Promyelocytic leukemia protein-induced growth suppression and cell death in liver cancer cells

Deciphering roles of TRIMs as promising targets in hepatocellular carcinoma: current advances and future directions

Tripartite motif (TRIM) family is assigned to RING-finger-containing ligases harboring the largest number of proteins in E3 ubiquitin ligating enzymes. E3 ubiquitin ligases target the specific substrate for proteasomal degradation via the ubiquitin-proteasome system (UPS), which seems to be a more effective and direct strategy for tumor therapy. Recent advances have demonstrated that TRIM genes associate with the occurrence and progression of hepatocellular carcinoma (HCC). TRIMs trigger or inhibit multiple biological activities like proliferation, apoptosis, metastasis, ferroptosis and autophagy in HCC dependent on its highly conserved yet diverse structures. Remarkably, autophagy is another proteolytic pathway for intracellular protein degradation and TRIM proteins may help to delineate the interaction between the two proteolytic systems. In depth research on the precise molecular mechanisms of TRIM family will allow for targeting TRIM in HCC treatment. We also highlight several potential directions warranted further development associated with TRIM family to provide bright insight into its translational values in hepatocellular carcinoma.

TRIM proteins in hepatocellular carcinoma

The tripartite motif (TRIM) protein family is a highly conserved group of E3 ligases with 77 members known in the human, most of which consist of a RING-finger domain, one or two B-box domains, and a coiled-coil domain. Generally, TRIM proteins function as E3 ligases to facilitate specific proteasomal degradation of target proteins. In addition, E3 ligase independent functions of TRIM protein were also reported. In hepatocellular carcinoma, expressions of TRIM proteins are both regulated by genetic and epigenetic mechanisms. TRIM proteins regulate multiple biological activities and signaling cascades. And TRIM proteins influence hallmarks of HCC. This review systematically demonstrates the versatile roles of TRIM proteins in HCC and helps us better understand the molecular mechanism of the development and progression of HCC.

The translational values of TRIM family in pan-cancers: From functions and mechanisms to clinics

Cancer is the second leading cause of human death across the world. Tripartite motif (TRIM) family, with E3 ubiquitin ligase activities in majority of its members, is reported to be involved in multiple cellular processes and signaling pathways. TRIM proteins have critical effects in the regulation of biological behaviors of cancer cells. Here, we discussed the current understanding of the molecular mechanism of TRIM proteins regulation of cancer cells. We also comprehensively reviewed published studies on TRIM family members as oncogenes or tumor suppressors in the oncogenesis, development, and progression of a variety of types of human cancers. Finally, we highlighted that certain TRIM family members are potential molecular biomarkers for cancer diagnosis and prognosis, and potential therapeutic targets.

Dual oncogenic and tumor suppressor roles of the promyelocytic leukemia gene in hepatocarcinogenesis associated with hepatitis B virus surface antigen

Proteasome-mediated degradation of promyelocytic leukemia tumor suppressor (PML) is upregulated in many viral infections and cancers. We previously showed that PML knockdown promotes early-onset hepatocellular carcinoma (HCC) in hepatitis B virus surface antigen (HBsAg)-transgenic mice. Here we report the effects of PML restoration on late-onset HBsAg-induced HCC. We compared protein expression patterns, genetic mutations and the effects of pharmacologically targeting PML in wild-type, PML-/-, PML+/+HBsAgtg/o and PML-/-HBsAgtg/o mice. PML-/- mice exhibited somatic mutations in DNA repair genes and developed severe steatosis and proliferative disorders, but not HCC. PML-/-HBsAgtg/o mice exhibited early mutations in cancer driver genes and developed hyperplasia, fatty livers and indolent adipose-like HCC. In PML+/+HBsAg-transgenic mice, HBsAg expression declined over time, and HBsAg-associated PML suppression was concomitantly relieved. Nevertheless, these mice accumulated mutations in genes contributing to oxidative stress pathways and developed aggressive late-onset angiogenic trabecular HCC. PML inhibition using non-toxic doses of arsenic trioxide selectively killed long-term HBsAg-affected liver cells in PML+/+HBsAgtg/o mice with falling HBsAg and rising PML levels, but not normal liver cells or early-onset HCC cells in PML-/-HBsAgtg/0 mice. These findings suggest dual roles for PML as a tumor-suppressor lost in early-onset HBsAg-induced hepatocarcinogenesis and as an oncogenic promoter in late-onset HBsAg-related HCC progression.

