REVIEW
◥CANCER
CDK4 and CDK6 kinases: From basic
science to cancer therapy
Anne Fassl, Yan Geng, Piotr Sicinski*
Cyclin-dependent kinases 4 and 6 (CDK4 and CDK6) and their activating partners, D-type cyclins,
link the extracellular environment with the core cell cycle machinery. Constitutive activation of cyclin
D–CDK4/6 represents the driving force of tumorigenesis in several cancer types. Small-molecule
inhibitors of CDK4/6 have been used with great success in the treatment of hormone receptor–positive
breast cancers and are in clinical trials for many other tumor types. Unexpectedly, recent work
indicates that inhibition of CDK4/6 affects a wide range of cellular functions such as tumor cell
metabolism and antitumor immunity. We discuss how recent advances in understanding CDK4/6 biology
are opening new avenues for the future use of cyclin D–CDK4/6 inhibitors in cancer treatment.
C
yclin D1, the activator of CDK4 and CDK6,
was discovered in the early 1990s ( 1 , 2 ).
TheroleofcyclinD1inoncogenesiswas
alreadyevidentatthetimeofitscloning,
as it was also identified as the protein
product of thePRAD1oncogene, which is rear-
ranged and overexpressed in parathyroid
adenomas ( 3 ), and of theBCL1oncogene,
which is rearranged in B-lymphocytic malig-
nancies ( 4 ). Subsequently, the remaining two
D-type cyclins, D2 and D3, were discovered on
the basis of their homology to cyclin D1 ( 1 ).
Cyclins serve as regulatory subunits of
cyclin-dependent kinases (CDKs) ( 5 ). Shortly
after the discovery of D-cyclins, CDK4 and
CDK6 were identified as their kinase partners
( 6 ). Mouse gene knockout studies revealed that
CDK4 and CDK6 play redundant roles in de-
velopment, and combined ablation of CDK4
and CDK6 was found to result in embryonic
lethality ( 7 ). The essentially identical phenotype
wasseenincyclinD–knockout mice, thereby
confirming the role of D-cyclins as CDK4/6
activators in vivo ( 8 ). Surprisingly, these analyses
revealed that many normal nontransformed
mammalian cell types can proliferate without
any cyclin D–CDK4/6 activity ( 7 , 8 ).
CDK4 and CDK6 are expressed at constant
levels throughout the cell cycle. By contrast,
D-cyclins are labile proteins that are transcrip-
tionally induced upon stimulation of cells with
growth factors. For this reason, D-cyclins are
regarded as links between the cellular envi-
ronment and the cell cycle machinery ( 6 ).
Cell cycle inhibitors play an important role
in regulating the activity of cyclin D–CDK4/6
(Fig. 1). The INK inhibitors (p16INK4A, p15INK4B,
p18INK4C, p19INK4D) bind to CDK4 or CDK6
and prevent their interaction with D-type
cyclins, thereby inhibiting cyclin D–CDK4/6
kinase activity. By contrast, KIP/CIP inhibitors
(p27KIP1, p57KIP2, p21CIP1), which inhibit the
activity of CDK2-containing complexes, serveas assembly factors for cyclin D–CDK4/6 ( 6 , 9 ).
This was demonstrated by the observation
that mouse fibroblasts devoid of p27KIP1and
p21CIP1fail to assemble cyclin D–CDK4/6 com-
plexes ( 10 ).
p27KIP1canbindcyclinD–CDK4/6 in an in-
hibitory or noninhibitory mode, depending on
p27KIP1phosphorylation status. Cyclin D–p27KIP1-
CDK4/6 complexes are catalytically inactive
unless p27KIP1is phosphorylated on Tyr^88 and
Tyr^89 ( 11 ). Two molecular mechanisms may
explain this switch. First, Tyr^88 /Tyr^89 phos-
phorylation may dislodge the helix of p27KIP1
from the CDK active site and allow adenosine
triphosphate (ATP) binding ( 12 ). Second, the
presence of tyrosine-unphosphorylated p27KIP1
within the cyclin D–CDK4 complex prevents
the activating phosphorylation of CDK4’s T-loop
by the CDK-activating kinase (CAK) ( 12 ). Brk
has been identified as a physiological kinase
of p27KIP1( 13 ); Abl and Lyn can phosphorylate
p27KIP1in vitro, but their in vivo importance
remains unclear ( 11 , 14 ).
The activity of cyclin D–CDK4/6 is also regu-
lated by proteolysis. Cyclin D1 is an unstableRESEARCH
Fasslet al.,Science 375 , eabc1495 (2022) 14 January 2022 1of19
Department of Cancer Biology, Dana-Farber Cancer Institute,
Department of Genetics, Blavatnik Institute, Harvard Medical
School, Boston, MA 02215, USA.
*Corresponding author. Email: [email protected]
Box 1. Clinical use of CDK4/6 inhibitors.Palbociclib
Approved by FDA in 2016, in combination with fulvestrant for the treatment of hormone
receptor–positive, HER2-negative (HR+/HER2–) advanced or metastatic breast cancer in
women with disease progression following endocrine therapy. Approved in 2017 for the
treatment of HR+/HER2–advanced or metastatic breast cancer in combination with an
aromatase inhibitor as initial endocrine-based therapy in postmenopausal women.
Palbociclib is administered at a dose of 125 mg (given orally) daily for 3 weeks followed
by 1 week off, or 200 mg daily for 2 weeks followed by 1 week off. The rate-limiting
toxicities are neutropenia, thrombocytopenia, and anemia.Ribociclib
Approved by FDA in 2017, in combination with an aromatase inhibitor as initial
endocrine-based therapy for the treatment of postmenopausal women with HR+/HER2–
advanced or metastatic breast cancer. In 2018, the FDA expanded the indication for
ribociclib in combination with an aromatase inhibitor for pre/perimenopausal women
with HR+/HER2–advanced or metastatic breast cancer, as initial endocrine-based therapy.
FDA also approved ribociclib in combination with fulvestrant for postmenopausal
women with HR+/HER2–advanced or metastatic breast cancer, as initial endocrine-based
therapy or following disease progression on endocrine therapy.
Ribociclib is administered at a dose of 600 mg (given orally) daily for 3 weeks followed
by 1 week off. The main toxicities are neutropenia and thrombocytopenia.Abemaciclib
Approved by FDA in 2017, in combination with fulvestrant for women with HR+/HER2–
advanced or metastatic breast cancer with disease progression following endocrine therapy.
In addition, abemaciclib was approved as monotherapy for women and men with HR+/HER2–
advanced or metastatic breast cancer with disease progression following endocrine therapy and
prior chemotherapy in the metastatic setting. Approved by FDA in 2018 in combination
with an aromatase inhibitor as initial endocrine-based therapy for postmenopausal women with
HR+/HER2–advanced or metastatic breast cancer. Approved by FDA in 2021 for adjuvant
treatment of early-stage HR+/HER2–breast cancer in combination with endocrine therapy.
Abemaciclib is administered at a dose of 200 mg (given orally) every 12 hours. The
dose-limiting toxicity is fatigue. Neutropenia is also observed but is not rate-limiting.
Other severe side effects include diarrhea and nausea.