Functional inactivation of the retinoblastoma protein requires sequential modification by at least two distinct cyclin-cdk complexes

Role of specific CDKs in regulating DNA damage repair responses and replication stress

Cyclins along with their catalytic units, Cyclin-dependent kinases (CDKs) regulate the cell cycle transition and transcription; and are essentially known as 'master regulators' in modulating DNA damage response (DDR) and replication stress. In addition to influencing DNA repair and damage signaling, CDKs also play a pivotal role in cell division fidelity and the maintenance of genomic integrity after DNA damage. In this review, we focus on the intricate ways by which specific CDKs mainly CDK7, CDK9, and CDK12/13, regulate the cell cycle progression and transcription and how their modulation can lead to lethal effects on the integrity of the genome. With a better knowledge of how these CDKs control the DDR and replication stress, it is now possible to combine CDK inhibitors with chemotherapeutic drugs that damage DNA in ways that can be applied in clinical settings as successful therapeutic strategies.

An image-based screen for secreted proteins involved in breast cancer G0 cell cycle arrest

Secreted proteins regulate the balance between cellular proliferation and G0 arrest and therefore play important roles in tumour dormancy. Tumour dormancy presents a significant clinical challenge for breast cancer patients, where non-proliferating, G0-arrested cancer cells remain at metastatic sites, below the level of clinical detection, some of which can re-enter proliferation and drive tumour relapse. Knowing which secreted proteins can regulate entry into and exit from G0 allows us to manipulate their signalling to prevent tumour relapse. To identify novel secreted proteins that can promote breast cancer G0 arrest, we performed a secretome-wide, image-based screen for proteins that increase the fraction of cells in G0 arrest. From a secretome library of 1282 purified proteins, we identified 29 candidates that promote G0 arrest in non-transformed and transformed breast epithelial cells. The assay we have developed can be adapted for use in other perturbation screens in other cell types. All datasets have been made available for re-analysis and our candidate proteins are presented for alternative bioinformatic refinement or further experimental follow up.

Cancer takes many paths through G1/S

In the commonly accepted paradigm for control of the mammalian cell cycle, sequential cyclin-dependent kinase (CDK) and cyclin activities drive the orderly transition from G1 to S phase. However, recent studies using different technological approaches and examining a broad range of cancer cell types are challenging this established paradigm. An alternative model is evolving in which cell cycles utilize different drivers and take different trajectories through the G1/S transition. We are discovering that cancer cells in particular can adapt their drivers and trajectories, which has important implications for antiproliferative therapies. These studies have helped to refine an understanding of how CDK inhibition impinges on proliferation and have significance for understanding fundamental features of cell biology and cancer.

Plexiform fibrohistiocytic tumor: A series of 10 case studies

OBJECTIVES

We sought to investigate the clinicopathologic features and differential diagnosis of plexiform fibrohistiocytic tumor (PFHT) and its pathogenesis.

METHODS

Ten cases of PFHT were collected from Xi Jing Hospital, Fourth Military Medical University, from September 2008 to December 2022 for clinical data as well as microscopic and immunohistochemical observation. CCND1 gene amplification and break were assayed by fluorescence in situ hybridization (FISH).

RESULTS

We report 10 cases of PFHT according to histologic classification. Seven cases were of histiocytoid type, and 3 had mucous degeneration in the nodules. One case was of fibroblastic type, which was mainly composed of fibroblast-like cells. Two cases were of mixed type. Immunohistochemically, the osteoclast-like multinucleated giant cells, histiocyte-like cells, and occasional spindle cells in the adjacent fascicles were reactive for CD68 (10/10), CD163 (5/8), CD10 (8/8), cyclin D1 (8/8), CDK4 (5/8), β-catenin (4/6), MITF (2/6), and PGP9.5 (4/5). Vimentin (9/9) was strongly positive in tumor cells and peripheral fibroblast-like cells. The positive index of Ki-67 was 5% to 40%, with an average of 20%. The FISH analysis showed neither amplification nor break of the CCND1 gene. All cases underwent surgical resection, and patients were followed up for 9 months to 11 years. Only 2 cases recurred.

CONCLUSIONS

Plexiform fibrohistiocytic tumor is a low-grade malignant soft tissue neoplasm. The diagnosis mainly depends on histopathologic and immunohistochemical markers. Cyclin D1 and CD10 expression has diagnostic value for the diagnosis and differential diagnosis of PFHT combined with its plexiform morphology. The overexpression of cyclin D1 suggests an involvement of cell cycle regulatory genes in the pathogenesis of PFHT.

Circular RNAs in Cell Cycle Regulation of Cancers

Cancer has been one of the most problematic health issues globally. Typically, all cancers share a common characteristic or cancer hallmark, such as sustaining cell proliferation, evading growth suppressors, and enabling replicative immortality. Indeed, cell cycle regulation in cancer is often found to be dysregulated, leading to an increase in aggressiveness. These dysregulations are partly due to the aberrant cellular signaling pathway. In recent years, circular RNAs (circRNAs) have been widely studied and classified as one of the regulators in various cancers. Numerous studies have reported that circRNAs antagonize or promote cancer progression through the modulation of cell cycle regulators or their associated signaling pathways, directly or indirectly. Mostly, circRNAs are known to act as microRNA (miRNA) sponges. However, they also hold additional mechanisms for regulating cellular activity, including protein binding, RNA-binding protein (RBP) recruitment, and protein translation. This review will discuss the current knowledge of how circRNAs regulate cell cycle-related proteins through the abovementioned mechanisms in different cancers.

Design, synthesis and evaluation of thieno[3,2-d]pyrimidine derivatives as novel potent CDK7 inhibitors

The targeting of cyclin-dependent kinase 7 (CDK7) has become a highly desirable therapeutic approach in the field of oncology due to its dual role in regulating essential biological processes, encompassing cell cycle progression and transcriptional control. We have previously identified a highly selective thieno[3,2-d]pyrimidine-based CDK7 inhibitor with demonstrated efficacy and safety in animal model. In this study, we sought to optimize the thieno[3,2-d]pyrimidine core to discover a novel series of CDK7 inhibitors with improved potency and pharmacokinetic (PK) properties. Through extensive structure-activity relationship (SAR) studies, compound 20 has emerged as the lead candidate due to its potent inhibitory activity against CDK7 and remarkable efficacy on MDA-MB-453 cells, a representative triple negative breast cancer (TNBC) cell line. Furthermore, 20 has demonstrated favorable oral bioavailability and exhibited highly desirable pharmacokinetic (PK) properties, making it a promising lead candidate for further structural optimization.

A narrative review on therapeutic potential of naringenin in colorectal cancer: Focusing on molecular and biochemical processes

Colorectal cancer (CRC) is a common and highly metastatic cancer affecting people worldwide. Drug resistance and unwanted side effects are some of the limitations of current treatments for CRC. Naringenin (NAR) is a naturally occurring compound found in abundance in various citrus fruits such as oranges, grapefruits, and tomatoes. It possesses a diverse range of pharmacological and biological properties that are beneficial for human health. Numerous studies have highlighted its antioxidant, anticancer, and anti-inflammatory activities, making it a subject of interest in scientific research. This review provides a comprehensive overview of the effects of NAR on CRC. The study's findings indicated that NAR: (1) interacts with estrogen receptors, (2) regulates the expression of genes related to the p53 signaling pathway, (3) promotes apoptosis by increasing the expression of proapoptotic genes (Bax, caspase9, and p53) and downregulation of the antiapoptotic gene Bcl2, (4) inhibits the activity of enzymes involved in cell survival and proliferation, (5) decreases cyclin D1 levels, (6) reduces the expression of cyclin-dependent kinases (Cdk4, Cdk6, and Cdk7) and antiapoptotic genes (Bcl2, x-IAP, and c-IAP-2) in CRC cells. In vitro CDK2 binding assay was also performed, showing that the NAR derivatives had better inhibitory activities on CDK2 than NAR. Based on the findings of this study, NAR is a potential therapeutic agent for CRC. Additional pharmacology and pharmacokinetics studies are required to fully elucidate the mechanisms of action of NAR and establish the most suitable dose for subsequent clinical investigations.

