Randomized, Open Label, Clinical Study of the Targeted Therapy, Palbociclib, to Treat Metastatic Breast Cancer


About this study

The primary objective of this study is to demonstrate that the combination of palbociclib with anti-HER2 therapy plus endocrine therapy is superior to anti-HER2-based therapy plus endocrine therapy alone in improving the outcomes of subjects with hormone receptor-positive, HER2+ metastatic breast cancer.

Participation eligibility

Participant eligibility includes age, gender, type and stage of disease, and previous treatments or health concerns. Guidelines differ from study to study, and identify who can or cannot participate. There is no guarantee that every individual who qualifies and wants to participate in a trial will be enrolled. Contact the study team to discuss study eligibility and potential participation.

Inclusion Criteria - Screening:

  • Signed Preliminary Screening Informed Consent Form obtained prior to any study specific assessments and procedures.
  • Age ≥ 18 years (or per national guidelines).
  • Patients must have histologically confirmed invasive breast cancer that is metastatic or not amenable for resection or radiation therapy with curative intent. Histological documentation of metastatic/recurrent breast cancer is not required if there is unequivocal evidence for recurrence of the breast cancer.
  • Patients must have histologically confirmed HER2+ and hormone receptor positive (ER+ and/or PR+), metastatic breast cancer. ER, PR and HER2 measurements should be performed according to institutional guidelines, in a CLIA-approved setting in the US or certified laboratories for Non-US regions. Cut-off values for positive/negative staining should be in accordance with current ASCO/CAP (American Society of Clinical Oncology/College of American Pathologists) guidelines.
  • Patients must agree to provide a representative formalin-fixed paraffin-embedded (FFPE) tumor tissue block (preferred) from primary breast or metastatic site (archival) OR at least 15 freshly cut unstained slides from such a block, along with a pathology report documenting HER2 positivity and hormone receptor positivity.
  • Patients should be willing to provide a representative tumor specimen obtained from recently biopsied metastatic disease if clinically feasible. This is recommended but optional tissue.

Inclusion Criteria - Randomization:

  • Signed Main Informed Consent Form obtained prior to any study specific assessments and procedures.
  • Age ≥ 18 years (or per national guidelines).
  • ECOG performance status 0-1.
  • Patients must be able and willing to swallow and retain oral medication without a condition that would interfere with enteric absorption.
  • Serum or urine pregnancy test must be negative within 7 days of randomization in women of childbearing potential. Pregnancy testing does not need to be pursued in patients who are judged as postmenopausal before randomization, as determined by local practice, or who have undergone bilateral oophorectomy, total hysterectomy, or bilateral tubal ligation. Women of childbearing potential and male patients randomized into the study must use adequate contraception for the duration of protocol treatment which is 6 months after the last treatment with palbociclib if they are in Arm A and for 7 months after last treatment with trastuzumab if in either Arm A or Arm B Adequate contraception is defined as one highly effective form (i.e., abstinence, (fe)male sterilization OR two effective forms (e.g. non-hormonal IUD and condom / occlusive cap with spermicidal foam / gel / film / cream / suppository).
  • Resolution of all acute toxic effects of prior induction anti-HER2-based chemotherapy regimen to NCI CTCAE version 4.0 Grade ≤1 (except alopecia or other toxicities not considered a safety risk for the patient at investigator's discretion) 12 weeks between last dose of chemotherapy-anti-HER2therapy and randomization are allowed. Endocrine therapy could start before study randomization.
  • Willingness and ability to comply with scheduled visits, treatment plan, laboratory tests, and other study procedures.

Prior Treatment Specifics

  • Patients may or may not have received neo/adjuvant therapy, but must have a disease-free interval from completion of anti-HER2 therapy to metastatic diagnosis ≥ 6 months.
  • Patients must have received an acceptable, standard, chemotherapy containing anti-HER2 based induction therapy for the treatment of metastatic breast cancer prior to study enrollment. For this study, chemotherapy is limited to a taxane or vinorelbine (only for trastuzumab-based regimen). Eligible patients are expected to have completed 6 cycles of chemotherapy containing anti-HER2-therapy treatment. A minimum of 4 cycles of treatment is acceptable for patients experiencing significant toxicity associated with treatment as long as they are without evidence of disease progression (i.e., CR, PR or SD).
  • The maximum number of cycles is 8. Patients can randomize immediately following completion of their induction therapy, or for those who have already completed induction, a gap of 12 weeks between their last infusion/dose of induction therapy and the C1D1 visit is permitted. Patients are eligible provided they are without evidence of disease progression by local assessment (i.e., CR, PR or SD).
  • Patients with a history or presence of asymptomatic CNS metastases are eligible provided they meet all of the following criteria:
    • Disease outside the CNS is present;
    • No evidence of interim progression between the completion of induction therapy and the screening radiographic study;
    • No history of intracranial hemorrhage or spinal cord hemorrhage;
    • Not requiring anti-convulsants for symptomatic control;
    • Minimum of 3 weeks between completion of CNS radiotherapy and Cycle 1 Day 1 and recovery from significant (Grade ≥ 3) acute toxicity with no ongoing requirement for corticosteroid.

