Inhibition of DNA damage repair
The impairment of DDR is an essential determinant of CDK4/6 inhibitor-associated radiosensitization in human cell lines. IR produces cytotoxic effects on DNA, which mainly results in DNA single-strand breaks (SSBs) and double-strand breaks (DSBs) [
53]. Unrepaired and inaccurate-repaired DNA DSBs are the main cause of radiation-induced cell death [
54]. Multiple preclinical studies have demonstrated that IR increases γ-H2AX and 53BP1 levels in a dose dependent manner and increases DNA DSBs [
30,
42,
44‐
47]. γ-H2AX is one of the earliest events of DDR with the induction of DSB [
55]. The abundance of γ-H2AX foci reaches a peak at 30 min after IR and returns to baseline levels approximately 24 h post-IR [
37]. In addition, 53BP1 plays a similar role to γ-H2AX in most cases [
45], which is also a crucial hallmark of IR-induced DSBs [
56]. Naz et al. [
44] investigated γ-H2AX foci in H460 and H1299 (NSCLC) cells following 7.5 Gy irradiation. The γ-H2AX level rapidly increased in H460 (7.9-fold) and H1299 (6.7-fold) cells 0.5 h after IR compared to untreated control. Additional studies also demonstrated that various post-irradiated cell lines showed substantial increases in γ-H2AX and (or) 53BP1 foci, which eventually led to increased DSBs [
37,
42,
45‐
47,
49].
γ-H2AX and 53BP1 levels decrease over time, reflecting the dynamic process of DDR. The combination of IR and CDK4/6 inhibitors causes marked retention of γ-H2AX and (or) 53BP1 levels, thus delaying DSB repair to enhance radiotherapy efficacy [
42,
44‐
47]. With 7.5 Gy and 10 μM abemaciclib, γ-H2AX levels were increased 3.92-fold at 24 h in H460 cells. However, the levels were only 0.97-fold in H1299 cells (radiation-resistant), which was basically equivalent to the level before irradiation [
44]. Huang et al. [
45] reported similar findings in HuCCT1 (CCA), Huh7, and Hep3B (HCC) cells. These post-IR cells treated with 5 μM palbociclib sustained higher levels of γ-H2AX and 53BP1 at 24 h compared with DMSO-treated cells. Several other studies also supported this view that CDK4/6 inhibitors exhibited cellular radiosensitivity by increasing unrepaired DSBs and inducing the delayed repair kinetics of DSBs [
42,
46,
47].
The combination of CDK4/6 inhibitors and IR not only increases DSBs and delays the DSB repair but also causes homologous recombination (HR) deficiency by decreasing the expression of Rad51 and ataxia telangiectasia mutated (ATM) kinase [
57]. HR and non-homologous end joining (NHEJ) are the two principal pathways in DSB repair, and the latter plays a dominant role in IR-induced DDR [
55,
58]. The DNA recombinase Rad51 is a pivotal component of the HR pathway, and the fraction of Rad51 foci increases during the HR process [
47]. ATM kinase, the upstream regulator of Rad51, is activated to enhance HR in response to DSBs [
55,
57]. In UT-SCC-24A (HNSCC) cells, palbociclib combined with IR reduced the expression of Rad51 approximately 3.5-fold [
47]. Palbociclib also impaired ATM kinase activation by reducing phosphorylation of ATM kinase and its downstream targets with 10 Gy irradiation in both HCC and CCA cells [
45]. However, impairment of the HR pathway appeared to be dependent on a functional p53 or RB status. Abemaciclib and IR decreased the formation of Rad51 foci markedly in H460 (p53-proficient) cells. Nevertheless, no significant changes were observed in H1299 (p53-deficient) cells [
44]. A possible explanation was that p53 is a key effector in IR-induced cellular response and the lack of p53 was associated with increased radioresistance [
59]. This hypothesis was confirmed by Fernández-Aroca et al. [
49], who reported that p53 was a critical determinant of palbociclib-associated radiosensitivity. Dean et al. [
37] shared similar findings in breast cancer given that the response of Rad51 to palbociclib with IR presented in an RB-dependent manner. Palbociclib pretreatment led to complete inhibition of Rad51 foci accumulation in MDA-MB-231 and Hs578t (RB-proficient) cells but not in MDA-MB-468 (RB-deficient) cells. Furthermore, 500 nM palbociclib caused an approximately 60% decrease in HR-mediated DSB repair and 2.5-fold increase in relative NHEJ activity, indicating that CDK4/6 inhibition augmented NHEJ efficiency [
37]. However, current data are not available to demonstrate that CDK4/6 inhibitors with IR enhance NHEJ activity while weakening the HR efficiency. More studies are currently underway to elucidate this relationship.
