The p73 gene is an anti-tumoral target of the RARbeta/gamma-selective retinoid tazarotene

Posttranslational modifications of E2F family members in the physiological state and in cancer: Roles, mechanisms and therapeutic targets

The E2F transcription factor family, whose members are encoded by the E2F1-E2F8 genes, plays pivotal roles in the cell cycle, apoptosis, metabolism, stemness, metastasis, aging, angiogenesis, tumor promotion or suppression, and other biological processes. The activity of E2Fs is regulated at multiple levels, with posttranslational modifications being an important regulatory mechanism. There are numerous types of posttranslational modifications, among which phosphorylation, acetylation, methylation, ubiquitination, SUMOylation, neddylation, and poly(ADP-ribosyl)ation are the most commonly studied in the context of the E2F family. Posttranslational modifications of E2F family proteins regulate their biological activity, stability, localization, and interactions with other biomolecules, affecting cell proliferation, apoptosis, DNA damage, etc., and thereby playing roles in physiological and pathological processes. Notably, these modifications do not always act alone but rather form an interactive regulatory network. Currently, several drugs targeting posttranslational modifications are being studied or clinically applied, in which the proteolysis-targeting chimera and molecular glue can target E2Fs. This review aims to summarize the roles and regulatory mechanisms of different PTMs of E2F family members in the physiological state and in cancer and to briefly discuss their clinical significance and potential therapeutic use.

Atypical Fibroxanthoma Treated with a Topical Combination of Imiquimod, Tazarotene, and 5-Fluorouracil

This case report describes an 80-year-old man who presented with a growing erythematous nodule with erosion, measuring 0.6 cm × 0.6 cm, on his right temple. This lesion was later diagnosed as atypical fibroxanthoma (AFX). Instead of undergoing Mohs surgery, the gold standard treatment, the patient opted to pursue a topical treatment regimen because of financial costs associated with surgical removal and repair. This topical regimen consisted of tazarotene cream, imiquimod cream, and 5-fluorouracil solution, applied for 30 days. The patient was directed to use this combination 5 days per week for 6 weeks. The specified dosage for each medication was a fifth of a packet of imiquimod 5% cream, an equivalent amount of tazarotene 0.1% cream, and a single drop of 5-fluorouracil 2% solution. These were combined on a bandage and placed on the lesion overnight. Following the treatment, a 3-week post-application examination revealed an erosion, 1.0 cm × 0.9 cm, amidst erythema. A subsequent incisional biopsy with histopathology and stains for CD10 and CD99, 3 weeks after treatment, and three punch biopsies with histopathology and stains for CD10 and CD99, 1-year post-treatment, confirmed the absence of AFX. AFX is a superficial variant of pleomorphic dermal sarcoma (PDS), which shares histologic similarities, yet the exact relationship between AFX/PDS and undifferentiated pleomorphic sarcoma is still not well understood. Previous studies have indicated a genomic similarity between AFX/PDS and cutaneous squamous cell carcinoma (cSCC), which suggests the potential efficacy of cSCC-targeted treatments for AFX/PDS. This case marks the first recorded instance of successful topical medical treatment of AFX, offering an alternative for patients who may opt out of surgical intervention. Continued research to assess the broader efficacy of this approach is encouraged.

The Use of Retinoids for the Prevention and Treatment of Skin Cancers: An Updated Review

Retinoids are natural and synthetic vitamin A derivatives that are effective for the prevention and the treatment of non-melanoma skin cancers (NMSC). NMSCs constitute a heterogenous group of non-melanocyte-derived skin cancers that impose substantial burdens on patients and healthcare systems. They include entities such as basal cell carcinoma and cutaneous squamous cell carcinoma (collectively called keratinocyte carcinomas), cutaneous lymphomas and Kaposi’s sarcoma among others. The retinoid signaling pathway plays influential roles in skin physiology and pathology. These compounds regulate diverse biological processes within the skin, including proliferation, differentiation, angiogenesis and immune regulation. Collectively, retinoids can suppress skin carcinogenesis. Both topical and systemic retinoids have been investigated in clinical trials as NMSC prophylactics and treatments. Desirable efficacy and tolerability in clinical trials have prompted health regulatory bodies to approve the use of retinoids for NMSC management. Acceptable off-label uses of these compounds as drugs for skin cancers are also described. This review is a comprehensive outline on the biochemistry of retinoids, their activities in the skin, their effects on cancer cells and their adoption in clinical practice.

p73: From the p53 shadow to a major pharmacological target in anticancer therapy

p73, along with p53 and p63, belongs to the p53 family of transcription factors. Besides the p53-like tumor suppressive activities, p73 has unique roles, namely in neuronal development and differentiation. In addition, the TP73 gene is rarely mutated in tumors. This makes p73 a highly appealing therapeutic target, particularly towards cancers with a null or disrupted p53 pathway. Distinct isoforms are transcribed from the TP73 locus either with (TAp73) and without (ΔNp73) the N-terminal transactivation domain. Conversely to TA tumor suppressors, ΔN proteins exhibit oncogenic properties by inhibiting p53 and TA protein functions. As such, p73 isoforms compose a puzzled and challenging regulatory pathway. This state-of-the-art review affords an update overview on p73 structure, biological functions and pharmacological regulation. Importantly, it addresses the relevance of p73 isoforms in carcinogenesis, highlighting their potential as drug targets in anticancer therapy. A critical discussion of major pharmacological approaches to promote p73 tumor suppressive activities, with relevant survival outcomes for cancer patients, is also provided.

