibodies but not of affinity-purified anti-cdc2 antibodies, during G1, inhibited entry into S phase. The specificity of these effects was demonstrated by the fact that a plasmid-driven cdk2 overexpression counteracted the inhibition. These results demonstrate that the cdk2 protein kinase is involved in the activation of DNA synthesis.