Enhancement of apoptosis
As mentioned above, radiotherapy induces massive DSBs. As the intracellular stimulus, IR-induced DNA damage mediates most radiosensitivity-associated pro-apoptotic effects [
60]. Several preclinical studies investigated apoptotic changes in combined therapy. Huang et al. [
45] showed that 8 Gy irradiation plus 20 μM palbociclib resulted in remarkably increased DNA fragmentations (a hallmark of apoptosis) in HCC and CCA cells compared with monotherapy. Another study also reached similar conclusions in NPC cells, demonstrating that palbociclib treatment after irradiation prominently elevated the proportion of apoptotic cells in comparison with IR alone (CNE-1, 19.6% versus 10.685%; CNE-2, 21.655% versus 12.635%). Mechanistically, they demonstrated that the combination augmented the mitochondrial reactive oxygen species (ROS) level, which is regarded as an apoptosis mediator in radiotherapy or chemotherapy, thus enhancing apoptosis [
46]. Interestingly, Hagen et al. [
61] reported that palbociclib induced anti-apoptosis effects in irradiated MCF10A (normal human mammary epithelial cell) and MDA-MB-231 (triple-negative breast cancer, TNBC) cells by reducing cleaved PARP levels. The reason why the investigators reached the opposite conclusion remains unclear, but the radiotherapy-drug combination strategy and the p53 status of different cell lines could be an explanation.
Blockade of cell cycle progression
Another critical determinant of radiosensitivity is cell cycle arrest since CDK4/6 inhibitors induce significant G
1-S arrest [
11]. Meanwhile, G
2-M phase cells are the most sensitive to radiation [
4]. Tai et al. [
48] found that 4 Gy irradiation in combination with ribociclib caused evident G
1-S arrest in HNSCC cells as the ratio of OML1 cells in the G
1 phase increased from 48.6 to 69.4% after treatment and similar effects were observed in radioresistant OML1-R cells. Xie et al. [
46] demonstrated that concurrent palbociclib with IR and radiotherapy followed by palbociclib in NPC cells conspicuously increased the G
2-M cell proportion and decreased radioresistant G
1 cells, suggesting that the combination therapy also caused G
2-M arrest. Furthermore, the combined regimens not only blocked G
1-S and G
2-M checkpoint but also suppressed mitosis. Gottgens et al. [
47] investigated the changes in phosphorylated histone 3 (p-H3) at Ser10 in UT-SCC-24A (HNSCC) cells in response to IR and palbociclib. Phospho-H3 (Ser10) is associated with chromosome condensation, which is a major event during mitosis [
62]. After IR alone, p-H3 (Ser10) levels dropped rapidly and returned to normal at 24 h. In contrast, p-H3 (Ser10) depleted quickly without rebound after combing palbociclib and IR, indicating a decreased number of mitotic cells and inhibition of mitosis. Intriguingly, Hagen et al. [
61] reported that knockdown of CDK4 did not affect cell cycle as no marked changes in cell cycle distribution were observed after IR in shCDK4 cells. A possible explanation is that silencing CDK4 and CDK4/6 inhibitors display different outcomes in terms of cell cycle progression. In general, CDK4/6 inhibitors in combination with IR provide a strong blockade of cell cycle progression, which is a critical mechanism of radiosensitivity.
Other potential mechanisms
In addition to the classic radiosensitization mechanisms mentioned above, CDK4/6 inhibitors also induce other biological phenotypes, providing mechanistic foundations for the combination of CDK4/6 inhibitors and radiotherapy.