Baseline Body Function Specifics

  • Absolute neutrophil count ≥ 1,000/mm^3
  • Platelets ≥ 100,000/mm^3
  • Hemoglobin ≥ 10g/dL
  • Total serum bilirubin ≤ ULN; or total bilirubin ≤ 3.0 × ULN with direct bilirubin within normal range in patients with documented Gilbert’s Syndrome.
  • Aspartate aminotransferase (AST or SGOT) and alanine aminotransferase (ALT or SGPT) ≤ 3 × institutional ULN (≤ x ULN if liver metastases are present).
  • Serum creatinine below the upper limit of normal (ULN) of the institutional normal range or creatinine clearance ≥ 60 mL/min/1.73 m^2 for patients with serum creatinine levels above institutional ULN.
  • Left ventricular ejection fraction (LVEF) ≥ 50% at baseline as determined by either ECHO or MUGA.

Exclusion Criteria - Randomization:

  • Concurrent therapy with other Investigational Products.
  • Prior therapy with any CDK 4/6 inhibitor.
  • History of allergic reactions attributed to compounds of chemical or biologic composition similar to palbociclib.
  • Patients receiving any medications or substances that are strong inhibitors or inducers of CYP3A isoenzymes within 7 days of randomization.
  • Uncontrolled current illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, diabetes, or psychiatric illness/social situations that would limit compliance with study requirements. Ability to comply with study requirements is to be assessed by each investigator at the time of screening for study participation.
  • Pregnant women, or women of childbearing potential without a negative pregnancy test (serum or urine) within 7 days prior to randomization, irrespective of the method of contraception used, are excluded from this study because the effect of palbociclib on a developing fetus is unknown. Breastfeeding must be discontinued prior to study entry.
  • Patients on combination antiretroviral therapy; i.e., those who are HIV-positive, are ineligible because of the potential for pharmacokinetic interactions or increased immunosuppression with palbociclib.
  • QTc interval > 480 msec, Brugada syndrome or known history of QTc prolongation or Torsade de Pointes.
  • Patients with clinically significant history of liver disease, including viral or other known hepatitis, current alcohol abuse, or cirrhosis.


Participating Mayo Clinic locations

Study statuses change often. Please contact the study team for the most up-to-date information regarding possible participation.

Mayo Clinic Location Status Contact

Scottsdale/Phoenix, Ariz.

Mayo Clinic principal investigator

Donald Northfelt, M.D.

Closed for enrollment

Contact information:

Nicholas Schroeder B.S.

(480) 342-3089


Rochester, Minn.

Mayo Clinic principal investigator

Matthew Goetz, M.D.

Contact us for the latest status

Contact information:

