Elucidating the Role of Chmp1 Overexpression in the Transport of Polyamines in Drosophila melanogaster

Polyamines are small organic cations that are essential for many biological processes such as cell proliferation and cell cycle progression. While the metabolism of polyamines has been well studied, the mechanisms by which polyamines are transported into and out of cells are poorly understood. Here, we describe a novel role of Chmp1, a vesicular trafficking protein, in the transport of polyamines using a well-defined leg imaginal disc assay in Drosophila melanogaster larvae. We show that Chmp1 overexpression had no effect on leg development in Drosophila, but does attenuate the negative impact on leg development of Ant44, a cytotoxic drug known to enter cells through the polyamine transport system (PTS), suggesting that the overexpression of Chmp1 downregulated the PTS. Moreover, we showed that the addition of spermine did not rescue the leg development in Chmp1-overexpressing leg discs treated with difluoromethylornithine (DFMO), an inhibitor of polyamine metabolism, while putrescine and spermidine did, suggesting that there may be unique mechanisms of import for individual polyamines. Thus, our data provide novel insight into the underlying mechanisms that are involved in polyamine transport and highlight the utility of the Drosophila imaginal disc assay as a fast and easy way to study potential players involved in the PTS.

The Role of Spermidine Synthase (SpdS) and Spermine Synthase (Sms) in Regulating Triglyceride Storage in Drosophila

Polyamines are small organic cations that are important for several biological processes such as cell proliferation, cell cycle progression, and apoptosis. The dysregulation of intracellular polyamines is often associated with diseases such as cancer, diabetes, and developmental disorders. Although polyamine metabolism has been well studied, the effects of key enzymes in the polyamine pathway on lipid metabolism are not well understood. Here, we determined metabolic effects resulting from the absence of spermidine synthase (SpdS) and spermine synthase (Sms) in Drosophila. While SpdS mutants developed normally and accumulated triglycerides, Sms mutants had reduced viability and stored less triglyceride than the controls. Interestingly, when decreasing SpdS and Sms, specifically in the fat body, triglyceride storage increased. While there was no difference in triglycerides stored in heads, thoraxes and abdomen fat bodies, abdomen fat body DNA content increased, and protein/DNA decreased in both SpdS- and Sms-RNAi flies, suggesting that fat body-specific knockdown of SpdS and Sms causes the production of smaller fat body cells and triglycerides to accumulate in non-fat body tissues of the abdomen. Together, these data provide support for the role that polyamines play in the regulation of metabolism and can help enhance our understanding of polyamine function in metabolic diseases.

The regulation of triglyceride storage by ornithine decarboxylase (Odc1) in Drosophila

Polyamines are low molecular weight, organic cations that play a critical role in many major cellular processes including cell cycle regulation and apoptosis, cellular division, tissue proliferation, and cellular differentiation; however, the functions of polyamines in regulating the storage of metabolic fuels such as triglycerides and glycogen is poorly understood. To address this question, we focused on the Drosophila homolog of ornithine decarboxylase (Odc1), the first rate-limiting enzyme in the synthesis of polyamines. Mutants in Odc1 are lethal, but heterozygotes were viable to adulthood. Odc1 heterozygotes appeared larger than their genetic background control flies and consistent with this observation, weighed more than the controls. However, the increased weight was not due to increased food consumption as heterozygotes ate less than the controls. Interestingly, Odc1 heterozygous flies had augmented triglyceride storage, and this lipid phenotype was due to increased triglyceride storage per cell and an increase in the number of fat cells produced. Odc1 heterozygous flies also displayed increased expression of the lipid synthesis genes fatty acid synthase (FASN) and acetyl-CoA carboxylase (ACC), suggesting increased lipid synthesis was the cause of the augmented triglyceride phenotype. These results provide a link between the expression of Odc1 and triglyceride storage suggesting that the polyamine pathway plays a role in regulating lipid metabolism.

Knocking down raptor in human keratinocytes affects ornithine decarboxylase in a post-transcriptional Manner following ultraviolet B exposure

Non-melanoma skin cancer (NMSC) is the most common form of cancer. Ultraviolet-B (UVB) radiation has been shown to be a complete carcinogen in the development of NMSC. The mammalian target of rapamycin complex 1 (mTORC1) is upregulated by UVB. Ornithine decarboxylase (ODC), the first enzyme of the polyamine biosynthetic pathway, is also upregulated in response to UVB. However, the interplay between these two pathways after UVB exposure remains unclear. The studies described here compare mRNA stability between normal human keratinocytes (HaCaT cells) and HaCaT cells with low levels of raptor to investigate whether the induction of ODC by UVB is dependent on mTORC1. We show that the knockdown of mTORC1 activity led to decreased levels of ODC protein both before and after exposure to 20 mJ/cm² UVB. ODC mRNA was less stable in cells with decreased mTORC1 activity. Polysome profiles revealed that the initiation of ODC mRNA translation did not change in UVB-treated cells. We have shown that the ODC transcript is stabilized by the RNA-binding protein human antigen R (HuR). To expand these studies, we investigated whether HuR functions to regulate ODC mRNA stability in human keratinocytes exposed to UVB. We show an increased cytoplasmic localization of HuR after UVB exposure in wild-type cells. The ablation of HuR via CRISPR/Cas9 did not alter the stability of the ODC message, suggesting the involvement of other trans-acting factors. These data suggest that in human keratinocytes, ODC mRNA stability is regulated, in part, by an mTORC1-dependent mechanism after UVB exposure.