Preclinical evaluation of CDK4 phosphorylation predicts high sensitivity of pleural mesotheliomas to CDK4/6 inhibition

Malignant pleural mesothelioma (MPM) is an aggressive cancer with limited therapeutic options. We evaluated the impact of CDK4/6 inhibition by palbociclib in 28 MPM cell lines including 19 patient-derived ones, using various approaches including RNA-sequencing. Palbociclib strongly and durably inhibited the proliferation of 23 cell lines, indicating a unique sensitivity of MPM to CDK4/6 inhibition. When observed, insensitivity to palbociclib was mostly explained by the lack of active T172-phosphorylated CDK4. This was associated with high p16INK4A (CDKN2A) levels that accompany RB1 defects or inactivation, or (unexpectedly) CCNE1 overexpression in the presence of wild-type RB1. Prolonged palbociclib treatment irreversibly inhibited proliferation despite re-induction of cell cycle genes upon drug washout. A senescence-associated secretory phenotype including various potentially immunogenic components was irreversibly induced. Phosphorylated CDK4 was detected in 80% of 47 MPMs indicating their sensitivity to CDK4/6 inhibitors. Its absence in some highly proliferative MPMs was linked to very high p16 (CDKN2A) expression, which was also observed in public datasets in tumours from short-survival patients. Our study supports the evaluation of CDK4/6 inhibitors for MPM treatment, in monotherapy or combination therapy.

Research progress on the pharmacological mechanism, in vivo metabolism and structural modification of Erianin

Erianin is an important bibenzyl compound in dendrobium and has a wide spectrum of pharmacological properties. Since Erianin was discovered, abundant results have been achieved in the in vitro synthesis, structural modification, and pharmacological mechanism research. Researchers have developed a series of simple and efficient in vitro synthesis methods to improve the shortcomings of poor water solubility by replacing the chemical structure or coating it in nanomaterials. Erianin has a broad anti-tumor spectrum and significant anti-tumor effects. In addition, Erianin also has pharmacological actions like immune regulation, anti-inflammatory, and anti-angiogenesis. A comprehensive understanding of the synthesis, metabolism, structural modification, and pharmacological action pathways of Erianin is of great value for the utilization of Erianin. Therefore, this review conducts a relatively systematic look back at Erianin from the above four aspects, to give a reference for the evolvement and further appliance of Erianin.

Molecular mechanisms underlying the regulation of tumour suppressor genes in lung cancer

Tumour suppressor genes play a cardinal role in the development of a large array of human cancers, including lung cancer, which is one of the most frequently diagnosed cancers worldwide. Therefore, extensive studies have been committed to deciphering the underlying mechanisms of alterations of tumour suppressor genes in governing tumourigenesis, as well as resistance to cancer therapies. In spite of the encouraging clinical outcomes demonstrated by lung cancer patients on initial treatment, the subsequent unresponsiveness to first-line treatments manifested by virtually all the patients is inherently a contentious issue. In light of the aforementioned concerns, this review compiles the current knowledge on the molecular mechanisms of some of the tumour suppressor genes implicated in lung cancer that are either frequently mutated and/or are located on the chromosomal arms having high LOH rates (1p, 3p, 9p, 10q, 13q, and 17p). Our study identifies specific genomic loci prone to LOH, revealing a recurrent pattern in lung cancer cases. These loci, including 3p14.2 (FHIT), 9p21.3 (p16INK4a), 10q23 (PTEN), 17p13 (TP53), exhibit a higher susceptibility to LOH due to environmental factors such as exposure to DNA-damaging agents (carcinogens in cigarette smoke) and genetic factors such as chromosomal instability, genetic mutations, DNA replication errors, and genetic predisposition. Furthermore, this review summarizes the current treatment landscape and advancements for lung cancers, including the challenges and endeavours to overcome it. This review envisages inspired researchers to embark on a journey of discovery to add to the list of what was known in hopes of prompting the development of effective therapeutic strategies for lung cancer.

PR55α-controlled protein phosphatase 2A inhibits p16 expression and blocks cellular senescence induction by γ-irradiation

Cellular senescence is a permanent cell cycle arrest that can be triggered by both internal and external genotoxic stressors, such as telomere dysfunction and DNA damage. The execution of senescence is mainly by two pathways, p16/RB and p53/p21, which lead to CDK4/6 inhibition and RB activation to block cell cycle progression. While the regulation of p53/p21 signaling in response to DNA damage and other insults is well-defined, the regulation of the p16/RB pathway in response to various stressors remains poorly understood. Here, we report a novel function of PR55α, a regulatory subunit of PP2A Ser/Thr phosphatase, as a potent inhibitor of p16 expression and senescence induction by ionizing radiation (IR), such as γ-rays. The results show that ectopic PR55α expression in normal pancreatic cells inhibits p16 transcription, increases RB phosphorylation, and blocks IR-induced senescence. Conversely, PR55α-knockdown by shRNA in pancreatic cancer cells elevates p16 transcription, reduces RB phosphorylation, and triggers senescence induction after IR. Furthermore, this PR55α function in the regulation of p16 and senescence is p53-independent because it was unaffected by the mutational status of p53. Moreover, PR55α only affects p16 expression but not p14 (ARF) expression, which is also transcribed from the same CDKN2A locus but from an alternative promoter. In normal human tissues, levels of p16 and PR55α proteins were inversely correlated and mutually exclusive. Collectively, these results describe a novel function of PR55α/PP2A in blocking p16/RB signaling and IR-induced cellular senescence.

Cell cycle plasticity underlies fractional resistance to palbociclib in ER+/HER2- breast tumor cells

The CDK4/6 inhibitor palbociclib blocks cell cycle progression in Estrogen receptor-positive, human epidermal growth factor 2 receptor-negative (ER+/HER2-) breast tumor cells. Despite the drug's success in improving patient outcomes, a small percentage of tumor cells continues to divide in the presence of palbociclib-a phenomenon we refer to as fractional resistance. It is critical to understand the cellular mechanisms underlying fractional resistance because the precise percentage of resistant cells in patient tissue is a strong predictor of clinical outcomes. Here, we hypothesize that fractional resistance arises from cell-to-cell differences in core cell cycle regulators that allow a subset of cells to escape CDK4/6 inhibitor therapy. We used multiplex, single-cell imaging to identify fractionally resistant cells in both cultured and primary breast tumor samples resected from patients. Resistant cells showed premature accumulation of multiple G1 regulators including E2F1, retinoblastoma protein, and CDK2, as well as enhanced sensitivity to pharmacological inhibition of CDK2 activity. Using trajectory inference approaches, we show how plasticity among cell cycle regulators gives rise to alternate cell cycle "paths" that allow individual tumor cells to escape palbociclib treatment. Understanding drivers of cell cycle plasticity, and how to eliminate resistant cell cycle paths, could lead to improved cancer therapies targeting fractionally resistant cells to improve patient outcomes.

Anticancer Effect of Theranekron® on Androgen-Dependent Prostate Cancer Cells

Objectives

Prostate cancer (PCa) is a significant health problem in men worldwide. Although there are numerous treatment choices for PCa, acquired resistance limits treatment success. Therefore, there is a need for new approaches as powerful resources for use in alternative or supportive therapeutic strategies for anticancer therapeutics. Theranekron® is a commercially available alcoholic extract of Tarantula cubensis. Recent studies have shown the potent anticancer effect of theranekron in human tumors, including PCa. Herein, we comparatively examined the antiproliferative activity of theranekron and its biochemical action on androgenic signaling and cell cycle-related cyclin proteins in androgen-dependent PCa cells, LNCaP, VCaP, and 22Rv1.

