Crosstalk of moderate ROS and PARP-1 contributes to sustainable proliferation of conditionally reprogrammed keratinocytes

Conditionally reprogrammed cell (CRC) technique is a promising model for biomedical and toxicological research. In the present study, our data first demonstrated an increased level of PARP-1 in conditionally reprogrammed human foreskin keratinocytes (CR-HFKs). We then found that PARP inhibitor ABT-888 (ABT), reactive oxygen species (ROS) scavenger N-acetyl-l-cysteine (NAC), or combination (ABT + NAC) were able to inhibit cell proliferation, ROS, PARP-1, and ROS related protein, NRF2, and NOX1. Interestingly, knockdown of endogenous PARP-1 significantly inhibited cell proliferation, indicating that the increased PARP-1 expression was critical for CR. Importantly, we found that a moderate level of ROS contributed the cell proliferation and increased PARP-1 since knockdown of PARP-1 also inhibited the ROS. The similar inhibition of cell proliferation, ROS, and expression of PARP-1 and NRF2 proteins was observed when CR-HFKs were treated with hydroquinone (HQ), a key component from skin-lightening products. Moreover, the treatment of HQ plus treatment of ABT, NAC, or combination can further inhibit cell proliferation, ROS, expression of PARP-1, and NRF2 proteins. PARP-1 knockdown inhibited the population doubling (PDL) and treatment of HQ inhibited the PDL further, as well as the change of ROS. Finally, we discovered that pathways including cyclin D1, NRF2, Rb and pRb, CHK2, and p53, were involved in cell proliferation inhibition with HQ. Taken together, our findings demonstrated that crosstalk between ROS and PARP-1 involves in the cell proliferation in CR-HFKs, and that inhibition of CR-HFK proliferation with HQ is through modulating G1 cell cycle arrest.

Abstract A27: Fidelity of a PDX-CR model for bladder cancer

Current efforts for cancer drug discovery have predominantly been deterred by the lack of appropriate preclinical cancer models that recapitulate the characteristics of this complex disease. Patient-derived xenografts (PDXs) are widely recognized as a more physiologically relevant preclinical model than organoids and standard cell lines, and often resemble the original tumor histology, genetic profile, and gene-expression patterns. Despite these benefits, PDX models are limited by their variable engraftment rate, lack of sustained growth in vitro, low throughput for drug screening, lower amenability to experimental manipulation, and high cost. In this study, we utilized conditional reprogramming (CR) technology to generate four CR cell (CRC) lines from bladder cancer PDXs. The CR cells were then employed to evaluate the genetic status and drug sensitivity and compared with the parental PDX tumors. All the established CRC lines maintained parental mutations and allele frequencies without clonal drift. Moreover, the drug responses of the parental PDX tumors in vivo were retained in the established CRC lines in vitro. Altogether, CR technology offers the ability to generate cell lines and expand PDX cells without compromising fundamental biologic properties of the model, thereby allowing for in vitro use to reduce animal usage, variability, and study cost. Perhaps more importantly, the CR cell lines established here can be used for personalized high-throughput drug screening as well as for studying drug-resistance mechanisms.

Citation Format: Abdul M. Mondal, Ai-Hong Ma, Guangzhao Li, Ewa Krawczyk, Lu Jie, Richard Schlegel, Chong-Xian Pan, Xuefeng Liu. Fidelity of a PDX-CR model for bladder cancer [abstract]. In: Proceedings of the AACR Special Conference on Bladder Cancer: Transforming the Field; 2019 May 18-21; Denver, CO. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(15_Suppl):Abstract nr A27.

