Proline metabolism and cancer: emerging links to glutamine and collagen

Metabolic PET Imaging in Oncology

OBJECTIVE

In this article, we provide a general overview of how cancer cells subvert critical metabolic pathways to support their growth and unchecked division. Furthermore, we outline how molecular imaging can diagnostically exploit the resulting differences between cancer and normal cells.

CONCLUSION

Molecular PET can provide valuable information about the metabolic dysregulation in cancer.

Cartilaginous Metabolomic Study Reveals Potential Mechanisms of Osteophyte Formation in Osteoarthritis

Osteophyte is one of the inevitable consequences of progressive osteoarthritis with the main characteristics of cartilage degeneration and endochondral ossification. The pathogenesis of osteophyte formation is not fully understood to date. In this work, metabolomic approaches were employed to explore potential mechanisms of osteophyte formation by detecting metabolic variations between extracts of osteophyte cartilage tissues (n = 32) and uninvolved control cartilage tissues (n = 34), based on the platform of ultraperformance liquid chromatography tandem quadrupole time-of-flight mass spectrometry, as well as the use of multivariate statistic analysis and univariate statistic analysis. The osteophyte group was significantly separated from the control group by the orthogonal partial least-squares discriminant analysis models, indicating that metabolic state of osteophyte cartilage had been changed. In total, 28 metabolic variations further validated by mass spectrum (MS) match, tandom mass spectrum (MS/MS) match, and standards match mainly included amino acids, sulfonic acids, glycerophospholipids, and fatty acyls. These metabolites were related to some specific physiological or pathological processes (collagen dissolution, boundary layers destroyed, self-restoration triggered, etc.) which might be associated with the procedure of osteophyte formation. Pathway analysis showed phenylalanine metabolism (PI = 0.168, p = 0.004) was highly correlative to this degenerative process. Our findings provided a direction for targeted metabolomic study and an insight into further reveal the molecular mechanisms of ostophyte formation.

The fluctuation of free amino acids in serum during acute ischemic stroke

Currently, little data exists regarding the involvement of free amino acids (AA) in the pathogenesis of ischemic stroke (IS). Thus, our objective was to study the degree of the degree of fluctuation of free amino acids level in serum during the acute phase of IS. The study consisted of eighteen patients (female/male: 10/8; age: 73.1 ± 4.1) with acute IS that was confirmed by way of computed tomography, while twelve sex and age matched individuals were assigned as control group. During the study period, the patients did not receive any supplemental amino acids therapy that could affect the obtained results. The venous blood was obtained after >3 hours fasting at two time-points; time-point 1 – at admission to the hospital; time-point 2 – on day 5 from stroke onset. The blood for control purposes was collected only once, and the blood collection at time-point 1 was done before thrombolytic treatment (nine patients). The amino acids were identified using the Amino Acids Analyser (AAA 400) by INGOS Corp., Praha, Czech Republic. Our results revealed a statistically significant increase of glutamate, cystine and methionine on day 1 of stroke, in comparison to control, whereas, proline level was decreased on day 1 of stroke – in comparison to control serum. On comparing day 5 to the initial day of IS, elevation was observed of levels of asparagine, glycine, tyrosine, arginine, threonine, valine, leucine and phenylalanine. It can be said, then, that ischemic stroke induces both essential and nonessential amino acid fluctuations. Moreover, the decrease in proline and glutamine serum level with the simultaneous increase in the concentration of branch chain amino acids, Glu and Thr suggests a violent mobilization of the body’s proteins. Thus, a decrease of Pro and a simultaneous increase of Glu serum level could be considered as a marker of acute IS.

Metabolic synthetic lethality in cancer therapy

Our understanding of cancer has recently seen a major paradigm shift resulting in it being viewed as a metabolic disorder, and altered cellular metabolism being recognised as a hallmark of cancer. This concept was spurred by the findings that the oncogenic mutations driving tumorigenesis induce a reprogramming of cancer cell metabolism that is required for unrestrained growth and proliferation. The recent discovery that mutations in key mitochondrial enzymes play a causal role in tumorigenesis suggested that dysregulation of metabolism could also be a driver of tumorigenesis. These mutations induce profound adaptive metabolic alterations that are a prerequisite for the survival of the mutated cells. Because these metabolic events are specific to cancer cells, they offer an opportunity to develop new therapies that specifically target tumour cells without affecting healthy tissue. Here, we will describe recent developments in metabolism-based cancer therapy, in particular focusing on the concept of metabolic synthetic lethality. This article is part of a Special Issue entitled Mitochondria in Cancer, edited by Giuseppe Gasparre, Rodrigue Rossignol and Pierre Sonveaux.

Metabolomics reveals the mechanisms for the cardiotoxicity of Pinelliae Rhizoma and the toxicity-reducing effect of processing

Pinelliae Rhizoma (PR) is a commonly used Chinese medicinal herb, but it has been frequently reported about its toxicity. According to the traditional Chinese medicine theory, processing can reduce the toxicity of the herbs. Here, we aim to determine if processing reduces the toxicity of raw PR, and to explore the underlying mechanisms of raw PR-induced toxicities and the toxicity-reducing effect of processing. Biochemical and histopathological approaches were used to evaluate the toxicities of raw and processed PR. Rat serum metabolites were analyzed by LC-TOF-MS. Ingenuity pathway analysis of the metabolomics data highlighted the biological pathways and network functions involved in raw PR-induced toxicities and the toxicity-reducing effect of processing, which were verified by molecular approaches. Results showed that raw PR caused cardiotoxicity, and processing reduced the toxicity. Inhibition of mTOR signaling and activation of the TGF-β pathway contributed to raw PR-induced cardiotoxicity, and free radical scavenging might be responsible for the toxicity-reducing effect of processing. Our data shed new light on the mechanisms of raw PR-induced cardiotoxicity and the toxicity-reducing effect of processing. This study provides scientific justifications for the traditional processing theory of PR, and should help in optimizing the processing protocol and clinical combinational application of PR.