The ODC 3'-Untranslated Region and 5'-Untranslated Region Contain cis-Regulatory Elements: Implications for Carcinogenesis

It has been hypothesized that both the 3′-untranslated region (3′UTR) and the 5′-untranslated region (5′UTR) of the ornithine decarboxylase (ODC) mRNA influence the expression of the ODC protein. Here, we use luciferase expression constructs to examine the influence of both UTRs in keratinocyte derived cell lines. The ODC 5′UTR or 3′UTR was cloned into the pGL3 control vector upstream or downstream of the luciferase reporter gene, respectively, and luciferase activity was measured in both non-tumorigenic and tumorigenic mouse keratinocyte cell lines. Further analysis of the influence of the 3′UTR on luciferase activity was accomplished through site-directed mutagenesis and distal deletion analysis within this region. Insertion of either the 5′UTR or 3′UTR into a luciferase vector resulted in a decrease in luciferase activity when compared to the control vector. Deletion analysis of the 3′UTR revealed a region between bases 1969 and 2141 that was inhibitory, and mutating residues within that region increased luciferase activity. These data suggest that both the 5′UTR and 3′UTR of ODC contain cis-acting regulatory elements that control intracellular ODC protein levels.

Destabilization of the ornithine decarboxylase mRNA transcript by the RNA-binding protein tristetraprolin

Ornithine decarboxylase (ODC) is the first and usually rate-limiting enzyme in the polyamine biosynthetic pathway. In a normal physiological state, ODC is tightly regulated. However, during neoplastic transformation, ODC expression becomes upregulated. The studies described here show that the ODC mRNA transcript is destabilized by the RNA-binding protein tristetraprolin (TTP). We show that TTP is able to bind to the ODC mRNA transcript in both non-transformed RIE-1 cells and transformed Ras12V cells. Moreover, using mouse embryonic fibroblast cell lines that are devoid of a functional TTP protein, we demonstrate that in the absence of TTP both ODC mRNA stability and ODC enzyme activity increase when compared to wild-type cells. Finally, we show that the ODC 3' untranslated region contains cis acting destabilizing elements that are affected by, but not solely dependent on, TTP expression. Together, these data support the hypothesis that TTP plays a role in the post-transcriptional regulation of the ODC mRNA transcript.

Skin Carcinogenesis Studies Using Mouse Models with Altered Polyamines

Nonmelanoma skin cancer (NMSC) is a major health concern worldwide. With increasing numbers in high-risk groups such as organ transplant recipients and patients taking photosensitizing medications, the incidence of NMSC continues to rise. Mouse models of NMSC allow us to better understand the molecular signaling cascades involved in skin tumor development in order to identify novel therapeutic strategies. Here we review the models designed to determine the role of the polyamines in NMSC development and maintenance. Elevated polyamines are absolutely required for tumor growth, and dysregulation of their biosynthetic and catabolic enzymes has been observed in NMSC. Studies using mice with genetic alterations in epidermal polyamines suggest that they play key roles in tumor promotion and epithelial cell survival pathways, and recent clinical trials indicate that pharmacological inhibitors of polyamine metabolism show promise in individuals at high risk for NMSC.

Structure-activity study for (bis)ureidopropyl- and (bis)thioureidopropyldiamine LSD1 inhibitors with 3-5-3 and 3-6-3 carbon backbone architectures

Methylation at specific histone lysine residues is a critical post-translational modification that alters chromatin architecture, and dysregulated lysine methylation/demethylation is associated with the silencing of tumor suppressor genes. The enzyme lysine-specific demethylase 1 (LSD1) complexed to specific transcription factors catalyzes the oxidative demethylation of mono- and dimethyllysine 4 of histone H3 (H3K4me and H3K4me2, respectively). We have previously reported potent (bis)urea and (bis)thiourea LSD1 inhibitors that increase cellular levels of H3K4me and H3K4me2, promote the re-expression of silenced tumor suppressor genes and suppress tumor growth in vitro. Here we report the design additional (bis)urea and (bis)thiourea LSD1 inhibitors that feature 3-5-3 or 3-6-3 carbon backbone architectures. Three of these compounds displayed single-digit IC50 values in a recombinant LSD1 assay. In addition, compound 6d exhibited an IC50 of 4.2μM against the Calu-6 human lung adenocarcinoma line, and 4.8μM against the MCF7 breast tumor cell line, in an MTS cell viability assay. Following treatment with 6b-6d, Calu-6 cells exhibited a significant increase in the mRNA expression for the silenced tumor suppressor genes SFRP2, HCAD and p16, and modest increases in GATA4 message. The compounds described in this paper represent the most potent epigenetic modulators in this series, and have potential for use as antitumor agents.