Materials and Methods

Human androgen-dependent PCa cells, LNCaP (CRL-1740TM), 22Rv1 (CRL-2505TM), and VCaP (CRL-2876TM) were used to evaluate the effect of theranekron in vitro. The impact of theranekron on cell viability was evaluated using a WST-1-based viability test. Its impact on AR, cyclin A2, cyclin B1, and cyclin E1 was examined by immunoblotting. To test the anti-malignant effect of theranekron on 3D tumor formation of PCa cells, soft agar assay was used.

Results

Our results indicated that theranekron treatment significantly reduced the viability of PCa cells. It remarkably decreased the protein levels of AR, cyclin A2, cyclin B1, and cyclin E1 in a dose-dependent manner. In addition, Theranekron administration strongly limited the 3D tumor formation of LNCaP, 22Rv1, and VCaP cells.

Conclusion

Our findings strongly suggest that theranekron may offer potent therapeutic efficacy against androgen-dependent PCa cells. Moreover, it may be a potent component for preventing acquired resistance to chemotherapeutics.

Discovery and optimization of thieno[3,2-d]pyrimidine derivatives as highly selective inhibitors of cyclin-dependent kinase 7

Targeting cyclin-dependent kinase 7 (CDK7) has emerged as a highly sought-after therapeutic strategy in oncology due to its duality of function in regulating biological processes, including cell cycle progression and transcriptional control. Herein, we describe the design, optimization and characterization of a series of thieno[3,2-d]pyrimidine derivatives as potent CDK7 inhibitors. The involvement of thiophene as core structure plays critical role in leading to the remarkable selectivity and incorporation of a fluorine atom into the piperidine ring enhances metabolic stability. Structure-activity relationship (SAR) study generated compound 36 as lead compound with potent inhibitory activity against CDK7 and good kinome selectivity in vitro. Compound 36 demonstrated strong efficacy against a triple negative breast cancer (TNBC) cell line-derived xenograft (CDX) mouse model upon oral administration at 5 mg/kg once daily. Therefore, it exhibits immense potential as a lead candidate for further exploration in the development of cancer therapy.

The Potent Novel CDK4/6 Inhibitor TQB3616 in Hormone Receptor Positive Breast Cancer: Preclinical Characterization with in vitro and Human Tumor Xenograft Models

PURPOSE

Inhibition of the cyclin-dependent kinase (CDK) 4/6-retinoblastoma (RB) pathway exerts a considerable inhibitory effect, preventing the spread and metastasis of breast cancer cells and promoting tumor regression. In this study, we examined the antitumor activity of TQB3616, a novel inhibitor of CDK4/6 activity, which showed a greater efficacy improvement in antitumor effects.

METHODS

TQB3616 group, abemaciclib group and endocrine or HER-2 targeted combination therapy group were set up respectively. The effects of drugs on cell proliferation activity, cell cycle, apoptosis, downstream protein expression and gene expression of HR positive (T47D, MCF-7) and HER-2 positive (BT474, MDA-MB-361) breast cancer cell lines were studied. The antiproliferative effect of TQB3616 was also measured in vivo.

RESULTS

TQB3616 showed a remarkable inhibitory effect on the proliferation of hormone receptor-positive breast cancer cells in vitro. In addition, TQB3616 combined with endocrine therapy or Human Epidermal Growth Factor Receptor 2 (HER2) targeted therapy showed significant synergistic antitumor activity in estrogen receptor (ER)-positive/HER2-negative or HER2-positive breast cancer. In contrast to abemaciclib, which targets the CDK4/6 pathway with proven efficacy, the oral agent TQB3616 not only induced G1 stalling, leading to a profound reduction in the level of RB protein phosphorylated at Ser807/811, but also showed enhanced tumor killing effects by promoting cell apoptosis. Oral administration of TQB3616 showed more potent antitumor activity than abemaciclib in an in vitro breast cancer xenograft model, causing significant tumor regression associated with sustained target inhibition in tumor tissue and manageable in vivo toxicity.

CONCLUSION

The results of this study indicate that TQB3616 is a novel CDK4/6 inhibitor, and its highly effective antitumor activity against breast cancer is expected to yield promising therapeutic effects in clinical studies.

The promise of combining CDK4/6 inhibition with hormonal therapy in the first-line treatment setting for metastatic or recurrent endometrial adenocarcinoma

Metastatic or recurrent endometrioid adenocarcinoma of the uterine corpus is often incurable with limited treatment options. First-line treatment often includes cytotoxic chemotherapy, which incurs significant toxicities for many patients. Endometrial cancer, specifically endometrioid cancer, is a hormone-sensitive disease and, while single-agent hormonal therapies have demonstrated clinical benefit, resistance to these agents often leads to the use of chemotherapy. There is a lack of approved endocrine treatment options in the metastatic setting for most recurrent endometrial cancers, representing an unmet clinical need. Emerging evidence suggests that hormonal therapy in combination with other targeted treatments, such as cyclin dependent kinase (CDK)4/6 inhibitors, is well tolerated and effective in select patient populations. We discuss the clinical evidence suggesting that the combination of CDK4/6 inhibitors and hormonal therapy has the potential to represent an important addition to the first-line treatment options for patients with low-grade advanced or recurrent endometrial cancer.

Growing and dividing: how O-GlcNAcylation leads the way

Cell cycle errors can lead to mutations, chromosomal instability, or death; thus, the precise control of cell cycle progression is essential for viability. The nutrient-sensing posttranslational modification, O-GlcNAc, regulates the cell cycle allowing one central control point directing progression of the cell cycle. O-GlcNAc is a single N-acetylglucosamine sugar modification to intracellular proteins that is dynamically added and removed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), respectively. These enzymes act as a rheostat to fine-tune protein function in response to a plethora of stimuli from nutrients to hormones. O-GlcNAc modulates mitogenic growth signaling, senses nutrient flux through the hexosamine biosynthetic pathway, and coordinates with other nutrient-sensing enzymes to progress cells through Gap phase 1 (G1). At the G1/S transition, O-GlcNAc modulates checkpoint control, while in S Phase, O-GlcNAcylation coordinates the replication fork. DNA replication errors activate O-GlcNAcylation to control the function of the tumor-suppressor p53 at Gap Phase 2 (G2). Finally, in mitosis (M phase), O-GlcNAc controls M phase progression and the organization of the mitotic spindle and midbody. Critical for M phase control is the interplay between OGT and OGA with mitotic kinases. Importantly, disruptions in OGT and OGA activity induce M phase defects and aneuploidy. These data point to an essential role for the O-GlcNAc rheostat in regulating cell division. In this review, we highlight O-GlcNAc nutrient sensing regulating G1, O-GlcNAc control of DNA replication and repair, and finally, O-GlcNAc organization of mitotic progression and spindle dynamics.

