Insights of RKIP-Derived Suppression of Prostate Cancer

Simple Summary

Despite an intensive research effort in the past few decades, prostate cancer (PC) remains a top cause of cancer death in men, particularly in the developed world. The major cause of fatality is the progression of local prostate cancer to metastasis disease. Treatment of patients with metastatic prostate cancer (mPC) is generally ineffective. Based on the discovery of mPC relying on androgen for growth, many patients with mPC show an initial response to the standard of care: androgen deprivation therapy (ADT). However, lethal castration resistant prostate cancers (CRPCs) commonly develop. It is widely accepted that intervention of metastatic progression of PC is a critical point of intervention to reduce PC death. Accumulative evidence reveals a role of RKIP in suppression of PC progression towards mPC. We will review current evidence and discuss the potential utilization of RKIP in preventing mPC progression.

Abstract

Prostate cancer (PC) is a major cause of cancer death in men. The disease has a great disparity in prognosis. Although low grade PCs with Gleason scores ≤ 6 are indolent, high-risk PCs are likely to relapse and metastasize. The standard of care for metastatic PC (mPC) remains androgen deprivation therapy (ADT). Resistance commonly occurs in the form of castration resistant PC (CRPC). Despite decades of research efforts, CRPC remains lethal. Understanding of mechanisms underpinning metastatic progression represents the overarching challenge in PC research. This progression is regulated by complex mechanisms, including those regulating PC cell proliferation, epithelial–mesenchymal transition (EMT), and androgen receptor (AR) signaling. Among this PC metastatic network lies an intriguing suppressor of PC metastasis: the Raf kinase inhibitory protein (RKIP). Clinically, the RKIP protein is downregulated in PC, and showed further reduction in mPC. In xenograft mouse models for PC, RKIP inhibits metastasis. In vitro, RKIP reduces PC cell invasion and sensitizes PC cells to therapeutic treatments. Mechanistically, RKIP suppresses Raf-MEK-ERK activation and EMT, and modulates extracellular matrix. In return, Snail, NFκB, and the polycomb protein EZH2 contribute to inhibition of RKIP expression. In this review, we will thoroughly analyze RKIP’s tumor suppression actions in PC.

RKIP Pleiotropic Activities in Cancer and Inflammatory Diseases: Role in Immunity

Simple Summary

The human body consists of tissues and organs formed by cells. In each cell there is a switch that allows the cell to divide or not. In contrast, cancer cells have their switch on which allow them to divide and invade other sites leading to death. Over two decades ago, Doctor Kam Yeung, University of Toledo, Ohio, has identified a factor (RKIP) that is responsible for the on/off switch which functions normally in healthy tissues but is inactive or absent in cancers. Since this early discovery, many additional properties have been ascribed to RKIP including its role in inhibiting cancer metastasis and resistance to therapeutics and its role in modulating the normal immune response. This review describes all of the above functions of RKIP and suggesting therapeutics to induce RKIP in cancers to inhibit their growth and metastases as well as inhibit its activity to treat non-cancerous inflammatory diseases.

Abstract

Several gene products play pivotal roles in the induction of inflammation and the progression of cancer. The Raf kinase inhibitory protein (RKIP) is a cytosolic protein that exerts pleiotropic activities in such conditions, and thus regulates oncogenesis and immune-mediated diseases through its deregulation. Herein, we review the general properties of RKIP, including its: (i) molecular structure; (ii) involvement in various cell signaling pathways (i.e., inhibition of the Raf/MEK/ERK pathway; the NF-kB pathway; GRK-2 or the STAT-3 pathway; as well as regulation of the GSK3Beta signaling; and the spindle checkpoints); (iii) regulation of RKIP expression; (iv) expression’s effects on oncogenesis; (v) role in the regulation of the immune system to diseases (i.e., RKIP regulation of T cell functions; the secretion of cytokines and immune mediators, apoptosis, immune check point inhibitors and RKIP involvement in inflammatory diseases); and (vi) bioinformatic analysis between normal and malignant tissues, as well as across various immune-related cells. Overall, the regulation of RKIP in different cancers and inflammatory diseases suggest that it can be used as a potential therapeutic target in the treatment of these diseases.

