A Cdk7-Cdk4 T-loop phosphorylation cascade promotes G1 progression

Abstract

Eukaryotic cell division is controlled by cyclin-dependent kinases (CDKs), which require phosphorylation by a CDK-activating kinase (CAK) for full activity. Chemical genetics uncovered requirements for the metazoan CAK Cdk7 in determining cyclin specificity and activation order of Cdk2 and Cdk1 during S and G2 phases. It was unknown if Cdk7 also activates Cdk4 and Cdk6 to promote passage of the restriction (R) point, when continued cell-cycle progression becomes mitogen independent, or if CDK-activating phosphorylation regulates G1 progression. Here we show that Cdk7 is a Cdk4- and Cdk6-activating kinase in human cells, required to maintain activity, not just to establish the active state, as is the case for Cdk1 and Cdk2. Activating phosphorylation of Cdk7 rises concurrently with that of Cdk4 as cells exit quiescence and accelerates Cdk4 activation in vitro. Therefore, mitogen signaling drives a CDK-activation cascade during G1 progression, and CAK might be rate-limiting for R point passage.

Figure 1. Chemical genetics uncovers a Cdk7 requirement in G1/S progression

A. Cdk7^as/as HCT116 cells were synchronized in G0/G1 by serum starvation and released into media containing increasing concentrations of serum (as indicated), with DMSO or 10 μM 3-MB-PP1 (as indicated), and nocodazole (added to trap cells in mitosis). Cells were collected 24 hr after release and cell-cycle distribution was analyzed by flow cytometry to determine DNA content. B. Cdk7^as/as cells were synchronized as in (A) and released into 1% serum-containing media with DMSO or 10 μM 3-MB-PP1. Cells were collected at indicated times and Rb phosphorylation was analyzed with indicated phospho-specific antibodies. C. Cdk2^as/as and Cdk7^as/as HCT116 cells were grown for 5 d in DMSO or the indicated concentration of 3-MB-PP1 and various concentrations of PD0332991. Cells were fixed and stained with crystal violet solution, and proliferation was quantified by solubilization and spectrophotometric measurement of dye-binding and expressed as a percentage of growth in the DMSO-treated controls. Each data point is the mean ± SEM of 3 replicates. See also Figure S1.

Figure 2. Cdk7 execution point correlates with timing of Rb phosphorylation by Cdk4/6

A. Cdk7^as/as cells were synchronized as in Figure 1 and released into 1% serum-containing medium (with nocodazole), to which DMSO was added at time 0 or 10 μM 3-MB-PP1 was added at the indicated times. Cells were collected 24 hr after release and cell-cycle distribution was analyzed by flow cytometry. B. Cdk7^WT/WT or Cdk7^as/as cells were synchronized as in (A) and released into 1% serum-containing medium. DMSO or 10 μM 3-MB-PP1 was added at the indicated times, together with bromodeoxyuridine (BrdU) to detect DNA synthesis occurring after drug treatment, and nocodazole to prevent additional rounds of DNA synthesis. Cells were fixed and stained for BrdU incorporation 24 hr after release. Each data point is the mean +/− SEM of three fields of cells. C. Cdk7^as/as cells were synchronized and released as in (A). The first lane contains extract from cells prior to serum addition; subsequent lanes contain extracts from cells to which DMSO or 10 μM 3-MB-PP1 was added at indicated times, collected 12 hr after release. Rb phosphorylation was analyzed with indicated phosphospecific antibodies. See also Figure S2.

Figure 3. Cdk4 and Cdk6 depend on Cdk7 activity for activation in vivo

A. Wild-type (WT/WT) or Cdk7^as/as cells were treated with DMSO or 10 μM 3-MB-PP1 for 8 hr, collected and lysed. Extracts were immunoprecipitated with anti-Cdk4 antibody, immunoblotted for Cdk4 and cyclin D, and tested for Rb kinase activity. Incorporation was quantified by Phosphorimager. B. Same as (A) except that immunoprecipitation was with anti-Cdk6 antibody. C. Cdk7^as/as cells were treated with DMSO or 10 μM 3-MB-PP1 for 8 hr, lysed and extracts were immunoprecipitated with anti-Cdk4 antibody. The immunoprecipitates and whole-cell extract were immunoblotted with antibodies specific for phosphorylated Cdk4-Thr172 (T172-P) and total Cdk4 (Ig LC: immunoglobulin light chain). D. Extracts were immunoprecipitated with anti-Cdk6 antibody as in (B) and treated with purified recombinant Cdk7/cyclin H/Mat1, immunoblotted for Cdk6 and tested for Rb kinase activity. Incorporation was quantified by Phosphorimager. See also Figure S3.