Conditional cell reprogramming for modeling host-virus interactions and human viral diseases

Conventional cancer and transformed cell lines are widely used in cancer biology and other fields within biology. These cells usually have abnormalities from the original tumor itself, but may also develop abnormalities due to genetic manipulation, or genetic and epigenetic changes during long‐term passages. Primary cultures may maintain lineage functions as the original tissue types, yet they have a very limited life span or population doubling time because of the nature of cellular senescence. Primary cultures usually have very low yields, and the high variability from any original tissue specimens, largely limiting their applications in research. Animal models are often used for studies of virus infections, disease modeling, development of antiviral drugs, and vaccines. Human viruses often need a series of passages in vivo to adapt to the host environment because of variable receptors on the cell surface and may have intracellular restrictions from the cell types or host species. Here, we describe a long‐term cell culture system, conditionally reprogrammed cells (CRCs), and its applications in modeling human viral diseases and drug discovery. Using feeder layer coculture in presence of Y‐27632 (conditional reprogramming, CR), CRCs can be obtained and rapidly propagated from surgical specimens, core or needle biopsies, and other minimally invasive or noninvasive specimens, for example, nasal cavity brushing. CRCs preserve their lineage functions and provide biologically relevant and physiological conditions, which are suitable for studies of viral entry and replication, innate immune responses of host cells, and discovery of antiviral drugs. In this review, we summarize the applications of CR technology in modeling host‐virus interactions and human viral diseases including severe acute respiratory syndrome coronavirus‐2 and coronavirus disease‐2019, and antiviral discovery.

Fidelity of a PDX-CR model for bladder cancer

Patient-derived xenografts (PDXs) are widely recognised as a more physiologically relevant preclinical model than standard cell lines, but are expensive and low throughput, have low engraftment rate and take a long time to develop. Our newly developed conditional reprogramming (CR) technology addresses many PDX drawbacks, but lacks many in vivo factors. Here we determined whether PDXs and CRCs of the same cancer origin maintain the biological fidelity and complement each for translational research and drug development. Four CRC lines were generated from bladder cancer PDXs. Short tandem repeat (STR) analyses revealed that CRCs and their corresponding parental PDXs shared the same STRs, suggesting common cancer origins. CRCs and their corresponding parental PDXs contained the same genetic alterations. Importantly, CRCs retained the same drug sensitivity with the corresponding downstream signalling activity as their corresponding parental PDXs. This suggests that CRCs and PDXs can complement each other, and that CRCs can be used for in vitro fast, high throughput and low cost screening while PDXs can be used for in vivo validation and study of the in vivo factors during translational research and drug development.

Δ133p53α, a natural p53 isoform, contributes to conditional reprogramming and long-term proliferation of primary epithelial cells

We previously developed the technique of conditional reprogramming (CR), which allows primary epithelial cells from fresh or cryopreserved specimens to be propagated long-term in vitro, while maintaining their genetic stability and differentiation potential. This method requires a combination of irradiated fibroblast feeder cells and a Rho-associated kinase (ROCK) inhibitor. In the present study, we demonstrate increased levels of full-length p53 and its natural isoform, Δ133p53α, in conditionally reprogrammed epithelial cells from primary prostate, foreskin, ectocervical, and mammary tissues. Increased Δ133p53α expression is critical for CR since cell proliferation is rapidly inhibited following siRNA knockdown of endogenous Δ133p53α. Importantly, overexpression of Δ133p53α consistently delays the onset of cellular senescence of primary cells when cultured under non-CR conditions in normal keratinocyte growth medium (KGM). More significantly, the combination of Δ133p53α overexpression and ROCK inhibitor, without feeder cells, enables primary epithelial cells to be propagated long-term in vitro. We also show that Δ133p53α overexpression induces hTERT expression and telomerase activity and that siRNA knockdown of hTERT causes rapid inhibition of cell proliferation, indicating a critical role of hTERT for mediating the effects of Δ133p53α. Altogether, these data demonstrate a functional and regulatory link between p53 pathways and hTERT expression during the conditional reprogramming of primary epithelial cells.