Broad-spectrum kinome profiling identifies CDK6 upregulation as a driver of lenvatinib resistance in hepatocellular carcinoma

Increasing evidence has demonstrated that drug resistance can be acquired in cancer cells by kinase rewiring, which is an obstacle for efficient cancer therapy. However, it is technically challenging to measure the expression of protein kinases on large scale due to their dynamic range in human proteome. We employ a lysine-targeted sulfonyl fluoride probe, named XO44, which binds to 133 endogenous kinases in intact lenvatinib-resistant hepatocellular carcinoma (HCC) cells. This analysis reveals cyclin-dependent kinase 6 (CDK6) upregulation, which is mediated by ERK/YAP1 signaling cascade. Functional analyses show that CDK6 is crucial in regulation of acquired lenvatinib resistance in HCC via augmentation of liver cancer stem cells with clinical significance. We identify a noncanonical pathway of CDK6 in which it binds and regulates the activity of GSK3β, leading to activation of Wnt/β-catenin signaling. Consistently, CDK6 inhibition by palbociclib or degradation by proteolysis targeting chimeras (PROTACs) is highly synergistic with lenvatinib in vitro. Interestingly, palbociclib not only exerts maximal growth suppressive effect with lenvatinib in lenvatinib-resistant HCC models but also reshapes the tumor immune microenvironment. Together, we unveil CDK6 as a druggable target in lenvatinib-resistant HCC and highlight the use of a chemical biology approach to understand nongenetic resistance mechanisms in cancer.

Immunological Phenotyping of Mice with a Point Mutation in Cdk4

Cyclin-dependent kinases (CDKs) play a crucial role in regulation of the mammalian cell cycle. CDK4 and CDK6 control the G1/S restriction checkpoint through their ability to associate with cyclin D proteins in response to growth factor signals. CDK4 deficiency in mice gives rise to a range of endocrine-specific phenotypes including diabetes, infertility, dwarfism, and atrophy of the anterior pituitary. Although CDK6 deficiency can cause thymic atrophy due to a block in the double-negative (DN) to double-positive (DP) stage of T cell development, there are no overt defects in immune cell development reported for CDK4-deficient mice. Here, we examined the impact of a novel N-ethyl-N-nitrosourea-induced point mutation in the gene encoding CDK4 on immune cell development. Mutant mice (Cdk4wnch/wnch) showed normal development and differentiation of major immune cell subsets in the thymus and spleen. Moreover, T cells from Cdk4wnch/wnch mice exhibited normal cytokine production in response to in vitro stimulation. However, analysis of the mixed bone marrow chimeras revealed that Cdk4wnch/wnch-derived T cell subsets and NK cells are at a competitive disadvantage compared to Cdk4+/+-derived cells in the thymus and periphery of recipients. These results suggest a possible role for the CDK4wnch mutation in the development of some immune cells, which only becomes apparent when the Cdk4wnch/wnch mutant cells are in direct competition with wild-type immune cells in the mixed bone marrow chimera.

SHCBP1 Promotes the Proliferation of Breast Cancer Cells by Inhibiting CXCL2

Breast cancer has the characteristics of high metastasis and recurrence and ranks first in incidence and mortality among female malignant tumors. Shc SH2-domain binding protein 1 (SHCBP1) is an important protein in intracellular signal transduction and cell division, but the role of SHCBP1 in breast cancers remains elusive. Here, we found that SHCBP1 deficiency inhibited the proliferation of breast cancer cells. Mechanistically, SHCPB1 significantly downregulates the mRNA level of CXCL2, which in turn activates the AKT and ERK signaling, while inactivates the p21 and p27 signaling. In addition, overexpression of SHCPB1 downregulates the protein levels of p21 and p27, which could be completely reversed by restoration of CXCL2 expression. Moreover, we analyzed the expression of both SHCPB1 and CXCL2, and found that SHCPB1 is highly expressed in breast cancer cells or tissues from breast cancer patients compared to normal breast cells or adjacent normal tissues, while CXCL2 is lowly expressed in breast cancer cells or tissues. Collectively, our study reveals that SHCBP1 plays an oncogenic role in breast cancer tumorigenesis partially through inhibiting the inflammatory response and ultimately activating the proliferation of breast cancers.

Cyclers' kinases in cell division: from molecules to cancer therapy

Faithful eucaryotic cell division requires spatio-temporal orchestration of multiple sequential events. To ensure the dynamic nature of these molecular and morphological transitions, a swift modulation of key regulatory pathways is necessary. The molecular process that most certainly fits this description is phosphorylation, the post-translational modification provided by kinases, that is crucial to allowing the progression of the cell cycle and that culminates with the separation of two identical daughter cells. In detail, from the early stages of the interphase to the cytokinesis, each critical step of this process is tightly regulated by multiple families of kinases including the Cyclin-dependent kinases (CDKs), kinases of the Aurora, Polo, Wee1 families, and many others. While cell-cycle-related CDKs control the timing of the different phases, preventing replication machinery errors, the latter modulate the centrosome cycle and the spindle function, avoiding karyotypic abnormalities typical of chromosome instability. Such chromosomal abnormalities may result from replication stress (RS) and chromosome mis-segregation and are considered a hallmark of poor prognosis, therapeutic resistance, and metastasis in cancer patients. Here, we discuss recent advances in the understanding of how different families of kinases concur to govern cell cycle, preventing RS and mitotic infidelity. Additionally, considering the growing number of clinical trials targeting these molecules, we review to what extent and in which tumor context cell-cycle-related kinases inhibitors are worth exploiting as an effective therapeutic strategy.

CDK2 regulates collapsed replication fork repair in CCNE1-amplified ovarian cancer cells via homologous recombination

CCNE1 amplification is a common alteration in high-grade serous ovarian cancer and occurs in 15-20% of these tumors. These amplifications are mutually exclusive with homologous recombination deficiency, and, as they have intact homologous recombination, are intrinsically resistant to poly (ADP-ribose) polymerase inhibitors or chemotherapy agents. Understanding the molecular mechanisms that lead to this mutual exclusivity may reveal therapeutic vulnerabilities that could be leveraged in the clinic in this still underserved patient population. Here, we demonstrate that CCNE1-amplified high-grade serous ovarian cancer cells rely on homologous recombination to repair collapsed replication forks. Cyclin-dependent kinase 2, the canonical partner of cyclin E1, uniquely regulates homologous recombination in this genetic context, and as such cyclin-dependent kinase 2 inhibition synergizes with DNA damaging agents in vitro and in vivo. We demonstrate that combining a selective cyclin-dependent kinase 2 inhibitor with a DNA damaging agent could be a powerful tool in the clinic for high-grade serous ovarian cancer.

Identification of Selective ATP-Competitive CMG Helicase Inhibitors for Cancer Intervention that Disrupt CMG-Replisome Function

The human CMG helicase (Cdc45-MCM-GINS) is a novel target for anti-cancer therapy due to tumor-specific weaknesses in CMG function induced by oncogenic changes and the need for CMG function during recovery from replicative stresses such as chemotherapy. Here, we developed an orthogonal biochemical screening approach and identified selective CMG inhibitors (CMGi) that inhibit ATPase and helicase activities in an ATP-competitive manner at low micromolar concentrations. Structure-activity information and in silico docking indicate that CMGi occupy ATP binding sites and channels within MCM subunits leading to the ATP clefts, which are likely used for ATP/ADP ingress or egress. CMGi inhibit cell growth and DNA replication using multiple molecular mechanisms. CMGi block helicase assembly steps that require ATP binding/hydrolysis by the MCM complex, specifically MCM ring assembly on DNA and GINS recruitment to DNA-loaded MCM hexamers. During S-phase, inhibition of MCM ATP binding/hydrolysis by CMGi causes a 'reverse allosteric' dissociation of Cdc45/GINS from the CMG that destabilizes the replisome and disrupts interactions with Ctf4, Mcm10, and DNA polymerase-α, -δ, -ε, resulting in DNA damage. These novel CMGi are selectively toxic toward tumor cells and define a new class of CMG helicase-targeted anti-cancer compounds with distinct mechanisms of action.

