Coming full circle: cyclin-dependent kinases as anti-cancer drug targets

Quantitative Imaging of B1 Cyclin Expression Across the Cell Cycle Using Green Fluorescent Protein Tagging and Epifluorescence

In this article, we report the number of cyclin B1 proteins tagged with enhanced green fluorescent protein (eGFP) in fixed U-2 OS cells across the cell cycle. We use a quantitative analysis of epifluorescence to determine the number of eGFP molecules in a nondestructive way, and integrated over the cell we find 10⁴ to 10⁵ molecules. Based on the measured number of eGFP tagged cyclin B1 proteins, knowledge of cyclin B1 dynamics through the cell cycle, and the cell morphology, we identify the stages of cells in the cell cycle. © 2020 The Authors. Cytometry Part A published by Wiley Periodicals LLC. on behalf of International Society for Advancement of Cytometry.

Unbalanced Growth, Senescence and Aging

Usually, cells balance their growth with their division. Coordinating growth inputs with cell division ensures the proper timing of division when sufficient cell material is available and affects the overall rate of cell proliferation. At a very fundamental level, cellular replicative lifespan-defined as the number of times a cell can divide, is a manifestation of cell cycle control. Hence, control of mitotic cell divisions, especially when the commitment is made to a new round of cell division, is intimately linked to replicative aging of cells. In this chapter, we review our current understanding, and its shortcomings, of how unbalanced growth and division, can dramatically influence the proliferative potential of cells, often leading to cellular and organismal aging phenotypes. The interplay between growth and division also underpins cellular senescence (i.e., inability to divide) and quiescence, when cells exit the cell cycle but still retain their ability to divide.

Compensatory induction of MYC expression by sustained CDK9 inhibition via a BRD4-dependent mechanism

CDK9 is the kinase subunit of positive transcription elongation factor b (P-TEFb) that enables RNA polymerase (Pol) II's transition from promoter-proximal pausing to productive elongation. Although considerable interest exists in CDK9 as a therapeutic target, little progress has been made due to lack of highly selective inhibitors. Here, we describe the development of i-CDK9 as such an inhibitor that potently suppresses CDK9 phosphorylation of substrates and causes genome-wide Pol II pausing. While most genes experience reduced expression, MYC and other primary response genes increase expression upon sustained i-CDK9 treatment. Essential for this increase, the bromodomain protein BRD4 captures P-TEFb from 7SK snRNP to deliver to target genes and also enhances CDK9's activity and resistance to inhibition. Because the i-CDK9-induced MYC expression and binding to P-TEFb compensate for P-TEFb's loss of activity, only simultaneously inhibiting CDK9 and MYC/BRD4 can efficiently induce growth arrest and apoptosis of cancer cells, suggesting the potential of a combinatorial treatment strategy.

Evaluation of cyclin-dependent kinase-like 1 expression in breast cancer tissues and its regulation in cancer cell growth

Cyclin-dependent kinase-like 1 (CDKL1) is a member of cell division control protein 2 (CDC2)-related serine-threonine protein kinase family, and it is likely to occur in malignant tumors, plays an important impact on the progress. This study aimed to evaluate the expression of CDKL1 in breast cancer and regulation in cancer cell growth. In the work, the CDKL1 mRNA level in fresh biopsy tissues from 186 breast cancer patients, with 98 benign tissues as negative control, and CDKL1 protein in 30 paraffin-embedded tissues from primary breast cancer patients were detected by the real-time reverse transcriptase-polymerase chain reaction (RT-PCR) assay and immunohistochemical staining, respectively. The roles of CDKL1 in cell growth were analyzed with CDKL1 short hairpin RNA (shRNA) inhibitor-transfected cells. CDKL1 was overexpressed in breast cancer patients and had a positive detection efficiency of 77% (144/186), which showed statistically significant difference compared with estrogen receptor (ER), progesterone receptor (PR), P53, vascular endothelial growth factor (VEGF), and E-cadherin (E-cad) (p<0.05). Inhibiting CDKL1 function with shRNA, MCF-7 cells exhibited obvious accumulation at the G2/M phase and increased sensitivity to cell cycle chemotherapeutic drugs. The results suggested that the CDKL1 gene could be a potential tumor marker for diagnosis and a gene target for therapy.

Apoptosis in polycystic kidney disease

Apoptosis is the process of programmed cell death. It is a ubiquitous, controlled process consuming cellular energy and designed to avoid cytokine release despite activation of local immune cells, which clear the cell fragments. The process occurs during organ development and in maintenance of homeostasis. Abnormalities in any step of the apoptotic process are associated with autoimmune diseases and malignancies. Polycystic kidney disease (PKD) is the most common inherited kidney disease leading to end-stage renal disease (ESRD). Cyst formation requires multiple mechanisms and apoptosis is considered one of them. Abnormalities in apoptotic processes have been described in various murine and rodent models of PKD as well as in human PKD kidneys. The purpose of this review is to outline the role of apoptosis in progression of PKD as well as to describe the mechanisms involved. This article is part of a Special Issue entitled: Polycystic Kidney Disease.