Abstract 922: Delta133p53 represses p53-inducible senescence genes and enhances the generation of human induced pluripotent stem cells

p53 functions to induce cellular senescence and apoptosis, which can be incompatible with self-renewal of pluripotent stem cells such as induced pluripotent stem cells (iPSC) and embryonic stem cells (ESC). On the other hand, p53 regulates DNA damage response and repair and thus plays an essential role in maintaining genomic integrity and suppressing malignant transformation in iPSC and ESC. It remains to be elucidated whether and how p53 and its regulators contribute to balanced regulation between the self-renewing capacity and the genomic and functional integrity in these pluripotent stem cells. We hypothesized the involvement of Δ133p53, a physiological p53 protein isoform that inhibits the activity of full-length p53 (FL-p53), and here examined 12 lines of human iPSC and their original fibroblasts, as well as 3 human ESC lines, for endogenous protein levels of Δ133p53 and FL-p53, and mRNA levels of p53 target genes of different functions. While FL-p53 levels in iPSC and ESC widely ranged from below to above those in the fibroblasts, all iPSC and ESC lines expressed elevated levels of Δ133p53. The p53-inducible genes that mediate cellular senescence (e.g., p21WAF1 and miR-34a), but not those for apoptosis and DNA repair, were downregulated in iPSC and ESC. Consistent with these endogenous expression profiles, overexpression of Δ133p53 in human fibroblasts preferentially repressed the p53-inducible senescence mediators and significantly enhanced their reprogramming to iPSC. The iPSC clones derived from Δ133p53-overexpressing fibroblasts, when injected into immunodeficient mice, formed well-differentiated, benign teratomas, suggesting that Δ133p53 overexpression is non- or less oncogenic than total inhibition of p53 activities. Overexpressed Δ133p53 prevented FL-p53 from binding to the regulatory regions of p21WAF1 and miR-34a, providing a mechanistic basis for its dominant-negative inhibition. This study supports the hypothesis that upregulation of Δ133p53 is an endogenous mechanism that facilitates human somatic cells to become pluripotent without malignant transformation.

Citation Format: Izumi Horikawa, Kye-yoon Park, Han Li, Kazunobu Isogaya, Yukiharu Hiyoshi, Katsuhiro Anami, Ana I. Robles, Abdul M. Mondal, Kaori Fujita, Manuel Serrano, Curtis C. Harris. Delta133p53 represses p53-inducible senescence genes and enhances the generation of human induced pluripotent stem cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 922. doi:10.1158/1538-7445.AM2017-922

Autophagic degradation of the inhibitory p53 isoform Δ133p53α as a regulatory mechanism for p53-mediated senescence

Δ133p53α, a p53 isoform that can inhibit full-length p53, is downregulated at replicative senescence in a manner independent of mRNA regulation and proteasome-mediated degradation. Here we demonstrate that, unlike full-length p53, Δ133p53α is degraded by autophagy during replicative senescence. Pharmacological inhibition of autophagy restores Δ133p53α expression levels in replicatively senescent fibroblasts, without affecting full-length p53. The siRNA-mediated knockdown of pro-autophagic proteins (ATG5, ATG7 and Beclin-1) also restores Δ133p53α expression. The chaperone-associated E3 ubiquitin ligase STUB1, which is known to regulate autophagy, interacts with Δ133p53α and is downregulated at replicative senescence. The siRNA knockdown of STUB1 in proliferating, early-passage fibroblasts induces the autophagic degradation of Δ133p53α and thereby induces senescence. Upon replicative senescence or STUB1 knockdown, Δ133p53α is recruited to autophagosomes, consistent with its autophagic degradation. This study reveals that STUB1 is an endogenous regulator of Δ133p53α degradation and senescence, and identifies a p53 isoform-specific protein turnover mechanism that orchestrates p53-mediated senescence.

p53 isoforms regulate aging- and tumor-associated replicative senescence in T lymphocytes