Reversible suspension culture of human vascular smooth muscle cells using the functional biopolymer FP003

Human vascular smooth muscle cells (SMCs) are adherent cells, and they cannot survive without scaffolds in suspension culture. Here, we aimed to establish a suspension culture of SMCs using the functional biopolymer FP003 and to investigate the proliferation status of the cells. When SMCs were suspension cultured with FP003, their proliferation was inhibited with a viability of 75% until day 15. When SMCs were re-plated on plastic plates after suspension culture with FP003 for 48 h, the SMCs proliferated as in a normal plate culture. The SMCs cultured in suspension with FP003 showed a relatively low phosphorylation of retinoblastoma protein, low expression of cyclin D1, high proportion of G0/G1 phase cells, low proportion of S phase cells, and no obvious signs of apoptosis, indicating that this culture system inhibited progression from the G1 to S phase. This growth arrest was a reversible property that showed no significant changes in the expressions of the marker proteins α-smooth muscle actin and smooth muscle myosin heavy chain. These results suggest that human SMCs can be stably cultured in suspension with FP003 without losing their characteristics when they are cultured on plastic plates again.

A Rare Case of Nodular Mantle Cell Lymphoma of the Gastrointestinal Tract Discovered During a Routine Colonoscopy With a Positive Response to R-CHOP Chemotherapy Regimen

This report describes the case of a 73-year-old female patient who presented with abdominal symptoms. A colonoscopy identified a cecal mass confirmed as mantle cell lymphoma (MCL). Imaging showed extensive lymph node involvement. The patient received rituximab, cyclophosphamide, hydroxydaunorubicin, vincristine, and prednisone (R-CHOP) chemotherapy, resulting in tumor reduction and adenopathy resolution. Despite a typically unfavorable prognosis associated with a high Ki-67 index, the patient responded well to chemotherapy and achieved a favorable outcome. This case highlights the importance of early detection, appropriate treatment which in our case was R-CHOP, and personalized management approaches in addressing MCL.

Hybrid-DIA: intelligent data acquisition integrates targeted and discovery proteomics to analyze phospho-signaling in single spheroids

Achieving sufficient coverage of regulatory phosphorylation sites by mass spectrometry (MS)-based phosphoproteomics for signaling pathway reconstitution is challenging, especially when analyzing tiny sample amounts. To address this, we present a hybrid data-independent acquisition (DIA) strategy (hybrid-DIA) that combines targeted and discovery proteomics through an Application Programming Interface (API) to dynamically intercalate DIA scans with accurate triggering of multiplexed tandem mass spectrometry (MSx) scans of predefined (phospho)peptide targets. By spiking-in heavy stable isotope labeled phosphopeptide standards covering seven major signaling pathways, we benchmark hybrid-DIA against state-of-the-art targeted MS methods (i.e., SureQuant) using EGF-stimulated HeLa cells and find the quantitative accuracy and sensitivity to be comparable while hybrid-DIA also profiles the global phosphoproteome. To demonstrate the robustness, sensitivity, and biomedical potential of hybrid-DIA, we profile chemotherapeutic agents in single colon carcinoma multicellular spheroids and evaluate the phospho-signaling difference of cancer cells in 2D vs 3D culture.

Rapid adaptation to CDK2 inhibition exposes intrinsic cell-cycle plasticity

CDK2 is a core cell-cycle kinase that phosphorylates many substrates to drive progression through the cell cycle. CDK2 is hyperactivated in multiple cancers and is therefore an attractive therapeutic target. Here, we use several CDK2 inhibitors in clinical development to interrogate CDK2 substrate phosphorylation, cell-cycle progression, and drug adaptation in preclinical models. Whereas CDK1 is known to compensate for loss of CDK2 in Cdk2-/- mice, this is not true of acute inhibition of CDK2. Upon CDK2 inhibition, cells exhibit a rapid loss of substrate phosphorylation that rebounds within several hours. CDK4/6 activity backstops inhibition of CDK2 and sustains the proliferative program by maintaining Rb1 hyperphosphorylation, active E2F transcription, and cyclin A2 expression, enabling re-activation of CDK2 in the presence of drug. Our results augment our understanding of CDK plasticity and indicate that co-inhibition of CDK2 and CDK4/6 may be required to suppress adaptation to CDK2 inhibitors currently under clinical assessment.

Overview of the therapeutic strategies for ER positive breast cancer

Estrogen Receptor is the driving transcription factor in about 75% of all breast cancers, which is the target of endocrine therapies, but drug resistance is a common clinical problem. ESR1 point mutations at the ligand binding domain are frequently identified in metastatic tumor and ctDNA (Circulating tumor DNA) derived from ER positive breast cancer patients with endocrine therapies. Although endocrine therapy and CDK4/6 inhibitor therapy have demonstrated preclinical and clinical benefits for breast cancer, the development of resistance remains a significant challenge and the detailed mechanisms, and potential therapeutic targets in advanced breast cancer yet to be revealed. Since a crosstalk between tumor and tumor microenvironment (TME) plays an important role to grow tumor and metastasis, this effect could serve as key regulators in the resistance of endocrine therapy and the transition of breast cancer cells to metastasis. In this article, we have reviewed recent progress in endocrine therapy and the contribution of TME to ER positive breast cancer.

An anti-cancer surveillance by the interplay between interferon-beta and retinoblastoma protein RB1

Interferon-beta (IFN-β), an extracellular cytokine that initiates signaling pathways for gene regulation, has been demonstrated to function as a tumor suppressor protein through lentiviral gene transduction. In this article, I review the relevant previous works and propose a cell cycle-based, tumor suppressor protein-mediated mechanism of anti-cancer surveillance. IFN-β induces a tumor cell cycle alteration that leads to S phase accumulation, senescence entry, and a loss of tumorigenicity in solid tumor cells. IFN-β does not show a significant cell cycle effect in their normal counterparts. Retinoblastoma protein RB1, another tumor suppressor protein, tightly controls the cell cycle and differentiation of normal cells, preventing them from being significantly impacted by the IFN-β effect. The interplay between IFN-β and RB1 acts as a mechanism of cell cycle-based, tumor suppressor protein-mediated anti-cancer surveillance that can selectively suppress solid tumor or proliferating transformed cells from the loss of control leading to cancer. This mechanism has important implications for the treatment of solid tumors.

A mass balance study of [14C]SHR6390 (dalpiciclib), a selective and potent CDK4/6 inhibitor in humans

SHR6390 (dalpiciclib) is a selective and effective cyclin-dependent kinase (CDK) 4/6 inhibitor and an effective cancer therapeutic agent. On 31 December 2021, the new drug application was approved by National Medical Product Administration (NMPA). The metabolism, mass balance, and pharmacokinetics of SHR6390 in 6 healthy Chinese male subjects after a single oral dose of 150 mg [14C]SHR6390 (150 µCi) in this research. The Tmax of SHR6390 was 3.00 h. In plasma, the t1/2 of SHR6390 and its relative components was approximately 17.50 h. The radioactivity B/P (blood-to-plasma) AUC0-t ratio was 1.81, indicating the preferential distribution of drug-related substances in blood cells. At 312 h after administration, the average cumulative excretion of radioactivity was 94.63% of the dose, including 22.69% in urine and 71.93% in stool. Thirteen metabolites were identified. In plasma, because of the low level of radioactivity, only SHR6390 was detected in pooled AUC0-24 h plasma. Stool SHR6390 was the main component in urine and stool. Five metabolites were identified in urine, and 12 metabolites were identified in stool. Overall, faecal clearance is the main method of excretion.

CDK4/6 Inhibitors in Pancreatobiliary Cancers: Opportunities and Challenges

Existing treatment strategies for pancreatobiliary malignancies are limited. Nowadays, surgery is the only path to cure these types of cancer, but only a small number of patients present with resectable tumors at the time of diagnosis. The notoriously poor prognosis, lack of diverse treatment options associated with pancreaticobiliary cancers, and their resistance to current therapies reflect the urge for the development of novel therapeutic targets. Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors have emerged as an attractive therapeutic strategy in a number of cancers since their approval for treatment in patients with ER+/HER- breast cancer in combination with antiestrogens. In this article, we discuss the therapeutic potential of CDK4/6 inhibitors in pancreatobiliary cancers, notably cholangiocarcinoma and pancreatic ductal adenocarcinoma.

