C-terminal deletion mutant p21(WAF1/CIP1) enhances E2F-1-mediated apoptosis in colon adenocarcinoma cells

C16, a PKR inhibitor, suppresses cell proliferation by regulating the cell cycle via p21 in colorectal cancer

Double-stranded RNA-activated protein kinase R (PKR) is highly expressed in colorectal cancer (CRC). However, the role of PKR in CRC remains unclear. The aim of this study was to clarify whether C16 (a PKR inhibitor) exhibits antitumor effects and to identify its target pathway in CRC. We evaluated the effects of C16 on CRC cell lines using the MTS assay. Enrichment analysis was performed to identify the target pathway of C16. The cell cycle was analyzed using flow cytometry. Finally, we used immunohistochemistry to examine human CRC specimens. C16 suppressed the proliferation of CRC cells. Gene Ontology (GO) analysis revealed that the cell cycle-related GO category was substantially enriched in CRC cells treated with C16. C16 treatment resulted in G1 arrest and increased p21 protein and mRNA expression. Moreover, p21 expression was associated with CRC development as observed using immunohistochemical analysis of human CRC tissues. C16 upregulates p21 expression in CRC cells to regulate cell cycle and suppress tumor growth. Thus, PKR inhibitors may serve as a new treatment option for patients with CRC.

p21 in Cancer Research

p21 functions as a cell cycle inhibitor and anti-proliferative effector in normal cells, and is dysregulated in some cancers. Earlier observations on p21 knockout models emphasized the role of this protein in cell cycle arrest under the p53 transcription factor activity. Although tumor-suppressor function of p21 is the most studied aspect of this protein in cancer, the role of p21 in phenotypic plasticity and its oncogenic/anti-apoptotic function, depending on p21 subcellular localization and p53 status, have been under scrutiny recently. Basic science and translational studies use precision gene editing to manipulate p21 itself, and proteins that interact with it; these studies have led to regulatory/functional/drug sensitivity discoveries as well as therapeutic approaches in cancer field. In this review, we will focus on targeting p21 in cancer research and its potential in providing novel therapies.

Inhibition of Müller glial cell division blocks regeneration of the light-damaged zebrafish retina

The adult zebrafish retina possesses a robust regenerative response. In the light-damaged retina, Müller glial cell divisions precede regeneration of rod and cone photoreceptors. Neuronal progenitors, which arise from the Müller glia, continue to divide and use the Müller glial cell processes to migrate to the outer nuclear layer and replace the lost photoreceptors. We tested the necessity of Müller glial cell division for photoreceptor regeneration. As knockdown tools were unavailable for use in the adult zebrafish retina, we developed a method to conditionally inhibit the expression of specific proteins by in vivo electroporation of morpholinos. We determined that two separate morpholinos targeted against the proliferating cell nuclear antigen (PCNA) mRNA reduced PCNA protein levels. Furthermore, injection and in vivo electroporation of PCNA morpholinos immediately prior to starting intense light exposure inhibited both Müller glial cell proliferation and neuronal progenitor marker Pax6 expression. PCNA knockdown additionally resulted in decreased expression of glutamine synthetase in Müller glia and Müller glial cell death, while amacrine and ganglion cells were unaffected. Finally, histological and immunological methods showed that long-term effects of PCNA knockdown resulted in decreased numbers of Müller glia and the failure to regenerate rod photoreceptors, short single cones, and long single cones. These data suggest that Müller glial cell division is necessary for proper photoreceptor regeneration in the light-damaged zebrafish retina and are consistent with the Müller glia serving as the source of neuronal progenitor cells in regenerating teleost retinas.

Differential effects of cell cycle regulatory protein p21(WAF1/Cip1) on apoptosis and sensitivity to cancer chemotherapy

p21(WAF1/Cip1) was initially identified as a cell cycle regulatory protein that can cause cell cycle arrest. It is induced by both p53-dependent and p53-independent mechanisms. This mini-review briefly discusses its currently known functions in apoptosis and drug sensitivity. As an inhibitor of cell proliferation, p21(WAF1/Cip1) plays an important role in drug-induced tumor suppression. Nevertheless, a number of recent studies have shown that p21(WAF1/Cip1) can assume both pro- or anti-apoptotic functions in response to anti-tumor agents depending on cell type and cellular context. This dual role of p21(WAF1/Cip1) in cancer cells complicates using p21(WAF1/Cip1) status to predict response to anti-tumor agents. However, it is possible to develop p21(WAF1/Cip1)-targeted reagents or p21(WAF1/Cip1) gene transfer techniques to have a beneficial effect within a well-defined therapeutic context. Better understanding of the roles of p21(WAF1/Cip1) in tumors should enable a more rational approach to anti-tumor drug design and therapy.

Transcriptional regulation during p21WAF1/CIP1-induced apoptosis in human ovarian cancer cells

In this study we used adenovirus vector-mediated transduction of either the p53 gene (rAd-p53) or the p21(WAF1/CIP1) gene (rAd-p21) to mimic both p53-dependent and -independent up-regulation of p21(WAF1/CIP1) within a human ovarian cancer cell line, 2774, and the derivative cell lines, 2774qw1 and 2774qw2. We observed that rAd-p53 can induce apoptosis in both 2774 and 2774qw1 cells but not in 2774qw2 cells. Surprisingly, overexpression of p21(WAF1/CIP1) also triggered apoptosis within these two cell lines. Quantitative reverse transcription-PCR analysis revealed that the differential expression of BAX, BCL2, and caspase 3 genes, specific in rAd-p53-induced apoptotic cells, was not altered in rAd-p21-induced apoptotic cells, suggesting p21(WAF1/CIP1)-induced apoptosis through a pathway distinguishable from p53-induced apoptosis. Expression analysis of 2774qw1 cells infected with rAd-p21 on 60,000 cDNA microarrays identified 159 genes in response to p21(WAF1/CIP1) expression in at least one time point with 2.5-fold change as a cutoff. Integration of the data with the parallel microarray experiments with rAd-p53 infection allowed us to extract 66 genes downstream of both p53 and p21(WAF1/CIP1) and 93 genes in response to p21(WAF1/CIP1) expression in a p53-independent pathway. The genes in the former set may play a dual role in both p53-dependent and p53-independent pathways, and the genes in the latter set gave a mechanistic molecular explanation for p53-independent p21(WAF1/CIP1)-induced apoptosis. Furthermore, promoter sequence analysis suggested that transcription factor E2F family is partially responsible for the differential expression of genes following p21(WAF1/CIP1). This study has profound significance toward understanding the role of p21(WAF1/CIP1) in p53-independent apoptosis.