Cellular senescence contributes to aging and decline in tissue function. p53 isoform switching regulates replicative senescence in cultured fibroblasts and is associated with tumor progression. Here, we found that the endogenous p53 isoforms Δ133p53 and p53β are physiological regulators of proliferation and senescence in human T lymphocytes in vivo. Peripheral blood CD8+ T lymphocytes collected from healthy donors displayed an age-dependent accumulation of senescent cells (CD28-CD57+) with decreased Δ133p53 and increased p53β expression. Human lung tumor-associated CD8+ T lymphocytes also harbored senescent cells. Cultured CD8+ blood T lymphocytes underwent replicative senescence that was associated with loss of CD28 and Δ133p53 protein. In poorly proliferative, Δ133p53-low CD8+CD28- cells, reconstituted expression of either Δ133p53 or CD28 upregulated endogenous expression of each other, which restored cell proliferation, extended replicative lifespan and rescued senescence phenotypes. Conversely, Δ133p53 knockdown or p53β overexpression in CD8+CD28+ cells inhibited cell proliferation and induced senescence. This study establishes a role for Δ133p53 and p53β in regulation of cellular proliferation and senescence in vivo. Furthermore, Δ133p53-induced restoration of cellular replicative potential may lead to a new therapeutic paradigm for treating immunosenescence disorders, including those associated with aging, cancer, autoimmune diseases, and HIV infection.

Downregulation of splicing factor SRSF3 induces p53β, an alternatively spliced isoform of p53 that promotes cellular senescence

Most human pre-mRNA transcripts are alternatively spliced, but the significance and fine-tuning of alternative splicing in different biological processes is only starting to be understood. SRSF3 (SRp20) is a member of a highly conserved family of splicing factors that have critical roles in key biological processes, including tumor progression. Here, we show that SRSF3 regulates cellular senescence, a p53-mediated process to suppress tumorigenesis, through TP53 alternative splicing. Downregulation of SRSF3 was observed in normal human fibroblasts undergoing replicative senescence, and was associated with the upregulation of p53β, an alternatively spliced isoform of p53 that promotes p53-mediated senescence. Knockdown of SRSF3 by short interfering RNA (siRNA) in early-passage fibroblasts induced senescence, which was associated with elevated expression of p53β at mRNA and protein levels. Knockdown of p53 partially rescued SRSF3-knockdown-induced senescence, suggesting that SRSF3 acts on p53-mediated cellular senescence. RNA pulldown assays demonstrated that SRSF3 binds to an alternatively spliced exon uniquely included in p53β mRNA through the consensus SRSF3-binding sequences. RNA crosslinking and immunoprecipitation assays (CLIP) also showed that SRSF3 in vivo binds to endogenous p53 pre-mRNA at the region containing the p53β-unique exon. Splicing assays using a transfected TP53 minigene in combination with siRNA knockdown of SRSF3 showed that SRSF3 functions to inhibit the inclusion of the p53β-unique exon in splicing of p53 pre-mRNA. These data suggest that downregulation of SRSF3 represents an endogenous mechanism for cellular senescence that directly regulates the TP53 alternative splicing to generate p53β. This study uncovers the role for general splicing machinery in tumorigenesis, and suggests that SRSF3 is a direct regulator of p53.

Positive feedback between p53 and TRF2 during telomere-damage signalling and cellular senescence

The telomere-capping complex shelterin protects functional telomeres and prevents the initiation of unwanted DNA-damage-response pathways. At the end of cellular replicative lifespan, uncapped telomeres lose this protective mechanism and DNA-damage signalling pathways are triggered that activate p53 and thereby induce replicative senescence. Here, we identify a signalling pathway involving p53, Siah1 (a p53-inducible E3 ubiquitin ligase) and TRF2 (telomere repeat binding factor 2; a component of the shelterin complex). Endogenous Siah1 and TRF2 were upregulated and downregulated, respectively, during replicative senescence with activated p53. Experimental manipulation of p53 expression demonstrated that p53 induces Siah1 and represses TRF2 protein levels. The p53-dependent ubiquitylation and proteasomal degradation of TRF2 are attributed to the E3 ligase activity of Siah1. Knockdown of Siah1 stabilized TRF2 and delayed the onset of cellular replicative senescence, suggesting a role for Siah1 and TRF2 in p53-regulated senescence. This study reveals that p53, a downstream effector of telomere-initiated damage signalling, also functions upstream of the shelterin complex.