Overexpression of RKIP and its cross-talk with several regulatory gene products in multiple myeloma

Multiple myeloma (MM) is a clonal plasma-cell neoplastic disorder arising from an indolent premalignant disease known as monoclonal gammopathy of undetermined significance (MGUS). MM is a biologically complex heterogeneous disease reflected by its variable clinical responses of patients receiving the same treatment. Therefore, a molecular identification of stage-specific biomarkers will support a more individualized precise diagnostic/prognostic approach, an effective therapeutic regime, and will assist in the identification of novel therapeutic molecular targets. The metastatic suppressor/anti-resistance factor Raf-1 kinase inhibitor protein (RKIP) is poorly expressed in the majority of cancers and is often almost absent in metastatic tumors. RKIP inhibits the Raf/MEK/ERK1/2 and the NF-κB pathways. Whereby all tumors examined exhibited low levels of RKIP, in contrast, our recent findings demonstrated that RKIP is overexpressed primarily in its inactive phosphorylated form in MM cell lines and patient-derived tumor tissues. The underlying mechanism of RKIP overexpression in MM, in contrast to other tumors, is not known. We examined transcriptomic datasets on Oncomine platform (Life Technologies) for the co-expression of RKIP and other gene products in both pre-MM and MM. The transcription of several gene products was found to be either commonly overexpressed (i.e., RKIP, Bcl-2, and DR5) or underexpressed (i.e., Bcl-6 and TNFR2) in both pre-MM and MM. Noteworthy, a significant inverse correlation of differentially expressed pro-apoptotic genes was observed in pre-MM: overexpression of Fas and TNF-α and underexpression of YY1 versus expression of anti-apoptotic genes in MM: overexpression of YY1 and underexpression of Fas and TNF-α. Based on the analysis on mRNA levels and reported studies on protein levels of the above various genes, we have constructed various schemes that illustrate the possible cross-talks between RKIP (active/inactive) and the identified gene products that underlie the mechanism of RKIP overexpression in MM. Clearly, such cross-talks would need to be experimentally validated in both MM cell lines and patient-derived tumor tissues. If validated, the differential molecular signatures between pre-MM and MM might lead to a more precise diagnosis/prognosis of the disease and disease stages and will also identify novel molecular therapeutic targets for pre-MM and MM.

Effect of Raf kinase inhibitor protein expression on malignant biological behavior and progression of colorectal cancer

The Raf kinase inhibitor protein (RKIP) is a novel metastasis suppressor. RKIP was previously found to have low expression in a colorectal cancer (CRC) patient cohort by immunohistochemistry. However, the role of RKIP in CRC remains undetermined. In the present study, immunohistochemistry was performed to compare RKIP expression between 129 paired stage II CRC and adjacent non-tumorous tissues. The correlations between clinical parameters, prognosis and RKIP expression were evaluated. To investigate the effect of RKIP on proliferation and metastasis, RKIP was overexpressed and knocked down in colon cancer cell lines. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), Transwell and wound-healing assays were performed. Murine models were established to confirm the influence of RKIP on malignant tumor phenotypes in vivo. Our results showed that RKIP expression was significantly decreased in the CRC tissues compared to the adjacent non‑cancerous tissues (p<0.001) and was correlated with the risk of relapse in stage II CRC (p<0.05). Overexpression of RKIP suppressed HCT116 cell metastasis in vitro and in vivo, whereas knockdown of RKIP expression in SW480 cells and its murine model increased metastatic ability (p<0.05). No effect of RKIP on cell proliferation in CRC was observed. These data suggest that RKIP is an important metastasis-suppressor gene in CRC. The re-expression of RKIP could be a potential therapeutic target for antimetastatic strategies for CRC.