The CMG helicase and cancer: a tumor "engine" and weakness with missing mutations

The replicative Cdc45-MCM-GINS (CMG) helicase is a large protein complex that functions in the DNA melting and unwinding steps as a component of replisomes during DNA replication in mammalian cells. Although the CMG performs this important role in cell growth, the CMG is not a simple bystander in cell cycle events. Components of the CMG, specifically the MCM precursors, are also involved in maintaining genomic stability by regulating DNA replication fork speeds, facilitating recovery from replicative stresses, and preventing consequential DNA damage. Given these important functions, MCM/CMG complexes are highly regulated by growth factors such as TGF-ß1 and by signaling factors such as Myc, Cyclin E, and the retinoblastoma protein. Mismanagement of MCM/CMG complexes when these signaling mediators are deregulated, and in the absence of the tumor suppressor protein p53, leads to increased genomic instability and is a contributor to tumorigenic transformation and tumor heterogeneity. The goal of this review is to provide insight into the mechanisms and dynamics by which the CMG is regulated during its assembly and activation in mammalian genomes, and how errors in CMG regulation due to oncogenic changes promote tumorigenesis. Finally, and most importantly, we highlight the emerging understanding of the CMG helicase as an exploitable vulnerability and novel target for therapeutic intervention in cancer.

Therapeutic effects of hAMSCs secretome on proliferation of MDA-MB-231 breast cancer cells by the cell cycle arrest in G1/S phase

BACKGROUND

Cancer refers to a disease resulting from the uncontrolled division and growth of abnormal cells. Among different cancer types, breast cancer is considered as one of the most commonly diagnosed cancers. Herein, we explored the therapeutic effects of human amniotic mesenchymal stromal cells (hAMSCs) secretome on breast cancer cells (MDA-MB-231) through analyzing cell cycle progression.

METHODS

We employed a co-culture system using 6-well Transwell plates and after 72 h, the cell cycle progression was evaluated in the hAMSCs-treated MDA-MB-231 cells through analyzing the expressions of RB, CDK4/6, cyclin D, CDK2, cyclin E, p16^/INK4a, p21^/WAF1/CIP1, and p27^/KIP1 using quantitative real-time PCR (qRT-PCR) and western blot method. Cell proliferation, apoptosis, and cell cycle progression were checked using an MTT assay, DAPI staining, and flow cytometry.

RESULTS

Our results indicated that elevation of RB, p21^/WAF1/CIP1, and p27^/KIP1 and suppression of RB hyperphosphorylation, p16^/INK4a, cyclin E, cyclin D1, CDK2, and CDK4/6 may contribute to inhibiting the proliferation of hAMSCs-treated MDA-MB-231 cells through cell cycle arrest in G1/S phase followed by apoptosis.

CONCLUSION

hAMSCs secretome may be an effective approach on breast cancer therapy through the inhibition of cell cycle progression.

Non-canonical approaches to targeting hypoxic tumors

Hypoxia is a common characteristic in solid cancers. Hypoxia-inducible factors (HIFs) are involved in various aspects of cancer, such as angiogenesis, metastasis and therapy resistance. Targeting the HIF pathway has been regarded as a challenging but promising strategy in cancer treatment with recent FDA approval of a HIF2α-inhibitor. During the past several decades, numerous efforts have been made to understand how HIFs participate in cancer development and progression along with how HIF signaling can be modulated to achieve anti-cancer effect. In this chapter, we will provide an overview of the role of hypoxia and HIFs in cancer, summarize the oxygen-dependent and independent mechanisms of HIF-1α regulation, and discuss emerging approaches targeting hypoxia and HIF signaling which possess therapeutic potential in cancer. We will emphasize on two signaling pathways, involving cyclin-dependent kinases (CDKs) and heat shock protein 90 (HSP90), which contribute to HIF-1α (and HIF-2α) stabilization in an oxygen-independent manner. Through reviewing their participation in malignant progression and the potential targeting strategies, we discuss the non-canonical approaches to target HIF signaling in cancer therapy.

Loss of VOPP1 Contributes to BET Inhibitor Acquired Resistance in Non-Small Cell Lung Cancer Cells

Inhibitors targeting bromodomain and extraterminal (BET) proteins are promising anticancer drugs. The emergence of drug resistance during treatments will impair their therapeutic effectiveness. To investigate the mechanisms of acquired resistance to BET inhibitors (BETi), we generated a series of drug-resistant sublines by exposing non-small cell lung cancer (NSCLC) NCI-H1975 cells to the BETi ABBV-075. These sublines displayed cross-resistance to other tested BETis, increased migration abilities, reduced growth rates accompanied by an increased proportion of cells in G1 phase and decreased apoptotic responses to BETis. Changes in RNA expression and gene mutation profiles in the resistant variants indicate that emergence of BETi resistance is multifactorial. Importantly, all the tested ABBV-075-resistant variants showed loss of vesicular overexpressed in cancer prosurvival protein 1 (VOPP1) and an increase in the antiapoptotic BCL-2 protein. By knockdown, knockout, and reconstitution of VOPP1 in resistant cells, their parental cells, and other NSCLC cells, we confirmed that the loss of VOPP1 contributed to BETi resistance. Moreover, knockout of VOPP1 in the parental cells caused the increased expression of BCL-2, and the latter directly mediated BETi resistance. Through combined treatments with BETis and BCL-2 inhibitors (BCL-2i), we demonstrated that BCL-2is synergistically sensitized resistant cells to BETis.

IMPLICATIONS

Based on these results, for the first time, we establish a causal link from VOPP1 loss to BCL-2 gain and then to BETi resistance, which provides new insights into BETi resistance and paves the way for further testing to circumvent BETi resistance.

Regulation of Cyclin D1 Degradation by Ubiquitin-Specific Protease 27X Is Critical for Cancer Cell Proliferation and Tumor Growth

Cyclin D1 (CCND1) is a critical regulator of cell proliferation and its overexpression has been linked to the development and progression of several malignancies. CCND1 overexpression is recognized as a major mechanism of therapy resistance in several cancers; tumors that rely on CCND1 overexpression to evade cancer therapy are extremely sensitive to its ablation. Therefore, targeting CCND1 is a promising strategy for preventing tumor progression and combating therapy resistance in cancer patients. Although CCND1 itself is not a druggable target, it can be targeted indirectly by inhibiting its regulators. CCND1 steady-state levels are tightly regulated by ubiquitin-mediated degradation, and defects in CCND1 ubiquitination are associated with increased CCND1 protein levels in cancer. Here, we uncover a novel function of ubiquitin-specific protease 27X (USP27X), a deubiquitinating enzyme (DUB), in regulating CCND1 degradation in cancer. USP27X binds to and stabilizes CCND1 in a catalytically dependent manner by negatively regulating its ubiquitination. USP27X expression levels correlate with the levels of CCND1 in several HER2 therapy-resistant breast cancer cell lines, and its ablation leads to a severe reduction of CCND1 protein levels, inhibition of tumor growth, and resensitization to targeted therapy. Together, the results presented in our study are the first to expose USP27X as a major CCND1 deubiquitinase and provide a mechanistic explanation for how this DUB fosters tumor growth.

IMPLICATIONS

As a deubiquitinating enzyme, USP27X is a druggable target. Our study illuminates new avenues for therapeutic intervention in CCND1-driven cancers.