First, long-term exposure to CDK4/6 inhibitors induces the cellular senescence phenotype in many cancer cell lines, such as breast cancer [
63], neuroblastoma [
64], and melanoma [
65]. It is not surprising as CDK4/6 inhibitors play a similar role to p16
Ink4a, and the p16
Ink4a-RB pathway is one of the most important mechanisms of cellular senescence [
66,
67]. Persistent CDK4/6 inhibition suppresses RB phosphorylation and downstream transcription activities to induce an irreversible arrest of cell proliferation, which is also known as cellular senescence [
68,
69]. After 8 days exposure of palbociclib, the fraction of senescence-associated β-galactosidase (SA-β-gal)-positive cells was significantly increased in 1205Lu (melanoma) cells [
65]. Ribociclib also yielded similar results in neuroblastoma cell lines [
64]. On the other hand, radiotherapy triggers premature senescence in solid tumor cell lines [
70]. This finding is attributed to the fact that IR causes massive lesions in DNA, therefore activates the ATM-Chk2-p53-p21 axis (senescence-associated DDR pathway), leading to persistent cell cycle arrest and cellular senescence [
68]. In addition, 8 or 10 Gy irradiation accelerated cellular senescence in TP53 wild-type tumor cell lines based on increased SA-β-gal positivity [
70,
71]. Accordingly, radiotherapy and CDK4/6 inhibitors may synergistically induce tumor cell senescence and further inhibit tumor progression.
Senescent cells secrete inflammatory cytokines, chemokines, and growth factors, which collectively comprise the the senescence-associated secretory phenotype (SASP) [
67,
68]. After treatment with palbociclib for 8 days, SASP components, such as interleukin-6 (IL-6), interleukin-8 (IL-8), and chemokine (C-X-C motif) ligand 1 (CXCL1), secreted by 1205Lu cells substantially increased [
65]. Furthermore, radiation triggers the release of cytoplasmic DNA and activates the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway, which plays a pivotal role in SASP production [
72,
73]. Thus, the combination of CDK4/6 inhibitors and radiation could theoretically induce SASP, which affects the immune response [
72]. On one hand, SASP recruits immune cells to stimulate the adaptive immune response and eliminate senescent tumor cells. On the other hand, SASP also attracts immunosuppressive cells and creates a protumorigenic environment [
72]. Although SASP is usually regarded as a “double-edged sword”, it still provides a novel perspective for radiosensitization mechanisms. Thus, the combination of CDK4/6 inhibitors and radiotherapy may exert radiosensitization effects through immunomodulation.
Indeed, several investigators have demonstrated that CDK4/6 inhibitors exhibited direct immunostimulatory effects in both tumor and immune cells [
74]. In tumor cells, CDK4/6 inhibitors suppressed the RB-E2F-DNMT1 axis, which activated endogenous retroviral elements and increased double-stranded RNA levels. This action subsequently induced a type III interferon response and upregulated tumor antigen presentation [
75]. On the other hand, radiotherapy enhances MHC class I expression by activating the mTOR pathway [
76]. Moreover, radiation elicits activation of dendritic cells (DCs) and enhances cross-presentation of tumor antigens [
73,
76]. Gupta et al. [
77] reported that the expression of CD70 and CD86 (co-stimulatory molecules) on DCs was significantly increased after 10 Gy irradiation. In immune cells, CDK4/6 inhibitors promoted T cell activation via enhancing nuclear factor of activated T cells (NFAT) transcriptional activity and interleukin-2 (IL-2) production [
78,
79]. On the other hand, radiation-induced T cell activation has been demonstrated in several preclinical studies, which is mediated by the induction of viral mimicry and activation of the cGAS-STING pathway [
80‐
82]. Although IR recruits regulatory T cells (Treg) and other immunosuppressive cells to the tumor microenvironment, CDK4/6 inhibitors markedly reduce the proliferation of Tregs [
75]. Collectively, CDK4/6 inhibitors and radiotherapy may synergistically exert an anti-tumor immune response by enhancing antigen presentation capacity and T cell activation. These potential mechanisms offer new perspectives for future exploration. The combination of CDK4/6 inhibitors and radiation may not only improve local tumor control but also enhance systemic disease control, providing the possibility for the triplet combination of CDK4/6 inhibitors, radiotherapy and immunotherapy.