Jean Jensen

(507) 538-1115


More information


  • In the PALOMA-3 study, the combination of the CDK4 and CDK6 inhibitor palbociclib and fulvestrant was associated with significant improvements in progression-free survival compared with fulvestrant plus placebo in patients with metastatic breast cancer. Identification of patients most suitable for the addition of palbociclib to endocrine therapy after tumour recurrence is crucial for treatment optimisation in metastatic breast cancer. We aimed to confirm our earlier findings with this extended follow-up and show our results for subgroup and biomarker analyses. Read More on PubMed
  • Using transgenic mouse models, cell line-based functional studies, and clinical specimens, we show that cyclin D1/CDK4 mediate resistance to targeted therapy for HER2-positive breast cancer. This is overcome using CDK4/6 inhibitors. Inhibition of CDK4/6 not only suppresses Rb phosphorylation, but also reduces TSC2 phosphorylation and thus partially attenuates mTORC1 activity. This relieves feedback inhibition of upstream EGFR family kinases, resensitizing tumors to EGFR/HER2 blockade. Consequently, dual inhibition of EGFR/HER2 and CDK4/6 invokes a more potent suppression of TSC2 phosphorylation and hence mTORC1/S6K/S6RP activity. The suppression of both Rb and S6RP enhances G1 arrest and a phenotype resembling cellular senescence. In vivo, CDK4/6 inhibitors sensitize patient-derived xenograft tumors to HER2-targeted therapies and delay tumor recurrence in a transgenic model of HER2-positive breast cancer. Read More on PubMed
  • In patients with metastatic breast cancer that is positive for human epidermal growth factor receptor 2 (HER2), progression-free survival was significantly improved after first-line therapy with pertuzumab, trastuzumab, and docetaxel, as compared with placebo, trastuzumab, and docetaxel. Overall survival was significantly improved with pertuzumab in an interim analysis without the median being reached. We report final prespecified overall survival results with a median follow-up of 50 months. Read More on PubMed
  • The processes that control the progression of ductal carcinoma in situ (DCIS) to invasive breast cancer remain poorly understood. Epidermal growth factor receptor 2 (ErbB2) overexpression is common in DCIS, as is disruption of the retinoblastoma tumor suppressor (RB) pathway. Here, we examined the cooperative impact of ErbB2 and RB deregulation on facets of disease progression. Our studies demonstrate that RB deficiency altered the expression of key molecules needed for proper cellular organization and epithelial cell-cell adhesion as part of a program related to the epithelial-to-mesenchymal transition (EMT). An increase in the invasive potential of ErbB2-overexpressing cells was observed upon RB depletion. Further, stable knockdown of RB resulted in invasive lesions in orthotopic xenograft assays, compared with DCIS-like lesions developing from RB-proficient cells. Conversely, the invasive phenotype observed in ErbB2-positive cancer models was inhibited through CDK4/6 inhibition in an RB-dependent manner. Finally, in a cohort of DCIS cases, we show that, although elevated levels of ErbB2 are associated with increased risk of a subsequent DCIS recurrence, it is not associated with progression to invasive disease. In contrast, RB loss in ErbB2-positive DCIS cases was associated with increased risk for invasive breast cancer. Taken together, these data demonstrate a key role for the RB pathway in invasion associated with breast tumor progression, and shed light on the key molecular events that promote the progression of DCIS to invasive disease. Read More on PubMed
  • To provide formal expert consensus-based recommendations to practicing oncologists and others on the management of brain metastases for patients with human epidermal growth factor receptor 2 (HER2) -positive advanced breast cancer. Read More on PubMed
  • Cancer incidence and mortality estimates for 25 cancers are presented for the 40 countries in the four United Nations-defined areas of Europe and for the European Union (EU-27) for 2012. Read More on PubMed
  • Cyclin-dependent kinases (CDKs) regulate cell proliferation and coordinate the cell cycle checkpoint response to DNA damage. Although inhibitors with varying selectivity to specific CDK family members have been developed, selective CDK4/6 inhibitors have emerged as the most attractive antineoplastic agents because of the importance of CDK4/6 activity in regulating cell proliferation and the toxic effects associated with inhibition of other CDKs (eg, CDK1 and CDK2). Read More on PubMed
  • Moderate differences in efficacy between adjuvant chemotherapy regimens for breast cancer are plausible, and could affect treatment choices. We sought any such differences. Read More on PubMed