A balancing act: orchestrating amino-truncated and full-length p73 variants as decisive factors in cancer progression

p73 is the older sibling of p53 and mimics most of its tumor-suppressor functions. Through alternative promoter usage and splicing, the TP73 gene generates more than two dozen isoforms of which N-terminal truncated DNp73 variants have a decisive role in cancer pathogenesis as they outweigh the positive effects of full-length TAp73 and p53 in acting as a barrier to tumor development. Beyond the prevailing view that DNp73 predominantly counteract cell cycle arrest and apoptosis, latest progress indicates that these isoforms acquire novel functions in epithelial-to-mesenchymal transition, metastasis and therapy resistance. New insight into the mechanisms underlying this behavior reinforced the expectation that DNp73 variants contribute to aggressive cellular traits through both loss of wild-type tumor-suppressor activity and gain-of-function, suggesting an equally important role in cancer progression as mutant p53. In this review, we describe the novel properties of DNp73 in the invasion metastasis cascade and outline the comprehensive p73 regulatome with an emphasis on molecular processes putting TAp73 out of action in advanced tumors. These intriguing insights provoke a new understanding of the acquisition of aggressive traits by cancer cells and may help to set novel therapies for a broad range of metastatic tumors.

Tazarotene induces apoptosis in human basal cell carcinoma via activation of caspase-8/t-Bid and the reactive oxygen species-dependent mitochondrial pathway

Previous studies suggest that tazarotene, a new member of the acetylenic class of RARβ/γ selective retinoids which is approved to treat a variety of skin diseases, exhibits an anti-proliferative effect in human basal cell carcinoma (BCC) by triggering caspase-dependent apoptosis. However, the detailed molecular mechanisms underlying the anti-tumor activity of tazarotene are poorly understood. This study aims at investigating the molecular mechanisms of tazarotene-induced apoptosis in human BCC cells. Our results are the first to demonstrate that tazarotene induces mitochondria-dependent cleavage of caspase-9 and -3 and PARP in BCC cells by producing reactive oxygen species (ROS) and activating caspase-8 through both ROS and death receptor signaling. These events are accompanied by a decrease in BCL-2 and BCL-xl anti-apoptotic proteins as well as by survivin and XIAP, two IAP family members. Furthermore, our results presented for the first time that tazarotene triggers a convergence of the intrinsic and extrinsic apoptotic pathways via the caspase-8-truncated Bid signaling pathway. Collectively, these data provide insights into the molecular mechanisms underlying tazarotene-induced apoptosis in human BCC cells, suggesting that this compound is a potential anti-skin cancer drug.

p73 plays a role in erythroid differentiation through GATA1 induction

The TP73 gene gives rise to transactivation domain-p73 isoforms (TAp73) as well as DeltaNp73 variants with a truncated N terminus. Although TAp73alpha and -beta proteins are capable of inducing cell cycle arrest, apoptosis, and differentiation, DeltaNp73 acts in many cell types as a dominant-negative repressor of p53 and TAp73. It has been proposed that p73 is involved in myeloid differentiation, and its altered expression is involved in leukemic degeneration. However, there is little evidence as to which p73 variants (TA or DeltaN) are expressed during differentiation and whether specific p73 isoforms have the capacity to induce, or hinder, this differentiation in leukemia cells. In this study we identify GATA1 as a direct transcriptional target of TAp73alpha. Furthermore, TAp73alpha induces GATA1 activity, and it is required for erythroid differentiation. Additionally, we describe a functional cooperation between TAp73 and DeltaNp73 in the context of erythroid differentiation in human myeloid cells, K562 and UT-7. Moreover, the impaired expression of GATA1 and other erythroid genes in the liver of p73KO embryos, together with the moderated anemia observed in p73KO young mice, suggests a physiological role for TP73 in erythropoiesis.