Cell growth and the cell cycle: New insights about persistent questions

Before a cell divides into two daughter cells, it typically doubles not only its DNA, but also its mass. Numerous studies in cells ranging from yeast to mammals have shown that cellular growth, stimulated by nutrients and/or growth factor signaling, is a prerequisite for cell cycle progression in most types of cells. The textbook view of growth-regulated cell cycles is that growth signaling activates the transcription of G1 Cyclin genes to induce cell proliferation, and also stimulates anabolic metabolism and cell growth in parallel. However, genetic knockout tests in model organisms indicate that this is not the whole story, and new studies show that additional, "smarter" mechanisms help to coordinate the cell cycle with growth itself. Here we summarize recent advances in this field, and discuss current models in which growth signaling regulates cell proliferation by targeting core cell cycle regulators via non-transcriptional mechanisms.

CDK4/6 initiates Rb inactivation and CDK2 activity coordinates cell-cycle commitment and G1/S transition

External signaling controls cell-cycle entry until cells irreversibly commit to the cell cycle to ensure faithful DNA replication. This process is tightly regulated by cyclin-dependent kinases (CDKs) and the retinoblastoma protein (Rb). Here, using live-cell sensors for CDK4/6 and CDK2 activities, we propose that CDK4/6 initiates Rb inactivation and CDK2 activation, which coordinates the timing of cell-cycle commitment and sequential G1/S transition. Our data show that CDK4/6 activation induces Rb inactivation and thereby E2F activation, driving a gradual increase in CDK2 activity. We found that rapid CDK4/6 inhibition can reverse cell-cycle entry until CDK2 activity reaches to high levels. This suggests that high CDK2 activity is required to initiate CDK2-Rb positive feedback and CDK4/6-indpendent cell-cycle progression. Since CDK2 activation also facilitates initiation of DNA replication, the timing of CDK2-Rb positive feedback is coupled with the G1/S transition. Our experiments, which acutely increased CDK2 activity by cyclin E1 overexpression, indicate that cells commit to the cell cycle before triggering DNA replication. Together, our data suggest that CDK4/6 inactivates Rb to begin E2F and CDK2 activation, and high CDK2 activity is necessary and sufficient to generate a bistable switch for Rb phosphorylation before DNA replication. These findings highlight how cells initiate the cell cycle and subsequently commit to the cell cycle before the G1/S transition.

Targeting the Retinoblastoma/E2F repressive complex by CDK4/6 inhibitors amplifies oncolytic potency of an oncolytic adenovirus

CDK4/6 inhibitors (CDK4/6i) and oncolytic viruses are promising therapeutic agents for the treatment of various cancers. As single agents, CDK4/6 inhibitors that are approved for the treatment of breast cancer in combination with endocrine therapy cause G1 cell cycle arrest, whereas adenoviruses induce progression into S-phase in infected cells as an integral part of the their life cycle. Both CDK4/6 inhibitors and adenovirus replication target the Retinoblastoma protein albeit for different purposes. Here we show that in combination CDK4/6 inhibitors potentiate the anti-tumor effect of the oncolytic adenovirus XVir-N-31 in bladder cancer and murine Ewing sarcoma xenograft models. This increase in oncolytic potency correlates with an increase in virus-producing cancer cells, enhanced viral genome replication, particle formation and consequently cancer cell killing. The molecular mechanism that regulates this response is fundamentally based on the reduction of Retinoblastoma protein expression levels by CDK4/6 inhibitors.

Neither CDK4/6 inhibitors nor oncolytic adenoviruses show high efficiency as monotherapy in the treatment of cancer. Authors show here that when combined, CDK4/6 inhibitors deplete Retinoblastoma protein levels, which leads to more efficient virus replication and an increase in oncolytic virus-producing cancer cells and thus to efficient anti-tumor response in mouse xenograft sarcoma models.

Structure-guided design and development of cyclin-dependent kinase 4/6 inhibitors: A review on therapeutic implications

Cyclin-dependent kinase 6 (EC 2.7.11.22) play significant roles in numerous biological processes and triggers cell cycle events. CDK6 controlled the transcriptional regulation. A dysregulated function of CDK6 is linked with the development of progression of multiple tumor types. Thus, it is considered as an effective drug target for cancer therapy. Based on the direct roles of CDK4/6 in tumor development, numerous inhibitors developed as promising anti-cancer agents. CDK4/6 inhibitors regulate the G1 to S transition by preventing Rb phosphorylation and E2F liberation, showing potent anti-cancer activity in several tumors, including HR+/HER2- breast cancer. CDK4/6 inhibitors such as abemaciclib, palbociclib, and ribociclib, control cell cycle, provoke cell senescence, and induces tumor cell disturbance in pre-clinical studies. Here, we discuss the roles of CDK6 in cancer along with the present status of CDK4/6 inhibitors in cancer therapy. We further discussed, how structural features of CDK4/6 could be implicated in the design and development of potential anti-cancer agents. In addition, the therapeutic potential and limitations of available CDK4/6 inhibitors are described in detail. Recent pre-clinical and clinical information for CDK4/6 inhibitors are highlighted. In addition, combination of CDK4/6 inhibitors with other drugs for the therapeutic management of cancer are discussed.

Cancer cell cycle dystopia: heterogeneity, plasticity, and therapy

The mammalian cell cycle has been extensively studied regarding cancer etiology, progression, and therapeutic intervention. The canonical cell cycle framework is supported by a plethora of data pointing to a relatively simple linear pathway in which mitogenic signals are integrated in a stepwise fashion to allow progression through G1/S with coordinate actions of cyclin-dependent kinases (CDK)4/6 and CDK2 on the RB tumor suppressor. Recent work on adaptive mechanisms and intrinsic heterogeneous dependencies indicates that G1/S control of the cell cycle is a variable signaling pathway rather than an invariant engine that drives cell division. These alterations can limit the effectiveness of pharmaceutical agents but provide new avenues for therapeutic interventions. These findings support a dystopian view of the cell cycle in cancer where the canonical utopian cell cycle is often not observed. However, recognizing the extent of cell cycle heterogeneity likely creates new opportunities for precision therapeutic approaches specifically targeting these states.

Reciprocal regulation of p21 and Chk1 controls the Cyclin D1-RB pathway to mediate senescence onset after G2 arrest

Senescence is an irreversible proliferation withdrawal that can be initiated after DNA damage-induced cell cycle arrest in G2 phase to prevent genomic instability. Senescence onset in G2 requires p53 and RB family tumour suppressors, but how they are regulated to convert a temporary cell cycle arrest into a permanent one remains unknown. Here, we show that a previously unrecognised balance between the CDK inhibitor p21 and Chk1 controls D-type cyclin-CDK activity during G2 arrest. In non-transformed cells, p21 activates RB in G2 by inhibiting Cyclin D1-CDK2/CDK4. The resulting G2 exit, which precedes appearance of senescence markers, is associated with a mitotic bypass, Chk1 downregulation and DNA damage foci reduction. In p53/RB-proficient cancer cells, compromised G2 exit correlates with sustained Chk1 activity, delayed p21 induction, untimely Cyclin E1 re-expression and genome reduplication. Conversely, Chk1 depletion promotes senescence by inducing p21 binding to Cyclin D1 and Cyclin E1-CDK complexes and down-regulating CDK6, whereas Chk2 knockdown enables RB phosphorylation and delays G2 exit. In conclusion, p21 and Chk2 oppose Chk1 to maintain RB activity, thus promoting DNA damage-induced senescence onset in G2.