Cytoplasmic accumulation of the RNA-binding protein HuR stabilizes the ornithine decarboxylase transcript in a murine nonmelanoma skin cancer model

Ornithine decarboxylase (ODC) is the first and usually rate-limiting enzyme in the polyamine biosynthetic pathway. Under normal physiological conditions, polyamine content and ODC enzyme activity are highly regulated. However, the induction of ODC activity is an early step in neoplastic transformation. The studies described here use normal mouse keratinocytes (C5N cells), and spindle carcinoma cells (A5 cells) to explore the regulation of ODC in nonmelanoma skin cancer development. Previous results have shown that induction of ODC activity is both necessary and sufficient for the promotion of skin tumors. We see a marked increase in ODC enzyme activity in A5 cells compared with C5N keratinocytes, which correlates with a 4-fold stabilization of ODC mRNA. These data suggest that ODC is post-transcriptionally regulated in skin tumor development. Thus, we sought to investigate whether the ODC transcript interacts with the RNA-binding protein HuR, which is known to bind to and stabilize its target mRNAs. We show that HuR is able to bind to the ODC 3'-UTR in A5 cells but not in C5N cells. Immunofluorescence results reveal that HuR is present in both the nucleus and cytoplasm of A5 cells, whereas C5N cells exhibit strictly nuclear localization of HuR. Knockdown experiments in A5 cells showed that when HuR is depleted, ODC RNA becomes less stable, and ODC enzyme activity decreases. Together, these data support the hypothesis that HuR plays a causative role in ODC up-regulation during nonmelanoma skin cancer development by binding to and stabilizing the ODC transcript.

Abstract 3161: The RNA binding protein HuR stabilizes the ornithine decarboxylase (ODC) transcript in non-melanoma skin cancer

Non-melanoma skin cancer (NMSC), comprised of both basal cell carcinoma and squamous cell carcinoma, is the most common malignancy in the United States. The work described here focuses on understanding the underlying mechanisms of this disease using a mouse keratinocyte model comprised of two cells lines that represent distinct stages in skin carcinogenesis, initiation and progression, denoted as C5N and A5 respectively. Previous results have shown that induction of ornithine decarboxylase (ODC) enzyme activity is both necessary and sufficient for the promotion of skin tumors. As expected, we saw a dramatic increase in ODC enzyme activity in A5 spindle carcinoma cells when compared to C5N keratinocytes. Interestingly, this increase in ODC enzyme activity correlates with an increase in ODC stability in A5 cells compared to C5N cells. Thus, we sought to investigate whether ODC mRNA interacted with the RNA binding protein HuR, which is known to bind to and stabilize its target mRNA transcripts. HuR has been shown by others to localize to the cytoplasm where it stabilizes numerous transcripts corresponding to proteins that are important in the development of a variety of malignancies. Moreover, studies involving human tumor samples from breast cancer, ovarian cancer, and Merkel cell carcinoma patients have shown a correlation between high cytoplasmic HuR presence and increased tumor grade. By conducting biotin-pulldown assays, which use a synthetic ODC transcript to test for RNA-protein interactions, as well as mRNP assays, which identify endogenous RNA-protein interactions, we confirmed that HuR is able to bind to the ODC 3’UTR in A5 cells but not in C5N cells. Immunofluorescence results showed that HuR localization between the two cell lines was different, with A5 cells displaying a diffuse HuR presence in both the nucleus and cytoplasm while C5N cells exhibited nuclear localization of HuR. In order to definitively link HuR to ODC mRNA stability, knockdown experiments were conducted using a short interfering RNA (siRNA) that was specific for the coding region of HuR. These knockdown experiments confirmed our hypothesis that HuR is able to bind to and stabilize ODC mRNA in A5 cells, as a 60% knockdown of HuR protein correlated with a profound decrease in ODC mRNA half-life and reduced intracellular ODC protein. These results suggest that the stabilization of ODC mRNA in A5 spindle carcinoma cells is regulated directly by changes in localization of the HuR protein. These data are novel, as they show for the first time that an RNA binding protein is able to bind to and regulate ODC mRNA stability. Knowledge from these and subsequent studies could be used to design RNA binding protein-targeted therapies to combat not only NMSC but other epithelial cancers and/or non-malignant anomalies that display an increased presence of cytoplasmic HuR.

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 3161.