  • The anti-human epidermal growth factor receptor 2 (HER2) humanized monoclonal antibody trastuzumab improves the outcome in patients with HER2-positive metastatic breast cancer. However, most cases of advanced disease eventually progress. Pertuzumab, an anti-HER2 humanized monoclonal antibody that inhibits receptor dimerization, has a mechanism of action that is complementary to that of trastuzumab, and combination therapy with the two antibodies has shown promising activity and an acceptable safety profile in phase 2 studies involving patients with HER2-positive breast cancer. Read More on PubMed
  • Alterations in cell cycle regulators have been implicated in human malignancies including breast cancer. PD 0332991 is an orally active, highly selective inhibitor of the cyclin D kinases (CDK)4 and CDK6 with ability to block retinoblastoma (Rb) phosphorylation in the low nanomolar range. To identify predictors of response, we determined the in vitro sensitivity to PD 0332991 across a panel of molecularly characterized human breast cancer cell lines. Read More on PubMed
  • The aim of this international phase II trial was to determine the efficacy and safety profile of weekly vinorelbine plus trastuzumab as first-line chemotherapy for women with HER 2-overexpressing metastatic breast cancer. Sixty-nine patients with tumours overexpressing HER 2 received vinorelbine: 30 mg m-2 week-1 and trastuzumab: 4 mg kg-1 on day 1 as a loading dose followed by 2 mg kg-1 week-1 starting on day 8. Sixty-two patients were evaluable for response and 69 patients were evaluable for toxicity. The overall response rate was 62.9%. The median time to response was 8.4 weeks, the median duration of response was 17.5 months, the median progression-free survival was 9.9 months (95% CI, 5.6-12.1) and the one-year progression-free survival was 39.1%. The median survival for all patients was 23.7 months (95% CI, 18.4-32.6). This regimen was safe: grade 3-4 neutropenia were observed over 17.7% of courses in 83.8% of patients, with only two episodes of febrile neutropenia (0.1%) in two patients (2.9%). Only one patient discontinued treatment due to grade 3 symptomatic cardiac dysfunction that resolved with therapy. Vinorelbine plus trastuzumab is one of the most active treatment regimens for patients with HER 2-positive metastatic breast cancer and demonstrates a very favourable safety profile allowing prolonged treatment with long-term survival. This study has been presented in part at the following conferences: The San Antonio Breast Cancer Symposium, San Antonio, TX, USA, 2003; The American Society of Clinical Oncology, Orlando, FL, USA, 2005. Read More on PubMed
  • Cyclin D1 is overexpressed in the majority of human breast cancers. We previously found that mice lacking cyclin D1 are resistant to mammary carcinomas triggered by the ErbB-2 oncogene. In this study, we investigated which function of cyclin D1 is required for ErbB-2-driven mammary oncogenesis. We report that the ability of cyclin D1 to activate cyclin-dependent kinase CDK4 underlies the critical role for cyclin D1 in breast cancer formation. We also found that the continued presence of CDK4-associated kinase activity is required to maintain breast tumorigenesis. We analyzed primary human breast cancers and found high cyclin D1 levels in a subset (approximately 25%) of ErbB-2-overexpressing tumors. We propose that this subset of breast cancer patients might benefit from inhibiting CDK4 kinase. Read More on PubMed
  • Cyclin D1 is a multifunctional protein that activates CDK4 and CDK6, titrates Cip/Kip CDK inhibitors to increase CDK2 activity, and modulates the function of certain transcription factors. To specifically test the importance of cyclin D1-associated kinase activity, we generated "knockin" mice expressing mutant cyclin D1 deficient in activating CDK4/6. The development of several cyclin D1-dependent compartments, including mammary glands, proceeds relatively normally in these animals, demonstrating that cyclin D1-associated kinase activity is largely dispensable for development of these tissues. Strikingly, knockin mice were resistant to breast cancers initiated by ErbB-2. These results demonstrate a differential requirement for cyclin D1-CDK4/6 kinase activity in development versus tumorigenesis and strongly support cyclin D1-dependent kinase activity as a specific therapeutic target in breast cancer. Read More on PubMed