Lessons Learned from Past Cyclin-Dependent Kinase Drug Discovery Efforts

Inhibition of cyclin-dependent kinases (CDKs) has become an effective therapeutic strategy for treating various diseases, especially cancer. Over almost three decades, although great efforts have been made to discover CDK inhibitors, many of which have entered clinical trials, only four CDK inhibitors have been approved. In the process of CDK inhibitor development, many difficulties and misunderstandings have hampered their discovery and clinical applications, which mainly include inadequate understanding of the biological functions of CDKs, less attention paid to pan- and multi-CDK inhibitors, nonideal isoform selectivity of developed selective CDK inhibitors, overlooking the metabolic stability of early discovered CDK inhibitors, no effective resistance solutions, and a lack of available combination therapy and effective biomarkers for CDK therapies. After reviewing the mechanisms of CDKs and the research progress of CDK inhibitors, this perspective summarizes and discusses these difficulties or lessons, hoping to facilitate the successful discovery of more useful CDK inhibitors.

Understanding Retinoblastoma Post-Translational Regulation for the Design of Targeted Cancer Therapies

Simple Summary

Rb1 is a regulator of cell cycle progression and genomic stability. This review focuses on post-translational modifications, their effect on Rb1 interactors, and their role in intracellular signaling in the context of cancer development. Finally, we highlight potential approaches to harness these post-translational modifications to design novel effective anticancer therapies.

Abstract

The retinoblastoma protein (Rb1) is a prototypical tumor suppressor protein whose role was described more than 40 years ago. Together with p107 (also known as RBL1) and p130 (also known as RBL2), the Rb1 belongs to a family of structurally and functionally similar proteins that inhibits cell cycle progression. Given the central role of Rb1 in regulating proliferation, its expression or function is altered in most types of cancer. One of the mechanisms underlying Rb-mediated cell cycle inhibition is the binding and repression of E2F transcription factors, and these processes are dependent on Rb1 phosphorylation status. However, recent work shows that Rb1 is a convergent point of many pathways and thus the regulation of its function through post-translational modifications is more complex than initially expected. Moreover, depending on the context, downstream signaling can be both E2F-dependent and -independent. This review seeks to summarize the most recent research on Rb1 function and regulation and discuss potential avenues for the design of novel cancer therapies.

Sodium salicylate and 5-aminosalicylic acid synergistically inhibit the growth of human colon cancer cells and mouse intestinal polyp-derived cells

As colon cancer is one of the most common cancers in the world, practical prevention strategies for colon cancer are needed. Recently, treatment with aspirin and/or 5-aminosalicylic acid-related agents was reported to reduce the number of intestinal polyps in patients with familial adenomatous polyposis. To evaluate the mechanism of aspirin and 5-aminosalicylic acid for suppressing the colon polyp growth, single and combined effects of 5-aminosalicylic acid and sodium salicylate (metabolite of aspirin) were tested in the two human colon cancer cells with different cyclooxygenase-2 expression levels and intestinal polyp-derived cells from familial adenomatous polyposis model mouse. The combination induced cell-cycle arrest at the G1 phase along with inhibition of cell growth and colony-forming ability in these cells. The combination reduced cyclin D1 via proteasomal degradation and activated retinoblastoma protein. The combination inhibited the colony-forming ability of mouse colonic mucosa cells by about 50% and the colony-forming ability of mouse intestinal polyp-derived cells by about 90%. The expression level of cyclin D1 in colon mucosa cells was lower than that in intestinal polyp-derived cells. These results suggest that this combination may be more effective in inhibiting cell growth of intestinal polyps through cyclin D1 down-regulation.

Cell Cycle Progression and Synchronization: An Overview

The cell cycle is the series of events that take place in a cell that drives it to divide and produce two new daughter cells. Through more than 100 years of efforts by scientists, we now have a much clearer picture of cell cycle progression and its regulation. The typical cell cycle in eukaryotes is composed of the G1, S, G2, and M phases. The M phase is further divided into prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis. Cell cycle progression is mediated by cyclin-dependent kinases (Cdks) and their regulatory cyclin subunits. However, the driving force of cell cycle progression is growth factor-initiated signaling pathways that controls the activity of various Cdk-cyclin complexes. Most cellular events, including DNA duplication, gene transcription, protein translation, and post-translational modification of proteins, occur in a cell-cycle-dependent manner. To understand these cellular events and their underlying molecular mechanisms, it is desirable to have a population of cells that are traversing the cell cycle synchronously. This can be achieved through a process called cell synchronization. Many methods have been developed to synchronize cells to the various phases of the cell cycle. These methods could be classified into two groups: synchronization methods using chemical inhibitors and synchronization methods without using chemical inhibitors. All these methods have their own merits and shortcomings.

Androgens promote vascular endothelial cell proliferation through activation of a ZIP9-dependent inhibitory G protein/PI3K-Akt/Erk/cyclin D1 pathway

While androgens have been reported to mediate cardiovascular endothelial cell proliferation, the potential involvement of membrane androgen receptors (mAR) has not been examined. Here we show ZIP9, a recently characterized mAR, mediates androgen-induced early proliferative events in human umbilical vein endothelial cells (HUVECs). Androgen treatment significantly increased cyclin D1 nuclear localization and proliferation, which were blocked by transfection with siRNA targeting ZIP9 but not the nuclear AR. Testosterone rapidly activated inhibitory G protein signaling, Erk, and Akt, and inhibition of these signaling members abrogated the ZIP9-mediated cyclin D1 and proliferative responses. Erk and Akt modulated cyclin D1 nuclear localization by upregulation of cyclin D1 mRNA and inhibition of GSK-3β activity, respectively. This is the first study to demonstrate a role for ZIP9 in HUVEC proliferation and indicates ZIP9 is a physiologically-relevant androgen receptor in the cardiovascular system that merits further study as a potential therapeutic target for treating cardiovascular disease.

Regulation of Cell Cycle Progression by Growth Factor-Induced Cell Signaling

The cell cycle is the series of events that take place in a cell, which drives it to divide and produce two new daughter cells. The typical cell cycle in eukaryotes is composed of the following phases: G1, S, G2, and M phase. Cell cycle progression is mediated by cyclin-dependent kinases (Cdks) and their regulatory cyclin subunits. However, the driving force of cell cycle progression is growth factor-initiated signaling pathways that control the activity of various Cdk–cyclin complexes. While the mechanism underlying the role of growth factor signaling in G1 phase of cell cycle progression has been largely revealed due to early extensive research, little is known regarding the function and mechanism of growth factor signaling in regulating other phases of the cell cycle, including S, G2, and M phase. In this review, we briefly discuss the process of cell cycle progression through various phases, and we focus on the role of signaling pathways activated by growth factors and their receptor (mostly receptor tyrosine kinases) in regulating cell cycle progression through various phases.

Palbociclib-mediated cell cycle arrest can occur in the absence of the CDK inhibitors p21 and p27

The use of CDK4/6 inhibitors in the treatment of a wide range of cancers is an area of ongoing investigation. Despite their increasing clinical use, there is limited understanding of the determinants of sensitivity and resistance to these drugs. Recent data have cast doubt on how CDK4/6 inhibitors arrest proliferation, provoking renewed interest in the role(s) of CDK4/6 in driving cell proliferation. As the use of CDK4/6 inhibitors in cancer therapies becomes more prominent, an understanding of their effect on the cell cycle becomes more urgent. Here, we investigate the mechanism of action of CDK4/6 inhibitors in promoting cell cycle arrest. Two main models explain how CDK4/6 inhibitors cause G1 cell cycle arrest, which differ in their dependence on the CDK inhibitor proteins p21 and p27. We have used live and fixed single-cell quantitative imaging, with inducible degradation systems, to address the roles of p21 and p27 in the mechanism of action of CDK4/6 inhibitors. We find that CDK4/6 inhibitors can initiate and maintain a cell cycle arrest without p21 or p27. This work clarifies our current understanding of the mechanism of action of CDK4/6 inhibitors and has implications for cancer treatment and patient stratification.