Cascades of transcription regulation during liver regeneration

An increasing demand for new strategies in cancer prevention and regenerative medicine requires a better understanding of molecular mechanisms that control cell proliferation in tissue-specific manner. Regenerating liver is a unique model allowing use of biochemical, genetic, and engineering tools to uncover molecular mechanisms and improve treatment of hepatic cancers, liver failure, and fibrotic disease. Molecular mechanisms of liver regeneration involve extra- and intracellular factors to activate transcription of genes normally silenced in quiescent liver. While many upstream signaling pathways of the regenerating liver have been extensively studied, our knowledge of the downstream effectors, transcription factors (TFs), remains limited. This review describes consecutive engagement of pre-existing and de novo synthesized TFs, as cascades that regulate expression of growth-related and metabolic genes during liver regeneration after partial hepatectomy in mice. Several previously recognized regulators of regenerating liver are described in the light of recently identified co-activator and co-repressor complexes that interact with primary DNA-binding TFs. Published results of gene expression and chromatin immunoprecipitation analyses, as well as studies of transgenic mouse models, are used to emphasize new potential regulators of transcription during liver regeneration. Finally, a more detailed description of newly identified transcriptional regulators of liver regeneration illustrates the tightly regulated balance of proliferative and metabolic responses to partial hepatectomy.

The human promyelocytic leukemia protein is a tumor suppressor for murine skin carcinogenesis

Expression of the PMLRARalpha fusion dominant-negative oncogene in the epidermis of transgenic mice resulted in spontaneous skin tumors attributed to changes in both the PML and RAR pathways [Hansen et al., Cancer Res 2003; 63:5257-5265]. To determine the contribution of PML to skin tumor susceptibility, transgenic mice were generated on an FVB/N background, that overexpressed the human PML protein in epidermis and hair follicles under the control of the bovine keratin 5 promoter. PML was highly expressed in the epidermis and hair follicles of these mice and was also increased in cultured keratinocytes where it was confined to nuclear bodies. While an overt skin phenotype was not detected in young transgenic mice, expression of keratin 10 (K10) was increased in epidermis and hair follicles and cultured keratinocytes. As mice aged, they exhibited extensive alopecia that was accentuated on the C57BL/6J background. Following skin tumor induction with 7, 12-dimethylbenz[a]anthracene (DMBA) as initiator and 12-O-tetradecanoylphorbol-13-acetate (TPA) as promoter, papilloma multiplicity and size were decreased in the transgenic mice by 35%, and the conversion of papillomas to carcinomas was delayed. Cultured transgenic keratinocytes underwent premature senescence and upregulated transcripts for p16 and Rb but not p19 and p53. Together, these changes suggest that PML participates in regulating the growth and differentiation of keratinocytes that likely influence its activity as a suppressor for tumor development.

PML has a predictive role in tumor cell permissiveness to interferon-sensitive oncolytic viruses

The oncotropic phenotypes of several viruses correlate with tumor-associated deficiencies within interferon (IFN) signaling pathways. This observation formed the conceptual basis for developing oncolytic viruses deleted for viral proteins that inhibit the host IFN-dependent antiviral response, such as herpes simplex virus type-1 infected cell protein-0 (ICP0) and vesicular stomatitis virus matrix protein. Many viruses have evolved means to disrupt promyelocytic leukemia protein (PML) nuclear bodies. For example, ICP0 promotes PML degradation to inhibit the antiviral activities of this IFN-stimulated gene. As PML is downregulated in a variety of tumors, we hypothesized ICP0-null herpes simplex type-1 viruses are selectively oncolytic in tumors with impaired PML expression. We illustrate that ICP0-null herpes simplex type-1 viruses target tumor cells that either possess impaired PML signaling or cannot upregulate PML because of impaired IFN responsiveness. Disruption of PML signaling through overexpression of the dominant-negative protein PML-retinoic acid receptor alpha in PML-positive cells renders them sensitive to oncolysis by ICP0-null herpes simplex virus type-1 and vesicular stomatitis virus M protein mutant viruses, whereas PML overexpression reverses this phenomenon. Together, these data illustrate that PML mediates an antiviral mechanism that predicts the tropism of IFN-sensitive oncolytic viruses. To our knowledge, these viruses are the first examples of anti-cancer therapeutics capable of targeting deficiencies in PML expression.