  • Previous work has shown that cyclin D1 expression is required for neu- and ras-induced, but not wnt- or c-myc-induced, breast tumorigenesis in mice. Although cyclin D1 binds and activates cyclin-dependent kinase 4 (Cdk4), thereby mediating activation of a program of E2F-dependent gene expression, it has been suggested that the oncogenic activities of cyclin D1 are independent of Cdk4. To determine whether Cdk4 expression is required for breast tumorigenesis in mice, we have generated compound mice ectopically expressing the neu or wnt oncogenes in the mammary glands of wild-type and Cdk4-/- mice. Our results show that Cdk4 expression is required for efficient neu-induced tumorigenesis but is dispensable for wnt-induced breast tumorigenesis. In contrast to results previously observed in the mammary glands of cyclin D1-/- virgin females, our results show defects in mammary gland development in Cdk4-/- virgin females, suggesting differences in compensatory mechanisms in the absence of either subunit of the cyclin D1/Cdk4 complex. These results suggest that drugs targeted to inhibit Cdk4 activities could be developed to specifically treat certain breast tumors as Cdk4 is not essential for viability. Read More on PubMed
  • Intact cyclin D1 functions are essential for transformation by erbB2 in tissue culture and murine models. Because cyclin D1 may alter cell proliferation through a variety of mechanisms, we used transgenic models and human tumor samples to particularly address the role of cyclin D1-cyclin-dependent kinases in transformation by erbB2. The p16 tumor suppressor specifically blocks cyclin-dependent kinase 4 and 6 activity. Here we show that an MMTV-p16 transgene blocked tumorigenesis by erbB2, demonstrating that deregulation of the cyclin-dependent kinase partner of cyclin D1 is an essential target of erbB2. ErbB2 overexpression was a determining factor in deregulation of cyclin D1-cdk4/6 interactions because neither transgenic cyclin D1 nor loss of p16 accelerated tumorigenesis in MMTV-erbB2-transgenic mice. ErbB2 was also a deciding factor in deregulation of cyclin D1-cdk4/6 in human tumors because no loss of pRb or p16 was found in tumors overexpressing erbB2, although erbB2-negative invasive breast adenocarcinomas frequently lacked expression of p16 or pRb. We conclude that deregulation of cyclin D1-Cdk4/6 interactions is a critical target of erbB2 function in human and mouse breast tumors, and erbB2's overexpression may be sufficient to deregulate cyclin D1-cdk4/6 activity in breast cancer. Read More on PubMed
  • Trastuzumab-based therapy improves survival for women with human epidermal growth factor receptor 2 (HER2)-positive advanced breast cancer. We conducted a multicenter phase II study to evaluate the efficacy and safety of trastuzumab combined with vinorelbine, and to assess cardiac surveillance algorithms and tumor markers as prognostic tools. Read More on PubMed
  • To determine the effect of elevated serum HER-2/neu on the response of metastatic breast cancer patients to an aromatase inhibitor versus an antiestrogen. Read More on PubMed
  • Cyclin D1 is downstream of erbB2 and is required for erbB2 transformation. Here we report thatcyclin D1 functions are essential, rate limiting for erbB2 transformation, and reciprocally increase erbB2 levels. This interaction depends on three cyclin D1 activities: cyclin-dependent kinase 4-dependent kinase activity, titration of p27, and an intrinsic transcriptional activity of cyclin D1. Drugs active against erbB2 and cyclin D1 (Herceptin and flavopiridol) were synergistically cytotoxic against erbB2-positive breast cancer cell lines. Addition of flavopiridol to Herceptin synergistically lowered erbB2 levels in these cells. Our data suggest the potential use of combinations of cyclin-dependent kinase inhibitors and Herceptin in breast cancer. Read More on PubMed
  • Overexpression of the ErbB2 receptor, a major component of the ErbB receptor signaling network, contributes to the development of a number of human cancers. ErbB2 presents itself, therefore, as a target for antibody-mediated therapies. In this respect, anti-ErbB2 monoclonal antibody 4D5 specifically inhibits the growth of tumor cells overexpressing ErbB2. We have analyzed the effect of 4D5-mediated ErbB2 inhibition on the cell cycle of the breast tumor cell line BT474. 4D5 treatment of BT474 cells resulted in a G(1) arrest, preceded by rapid dephosphorylation of ErbB2, inhibition of cytoplasmic signal transduction pathways, accumulation of the cyclin-dependent kinase inhibitor p27(Kip1), and inactivation of cyclin-Cdk2 complexes. Time courses demonstrated that 4D5 treatment redirects p27(Kip1) onto Cdk2 complexes, an event preceding increased p27(Kip1) expression; this correlates with the downregulation of c-Myc and D-type cyclins (proteins involved in p27(Kip1) sequestration) and the loss of p27(Kip1) from Cdk4 complexes. Similar events were observed in ErbB2-overexpressing SKBR3 cells, which exhibited reduced proliferation in response to 4D5 treatment. Here, p27(Kip1) redistribution resulted in partial Cdk2 inactivation, consistent