Figure 4. Cdk7 is required to maintain Cdk4 and Cdk6 activity, but only to establish active state of Cdk1 and Cdk2

A. Wild-type or Cdk7^as/as cells were treated with DMSO or 10 μM 3-MB-PP1 for the indicated times and collected immediately after treatment for extract preparation. Extracts were immunoprecipitated with anti-Cdk4 antibody, immunoblotted for Cdk4 (bottom) and tested for Rb kinase activity (top). (Note: Sample corresponding to the 3-hr 3-MB-PP1 treatment of Cdk7^as/as cells was loaded out of order and repositioned to conform to the immunoblot, but all data come from the same autoradiographic exposure.) Incorporation was quantified by Phosphorimager; each 3-MB-PP1-treated sample was normalized to corresponding DMSO-treated sample. B. Same as in (A) except that immunoprecipitation was with anti-Cdk6 antibody. C. Same as in (A) except that immunoprecipitation was with anti-Cdk2 antibody. D. Cells were treated as in (A) and whole-cell extracts were immunoblotted for phosphorylated and total Cdk4, Cdk2 and Cdk1, as indicated.

Figure 5. Cdk7 T-loop phosphorylation stimulates activation of Cdk4

A. Purified Cdk4/cyclin D was treated with T-loop phosphorylated Cdk7/cyclin H/Mat1 (P-K7/H/M), unphosphorylated Cdk7/cyclin H/Mat1 (K7/H/M), or phosphorylated Cdk7/cyclin H (P-K7/H) for 30 min and tested for Rb kinase activity. Incorporation was quantified in arbitrary units (A.U.) by Phosphorimager. B. Same as (A) except Cdk2/cyclin A was activated by Cdk7 isoforms. C. Purified Cdk4/cyclin D or Cdk2/cyclin A was treated with 50 or 6.25 ng, respectively, of phosphorylated Cdk7/cyclin H/Mat1 (P-K7/H/M) or phosphorylated Cdk7/cyclin H (P-K7/H), as indicated, for the indicated times and tested for Rb kinase activity. Incorporation was quantified by Phosphorimager. See also Figure S4.

Figure 6. Cdk7 and Cdk4 T-loop phosphorylation occur with similar kinetics during cell cycle reentry

A. HCT116 cells were synchronized by serum starvation and released into fresh medium containing 10% serum. Extracts were prepared in RIPA buffer at indicated times after release, and immunoblotted for Cdk7 phosphorylated at Thr170 (T170-P), total Cdk7, cyclin H, Mat1, Cdk4 phosphorylated at Thr172 (T172 -P), total Cdk4 and cyclin D. Rb kinase activity (³²P-Rb) and cyclin D recovery were measured in cyclin D1 immunoprecipitates from the same extracts. B. RPE-hTERT cells were synchronized in G0 by contact inhibition and released into fresh, 10% serum-containing medium. Extracts were prepared in RIPA buffer at the indicated times and immunoblotted for the same proteins measured in (A). See also Figure S5.

Figure 7. Changing modes of CDK activation during cell-cycle progression: from analog sensor to binary switch?

A CDK T-loop phosphorylation cascade operates during G1. Maximal rates of Cdk4 T-loop phosphorylation require the phosphorylated form of Cdk7, which is induced by mitogens. We propose that the increased activity of Cdk7 promotes activation of Cdk4 in the face of an opposing phosphatase. The Cdk2 T loop is exposed to phosphatases only prior to cyclin-binding. Assembly and T-loop phosphorylation of Cdk1 are coupled, and apparently unopposed by cellular phosphatases.