Retinoic acid increases aquaporin 3 expression in normal human skin

We have investigated the effects of all-trans retinoic acid (ATRA) on aquaporin 3 (AQP3) expression and function both in vitro and ex vivo. ATRA treatment provoked a rapid accumulation of AQP3 transcripts in cultured normal human epidermal keratinocytes (NHEK). This increase was still observed 24 hours after application of ATRA. The induction of AQP3 gene was accompanied by an augmentation of immunoreactivity. Using a selective agonist, we demonstrated that the effect of ATRA was predominantly mediated by retinoic acid receptor subtype gamma (RARgamma). Incubation of NHEK in ATRA for 24, 48, and 72 hours stimulated glycerol influx, suggesting that the increase in AQP3 gene and protein expression was followed by an enhancement of biological activity. Topical application of ATRA for 24 hours on skin explants induced significant epidermal expression of AQP3 and strong immunoreactivity in the epidermal basal layers. Collectively, the present results show that ATRA increased AQP3 expression and enhanced biological activity in human skin.

Fenretinide-induced apoptosis of Huh-7 hepatocellular carcinoma is retinoic acid receptor beta dependent

BACKGROUND

Retinoids are used to treat several types of cancer; however, their effects on liver cancer have not been fully characterized. To investigate the therapeutic potential of retinoids on hepatocellular carcinoma (HCC), the present study evaluates the apoptotic effect of a panel of natural and synthetic retinoids in three human HCC cell lines as well as explores the underlying mechanisms.

METHODS

Apoptosis was determined by caspase-3 cleavage using western blot, DNA double-strand breaks using TUNEL assay, and phosphatidylserine translocation using flow cytometry analysis. Gene expression of nuclear receptors was assessed by real-time PCR. Transactivation assay and chromatin immunoprecipitation (ChIP) were conducted to evaluate the activation of RXRalpha/RARbeta pathway by fenretinide. Knockdown of RARbeta mRNA expression was achieved by siRNA transfection.

RESULTS

Our data revealed that fenretinide effectively induces apoptosis in Huh-7 and Hep3B cells. Gene expression analysis of nuclear receptors revealed that the basal and inducibility of retinoic acid receptor beta (RARbeta) expression positively correlate with the susceptibility of HCC cells to fenretinide treatment. Furthermore, fenretinide transactivates the RXRalpha/RARbeta-mediated pathway and directly increases the transcriptional activity of RARbeta. Knockdown of RARbeta mRNA expression significantly impairs fenretinide-induced apoptosis in Huh-7 cells.

CONCLUSION

Our findings reveal that endogenous expression of retinoids receptor RARbeta gene determines the susceptibility of HCC cells to fenretinide-induced apoptosis. Our results also demonstrate fenretinide directly activates RARbeta and induces apoptosis in Huh-7 cells in a RARbeta-dependent manner. These findings suggest a novel role of RARbeta as a tumor suppressor by mediating the signals of certain chemotherapeutic agents.

13-cis Retinoic acid induces apoptosis and cell cycle arrest in human SEB-1 sebocytes

Isotretinoin (13-cis retinoic acid (13-cis RA)) is the most potent inhibitor of sebum production, a key component in the pathophysiology of acne, yet its mechanism of action remains largely unknown. The effects of 13-cis RA, 9-cis retinoic acid (9-cis RA), and all-trans retinoic acid (ATRA) on cell proliferation, apoptosis, and cell cycle proteins were examined in SEB-1 sebocytes and keratinocytes. 13-cis RA causes significant dose-dependent and time-dependent decreases in viable SEB-1 sebocytes. A portion of this decrease can be attributed to cell cycle arrest as evidenced by decreased DNA synthesis, increased p21 protein expression, and decreased cyclin D1. Although not previously demonstrated in sebocytes, we report that 13-cis RA induces apoptosis in SEB-1 sebocytes as shown by increased Annexin V-FITC staining, increased TUNEL staining, and increased cleaved caspase 3 protein. Furthermore, the ability of 13-cis RA to induce apoptosis cannot be recapitulated by 9-cis RA or ATRA, and it is not inhibited by the presence of a retinoid acid receptor (RAR) pan-antagonist AGN 193109. Taken together these data indicate that 13-cis RA causes cell cycle arrest and induces apoptosis in SEB-1 sebocytes by a RAR-independent mechanism, which contributes to its sebosuppressive effect and the resolution of acne.

p63-Specific Activation of the BPAG-1e Promoter

p63, a member of the p53 superfamily, is an essential cell fate determinant for stratified epithelium. Deficiency of p63 leads to lack of differentiated epithelium from the skin and the presence of trace undifferentiated cells left in the dermis. We found that transcriptionally active isoforms of p63, TAp63beta and TAp63gamma, activated the skin-specific promoter of bullous pemphigoid antigen 1 (BPAG-1). The p63-response element was localized between bases -177 and -153 upstream of exon 1 in the BPAG-1e promoter, whereas regions surrounding the response element suppressed transcriptional responses to p53 and TAp73beta, resulting in p63-specific activation of the promoter. This represents a novel molecular mechanism by which target gene induction by p63 is distinguished from induction by other p53 family members.