with a G1 accumulation. Moreover, p27(Kip1) protein levels remained constant. Antisense-mediated inhibition of p27(Kip1) expression in 4D5-treated BT474 cells further demonstrated that in the absence of p27(Kip1) accumulation, p27(Kip1) redirection onto Cdk2 complexes is sufficient to inactivate Cdk2 and establish the G(1) block. These data suggest that ErbB2 overexpression leads to potentiation of cyclin E-Cdk2 activity through regulation of p27(Kip1) sequestration proteins, thus deregulating the G(1)/S transition. Moreover, through comparison with an ErbB2-overexpressing cell line insensitive to 4D5 treatment, we demonstrate the specificity of these cell cycle events and show that ErbB2 overexpression alone is insufficient to determine the cellular response to receptor inhibition. Read More on PubMed
  • The neu (c-erbB-2) proto-oncogene encodes a tyrosine kinase receptor that is overexpressed in 20 to 30% of human breast tumors. Herein, cyclin D1 protein levels were increased in mammary tumors induced by overexpression of wild-type Neu or activating mutants of Neu in transgenic mice and in MCF7 cells overexpressing transforming Neu. Analyses of 12 Neu mutants in MCF7 cells indicated important roles for specific C-terminal autophosphorylation sites and the extracellular domain in cyclin D1 promoter activation. Induction of cyclin D1 by NeuT involved Ras, Rac, Rho, extracellular signal-regulated kinase, c-Jun N-terminal kinase, and p38, but not phosphatidylinositol 3-kinase. NeuT induction of the cyclin D1 promoter required the E2F and Sp1 DNA binding sites and was inhibited by dominant negative E2F-1 or DP-1. Neu-induced transformation was inhibited by a cyclin D1 antisense or dominant negative E2F-1 construct in Rat-1 cells. Growth of NeuT-transformed mammary adenocarcinoma cells in nude mice was blocked by the cyclin D1 antisense construct. These results demonstrate that E2F-1 mediates a Neu-signaling cascade to cyclin D1 and identify cyclin D1 as a critical downstream target of neu-induced transformation. Read More on PubMed
  • This is the first published report on the validation of the Functional Assessment of Cancer Therapy-Breast (FACT-B), a 44-item self-report instrument designed to measure multidimensional quality of life (QL) in patients with breast cancer. The FACT-B consists of the FACT-General (FACT-G) plus the Breast Cancer Subscale (BCS), which complements the general scale with items specific to QL in breast cancer. The FACT-B was developed with an emphasis on patients' values and brevity and is available in nine languages. Read More on PubMed
  • p16ink4 has been implicated as a tumor suppressor that is lost from a variety of human tumors and human cell lines. p16ink4 specifically binds and inhibits the cyclin-dependent kinases 4 and 6. In vitro, these kinases can phosphorylate the product of the retinoblastoma tumor suppressor gene. Thus, p16ink4 could exert its function as tumor suppressor through inhibition of phosphorylation and functional inactivation of the retinoblastoma protein. Here we show that overexpression of p16ink4 in certain cell types will lead to an arrest in the G1 phase of the cell cycle. In addition, we show that p16ink4 can only suppress the growth of human cells that contain functional pRB. Moreover, we have compared the effect of p16ink4 expression on embryo fibroblasts from wild-type and RB homozygous mutant mice. Wild-type embryo fibroblasts are inhibited by p16ink4, whereas the RB nullizygous fibroblasts are not. These data not only show that the presence of pRB is crucial for growth suppression by p16ink4 but also indicate that the pRB is the critical target acted upon by cyclin D-dependent kinases in the G1 phase of the cell cycle. Read More on PubMed
  • D-type cyclins, in association with the cyclin-dependent kinases Cdk4 or Cdk6, promote progression through the G1 phase of the cell cycle by phosphorylating the retinoblastoma protein (RB). The activities of Cdk4 and Cdk6 are constrained by inhibitors such as p16, the product of the CDKN2 gene on human chromosome 9p21 (refs 12-14). The frequent deletion or mutation of CDKN2 in tumour cells suggests that p16 acts as a tumour suppressor. We show that wild-type p16 arrests normal diploid cells in late G1, whereas a tumour-associated mutant of p16 does not. Significantly, the ability of p16 to induce cell-cycle arrest is lost in cells lacking functional RB, including primary fibroblasts from Rb-/- mouse embryos. Thus, loss of p16, overexpression of D-cyclins and loss of RB have similar effects on G1 progression, and may represent a common pathway to tumorigenesis. Read More on PubMed