Pondering the puzzle of PML (promyelocytic leukemia) nuclear bodies: can we fit the pieces together using an RNA regulon?

The promyelocytic leukemia protein PML and its associated nuclear bodies are hot topics of investigation. This interest arises for multiple reasons including the tight link between the integrity of PML nuclear bodies and several disease states and the impact of the PML protein and PML nuclear bodies on proliferation, apoptosis and viral infection. Unfortunately, an understanding of the molecular underpinnings of PML nuclear body function remains elusive. Here, a general overview of the PML field is provided and is extended to discuss whether some of the basic tenets of "PML-ology" are still valid. For instance, recent findings suggest that some components of PML nuclear bodies form bodies in the absence of the PML protein. Also, a new model for PML nuclear body function is proposed which provides a unifying framework for its effects on diverse biochemical pathways such as Akt signaling and the p53-Mdm2 axis. In this model, the PML protein acts as an inhibitor of gene expression post-transcriptionally via inhibiting a network node in the eIF4E RNA regulon. An example is given for how the PML RNA regulon model provided the basis for the development of a new anti-cancer strategy being tested in the clinic.

Promyelocytic leukemia protein induces apoptosis due to caspase-8 activation via the repression of NFkappaB activation in glioblastoma

Promyelocytic leukemia (PML) protein plays an essential role in the induction of apoptosis; its expression is reduced in various cancers. As the functional roles of PML in glioblastoma multiforme (GBM) have not been clarified, we assessed the expression of PML protein in GBM tissues and explored the mechanisms of PML-regulated cell death in GBM cells. We examined the PML mRNA level and the expression of PML protein in surgical GBM specimens. PML-regulated apoptotic mechanisms in GBM cells transfected with plasmids expressing the PML gene were examined. The protein expression of PML was significantly lower in GBM than in non-neoplastic tissues; approximately 10% of GBM tissues were PML-null. The PML mRNA levels were similar in both tissue types. The overexpression of PML activated caspase-8 and induced apoptosis in GBM cells. In these cells, PML decreased the expression of transactivated forms of NFkappaB/p65, and c-FLIP gene expression was suppressed. Therefore, PML-induced apoptosis resulted from the suppression of the transcriptional activity of NFkappaB/p65. PML overexpression decreased phosphorylated IkappaBalpha and nuclear NFkappaB/p65 and increased the expression of the suppressor of cytokine signaling (SOCS-1). A proteasome inhibitor blocked the reduction of activated p65 by PML. The reduction of PML is associated with the pathogenesis of GBM. PML induces caspase-8-dependent apoptosis via the repression of NFkappaB activation by which PML facilitates the proteasomal degradation of activated p65 and the sequestration of p65 with IkappaBalpha in the cytoplasm. This novel mechanism of PML-regulated apoptosis may represent a therapeutic target for GBM.

Retinoic acid attenuates promyelocytic leukemia protein-induced cell death in breast cancer cells by activation of the ubiquitin–proteasome pathway

All-trans-retinoic acid and the tumor suppressor promyelocytic leukemia protein (PML) are potent regulators of the growth of cancer cells. This study investigates the individual and combined effects of PML, when overexpressed by the recombinant PML adenovirus, and all-trans-retinoic acid on the proliferation of human estrogen-receptor negative SKBR-3 and estrogen-receptor positive MCF-7 breast cancer cell lines. All-trans-retinoic acid caused a significant degree of cell death in SKBR-3 cells and MCF-7 cells, and PML elicited a similar incidence of or slightly more cell death in MCF-7 cells. Dual-treated cells displayed significantly less cell death than did single-treated cells in the same cell line. We concluded that PML and all-trans-retinoic acid cause cell death by different pathways: PML activates ERK1/2, p38 MAPK, and p21; arrests the cell cycle; and later causes cell death; and all-trans-retinoic acid activates proteasome function, caspase cleavage, and apoptosis. The combined use of all-trans-retinoic acid and PML gene therapy may not be the best treatment for patients with cancer, because the ubiquitinylation of PML and its subsequent proteasome-dependent degradation by retinoic acids occur before overexpressed PML exhibits tumor-suppressive activity.