Combined treatment of PC-3 cells with ultrasound and microbubbles suppresses invasion and migration

The aim of the present study was to investigate whether ultrasound treatment combined with microbubbles inhibits cell invasion and migration in androgen-independent prostate cancer (PCa) cells and to identify the probable mechanism. Ultrasound was used in continuous wave mode at a frequency of 21 kHz and with a spatial-average temporal-average intensity of 46 mW/cm². Ultrasound combined with microbubbles (200 μl; SonoVue) was administered to androgen-independent human PCa PC-3 cells for 30 sec. The PC-3 cells were divided into three groups: The control group, the ultrasound group (US) and the ultrasound combined with microbubbles group (US + MB). Following treatment for 12, 24, 48 and 72 h, cell counting kit-8 was used to assess cell viability. Cell invasion and migration was measured 12 h after treatment using Transwell migration assays. Quantitative polymerase chain reaction and western blot analysis were used to evaluate the expression of the migration-associated proteins, matrix metalloproteinase (MMP)-2 and MMP-9. Cell reproduction levels in the US and US + MB groups were significantly suppressed when compared with the control group (P<0.01) following 24 h of treatment and this suppression was significantly higher in the US + MB group than in the US group (P<0.01). However, no significant differences in cell reproduction levels between the three groups were identified at 12 h (P>0.05). Ultrasound combined with microbubbles significantly suppressed the level of invasion and migration in the PC-3 cells compared with the control group (190.83±14.63 vs. 509.67±18.62, P<0.01; and 86.67±10.60 vs. 271.33±65.14; P<0.01, respectively). Furthermore, combined treatment with ultrasound and microbubbles suppressed the expression of MMP-2 and MMP-9. In conclusion, it was found that ultrasound combined with microbubbles suppressed invasion and migration in human PCa PC-3 cells via downregulation of MMP-2 and MMP-9.

RKIP: much more than Raf kinase inhibitory protein

From its discovery as a phosphatidylethanolamine-binding protein in bovine brain to its designation as a physiological inhibitor of Raf kinase protein, RKIP has emerged as a critical molecule for maintaining subdued, well-orchestrated cellular responses to stimuli. The disruption of RKIP in a wide range of pathologies, including cancer, Alzheimer's disease, and pancreatitis, makes it an exciting target for individualized therapy and disease-specific interventions. This review attempts to highlight recent advances in the RKIP field underscoring its potential role as a master modulator of many pivotal intracellular signaling cascades that control cellular growth, motility, apoptosis, genomic integrity, and therapeutic resistance. Specific biological and functional niches are highlighted to focus future research towards an enhanced understanding of the multiple roles of RKIP in health and disease.

Quantitative proteomic analysis in metastatic renal cell carcinoma reveals a unique set of proteins with potential prognostic significance

Metastatic renal cell carcinoma (RCC) is one of the most treatment-resistant malignancies, and patients have a dismal prognosis, with a <10% five-year survival rate. The identification of markers that can predict the potential for metastases will have a great effect in improving patient outcomes. In this study, we used differential proteomics with isobaric tags for relative and absolute quantitation (iTRAQ) labeling and LC-MS/MS analysis to identify proteins that are differentially expressed in metastatic and primary RCC. We identified 1256 non-redundant proteins, and 456 of these were quantified. Further analysis identified 29 proteins that were differentially expressed (12 overexpressed and 17 underexpressed) in metastatic and primary RCC. Dysregulated protein expressions of profilin-1 (Pfn1), 14-3-3 zeta/delta (14-3-3ζ), and galectin-1 (Gal-1) were verified on two independent sets of tissues by means of Western blot and immunohistochemical analysis. Hierarchical clustering analysis showed that the protein expression profile specific for metastatic RCC can distinguish between aggressive and non-aggressive RCC. Pathway analysis showed that dysregulated proteins are involved in cellular processes related to tumor progression and metastasis. Furthermore, preliminary analysis using a small set of tumors showed that increased expression of Pfn1 is associated with poor outcome and is a potential prognostic marker in RCC. In addition, 14-3-3ζ and Gal-1 also showed higher expression in tumors with poor prognosis than in those with good prognosis. Dysregulated proteins in metastatic RCC represent potential prognostic markers for kidney cancer patients, and a greater understanding of their involved biological pathways can serve as the foundation of the development of novel targeted therapies for metastatic RCC.