  • D-type cyclins couple extracellular signals to the biochemical machinery that governs progression through G1 phase of the mammalian cell division cycle. Induced by growth factor stimulation, D-type cyclins assemble with cyclin-dependent kinases CDK4 and CDK6 to form holoenzymes that facilitate exit from G1 by phosphorylating key substrates, including the retinoblastoma protein. Activation of the holoenzymes is antagonized by polypeptide inhibitors of CDK activity, which are induced by antiproliferative signals. Once cells pass a late G1 restriction point, cyclin-D-dependent kinases are unnecessary for completion of the cell cycle, implying that their primary role is to sense the cell's readiness to replicate DNA and to enforce the commitment to enter S phase. Read More on PubMed
  • Using a yeast interaction screen to search for proteins that interact with cyclin D1-Cdk4, we identified a 27 kDa mouse protein related to the p21 cyclin-Cdk inhibitor. p27 interacts strongly with D-type cyclins and Cdk4 in vitro and more weakly with cyclin E and Cdk2. In mouse fibroblasts, p27 is associated predominantly with cyclin D1-Cdk4. Recombinant p27 is a potent inhibitor of cyclin D1-Cdk4 and cyclin A-Cdk2 protein kinase activity and a weaker inhibitor of cyclin B1-Cdc2. Overexpression of p27 in Saos-2 cells causes G1 arrest. p27 protein levels do not change as serum-stimulated quiescent mouse fibroblasts progress through the cell cycle. p27 is identical to p27Kip1, a cyclin-Cdk inhibitor present in TGF beta-treated cells. p27 has the hallmarks of a negative regulator of G1 progression and may mediate TGF beta-induced G1 arrest. Read More on PubMed
  • Cell-cell contact and TGF-beta can arrest the cell cycle in G1. Mv1Lu mink epithelial cells arrested by either mechanism are incapable of assembling active complexes containing the G1 cyclin, cyclin E, and its catalytic subunit, Cdk2. These growth inhibitory signals block Cdk2 activation by raising the threshold level of cyclin E necessary to activate Cdk2. In arrested cells the threshold is set higher than physiological cyclin E levels and is determined by an inhibitor that binds to cyclin E-Cdk2 complexes. A 27-kD protein that binds to and prevents the activation of cyclin E-Cdk2 complexes can be purified from arrested cells but not from proliferating cells, using cyclin E-Cdk2 affinity chromatography. p27 is present in proliferating cells, but it is sequestered and unavailable to interact with cyclin E-Cdk2 complexes. Cyclin D2-Cdk4 complexes bind competitively to and down-regulate the activity of p27 and may thereby act in a pathway that reverses Cdk2 inhibition and enables G1 progression. Read More on PubMed
  • The key cell-cycle regulator Cdc2 belongs to a family of cyclin-dependent kinases in higher eukaryotes. Dominant-negative mutations were used to address the requirement for kinases of this family in progression through the human cell cycle. A dominant-negative Cdc2 mutant arrested cells at the G2 to M phase transition, whereas mutants of the cyclin-dependent kinases Cdk2 and Cdk3 caused a G1 block. The mutant phenotypes were specifically rescued by the corresponding wild-type kinases. These data reveal that Cdk3, in addition to Cdc2 and Cdk2, executes a distinct and essential function in the mammalian cell cycle. Read More on PubMed
  • The cyclin-dependent kinase Cdk2 associates with cyclins A, D, and E and has been implicated in the control of the G1 to S phase transition in mammals. To identify potential Cdk2 regulators, we have employed an improved two-hybrid system to isolate human genes encoding Cdk-interacting proteins (Cips). CIP1 encodes a novel 21 kd protein that is found in cyclin A, cyclin D1, cyclin E, and Cdk2 immunoprecipitates. p21CIP1 is a potent, tight-binding inhibitor of Cdks and can inhibit the phosphorylation of Rb by cyclin A-Cdk2, cyclin E-Cdk2, cyclin D1-Cdk4, and cyclin D2-Cdk4 complexes. Cotransfection experiments indicate that CIP1 and SV40 T antigen function in a mutually antagonistic manner to control cell cycle progression. Read More on PubMed
  • 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. Read More on PubMed
  • In the course of developing a standardised, non-disease-specific instrument for describing and valuing health states (based on the items in Table 1), the EuroQol Group (whose members are listed in the Appendix) conducted postal surveys in England, The Netherlands and Sweden which indicate a striking similarity in the relative valuations attached to 14 different health states. The data were collected using a visual analogue scale similar to a thermometer. The EuroQol instrument is intended to complement other quality-of-life measures and to facilitate the collection of a common data set for reference purposes. Others interested in participating in the extension of this work are invited to contact the EuroQol Group. Read More on PubMed

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