Abstract 3199: p53 represses TRF2 through E3 ubiquitin ligase Siah-1: Feedback regulation in telomere-initiated damage signaling

p53 plays critical roles in tumor suppression, stem cell functions and aging in vivo. The telomere-capping protein complex (shelterin) prevents functional telomeres from undergoing erosion or end-to-end fusion and from initiating unwanted DNA damage response. Uncapped, dysfunctional telomeres at the end of cellular replicative lifespan lose this protective mechanism and trigger telomere-initiated DNA damage signaling to activate p53 and thereby induce cellular senescence. Here we report that p53 in turn controls a component of the shelterin complex, TRF2, through Siah-1, a p53-inducible E3 ubiquitin ligase. Endogenous TRF2 and Siah-1 were repressed and induced, respectively, in normal human fibroblasts at replicative senescence, when p53 was physiologically activated. Spontaneous allelic loss, shRNA-mediated knockdown, dominant-negative inhibition, nutlin-3a activation and overexpression of p53 all showed that p53 induced Siah-1 and repressed TRF2. TRF2 was subject to proteasomal degradation in a p53-dependent manner. Anti-TRF2 antibody-immunoprecipitated proteins were found to undergo p53- and Siah-1-mediated ubiquitination. In vitro and in vivo ubiquitination experiments showed that the E3 ligase activity of Siah-1 was responsible for TRF2 ubiquitination. Biologically, Siah-1 knockdown or TRF2 overexpression delayed the onset of replicative senescence, suggesting that the proteolytic control of TRF2 cooperates with the transcriptional regulation of p53 target genes (e.g., p21WAF1 and microRNA-34a) to regulate p53-mediated replicative senescence. This study reveals that p53, which is a downstream effector of the DNA damage signaling from uncapped telomeres, also functions upstream to regulate the telomere-capping complex, and suggests that the p53-Siah-1-TRF2 pathway takes an integral part in orchestrating the DNA damage response at telomeres. Given that TRF2 inhibits a p53-activating kinase ATM at telomeres, a positive feedback loop involving TRF2, ATM and p53 may function to amplify DNA damage-induced and p53-mediated cellular responses.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3199.

Abstract 2915: p53 isoforms Δ133p53 and p53β are endogenous regulators of replicative cellular senescence

The finite proliferative potential of normal human cells leads to replicative cellular senescence, which is a critical barrier to tumor progression in vivo. The p53 signaling pathway plays central roles in the regulation of cellular senescence. Humans, as well as Drosophila and zebrafish, have p53 isoforms; however, their regulation and function are poorly understood. We here examine the expression profiles of two human p53 isoforms, p53β (lacking the C-terminal oligomerization domain due to an alternative mRNA splicing) and Δ133p53 (lacking the N-terminal transactivation and proline-rich domains due to the transcription from an alternative promoter in intron 4), during in vitro and in vivo cellular senescence and their biological activities in regulating cellular senescence. Induced p53β and diminished Δ133p53 were associated with replicative senescence, but not oncogene-induced senescence, in normal human fibroblasts. The replicatively senescent fibroblasts also expressed increased levels of miR-34a, a p53-induced microRNA, the antisense inhibition of which delayed the onset of replicative senescence. The siRNA-mediated knockdown of endogenous Δ133p53 induced cellular senescence, which was attributed to the regulation of p21WAF1 and other p53 transcriptional target genes. In overexpression experiments, while p53β cooperated with full-length p53 to accelerate cellular senescence, Δ133p53 repressed miR-34a expression and extended cellular replicative lifespan, providing a functional connection of this microRNA to the p53 isoform-mediated regulation of senescence. The senescence-associated signature of p53 isoform expression (i.e., elevated p53β and reduced Δ133p53) was observed in vivo in colon adenomas with senescent phenotypes. The decreased p53β and increased Δ133p53 expression found in colon carcinomas might signal an escape from the senescence barrier during the progression from premalignant to malignant tumors in vivo. This study shows that natural p53 isoforms constitute an endogenous regulatory mechanism for p53-mediated replicative senescence and may open up a new p53-based, senescence-mediated strategy to manipulate carcinogenesis and aging. The molecular details of the senescence-associated Δ133p53 repression and p53β induction are currently under investigation.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2915.