From Krebs to clinic: glutamine metabolism to cancer therapy

The resurgence of research into cancer metabolism has recently broadened interests beyond glucose and the Warburg effect to other nutrients, including glutamine. Because oncogenic alterations of metabolism render cancer cells addicted to nutrients, pathways involved in glycolysis or glutaminolysis could be exploited for therapeutic purposes. In this Review, we provide an updated overview of glutamine metabolism and its involvement in tumorigenesis in vitro and in vivo, and explore the recent potential applications of basic science discoveries in the clinical setting.

Can specific nutrients stimulate bowel wound healing?

PURPOSE OF REVIEW

The purpose of review is to provide an overview on specific nutrients which play an important role in bowel wound healing, and to judge the efficacy of supplementation to derive recommendations for clinical practice.

RECENT FINDINGS

Glutamine, arginine, butyrate, ω-3 fatty acids, nucleotides, and several micronutrients are involved in bowel wound healing. However, with regard to clinical trials, the efficacy of supplementation of specific nutrients on bowel wound healing in patients with inflammatory bowel diseases has not been clarified yet. In patients undergoing colon surgery, sufficient evidence exists that the perioperative supply of an enteral immunomodulating formula enriched with arginine, nucleotides, and ω-3 fatty acids may improve intestinal wound healing, considering the lower risk of wound infections, wound dehiscence, and intra-abdominal abscess.

SUMMARY

Even if a range of nutrients are involved in bowel wound healing, only perioperative supply of an enteral immunomodulating formula to cancer patients undergoing colon surgery, can be recommended. Further randomized controlled trials are needed to elucidate the efficacy of individual nutrients on intestinal wound healing.

Nonessential amino acid metabolism in breast cancer

Interest in studying cancer metabolism has risen in recent years, as it has become evident that the relationship between cancer and metabolic pathways could reveal novel biomarkers and therapeutic targets. Metabolic starvation therapy is particularly promising due to its low toxicity. Nonessential amino acids are promising metabolites for such therapy because they become essential in many tumor cells, including breast cancer cells. This review will focus on four nonessential amino acid metabolism pathways: glutamine-glutamate, serine-glycine, cysteine, and arginine-proline metabolism. Recent studies of these amino acids have revealed metabolic enzymes that have the potential to be effective as cancer therapy targets or biomarkers for response to metabolic starvation therapy. The review will also discuss features of nonessential amino acid metabolism that merit further investigation to determine their relevancy to breast cancer treatment.

Combination of Metabolomics with Cellular Assays Reveals New Biomarkers and Mechanistic Insights on Xenoestrogenic Exposures in MCF-7 Cells

The disruptive potential of xenoestrogens like bisphenol A (BPA) lies in their 17β-estradiol (E2)-like binding to estrogen receptors (ERs) followed by concomitant modulation of ER target gene expression. Unsurprisingly, most endocrine testing systems focus on the quantification of canonical transcripts or ER-sensitive reporters. However, only little information is available about the corresponding metabolomic changes in vitro. This knowledge gap becomes particularly relevant in the context of potential mixture effects, for example, as a consequence of coexposure to potentially estrogenically active pollutants (e.g., Cd²⁺). Such effects are often difficult to dissect with molecular tools, especially with regard to potential physiological relevance. Metabolomic biomarkers are well-suited to address this latter aspect as they provide a comprehensive readout of whole-cell physiology. Applying a targeted metabolomics approach (FIA-MS/MS), this study looked for biomarkers indicative of xenoestrogenic exposure in MCF-7 cells. Cells were treated with E2 and BPA in the presence or absence of Cd²⁺. Statistical analysis revealed a total of 11 amino acids and phospholipids to be related to the compound's estrogenic potency. Co-exposure to Cd²⁺ modulated the estrogenic profile. However, the corresponding changes were found to be moderate with cellular assays such as the E-screen failing to record any Cd²⁺-specific estrogenic effects. Overall, metabolomics analysis identified proline as the most prominent estrogenic biomarker. Its increase could clearly be related to estrogenic exposure and concomitant ERα-mediated induction of proliferation. Involvement of the latter was confirmed by siRNA-mediated knockdown studies as well as by receptor inhibition. Further, the underlying signaling was also found to involve the oncoprotein MYC. Taken together, this study provides insights into the underlying mechanisms of xenoestrogenic effects and exemplify the strength of the complementary use of metabolomics and cellular and molecular assays.

Dose-Dependent Metabolic Reprogramming and Differential Gene Expression in TCDD-Elicited Hepatic Fibrosis

We have previously shown that in response to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-elicited NAFLD progression, central carbon, glutaminolysis, and serine/folate metabolism are reprogrammed to support NADPH production and ROS defenses. To further investigate underlying dose-dependent responses associated with TCDD-induced fibrosis, female C57BL/6 mice were gavaged with TCDD every 4 days (d) for 28 d or 92 d. RNA-Seq, ChIP-Seq (2 h), and 28 d metabolomic (urine, serum, and hepatic extract) analyses were conducted with complementary serum marker assessments at 92 d. Additional vehicle and 30 µg/kg treatment groups were allowed to recover for 36 d following the 92-d treatment regimen to examine recovery from TCDD-elicited fibrosis. Histopathology revealed dose-dependent increases in hepatic fat accumulation, inflammation, and periportal collagen deposition at 92 days, with increased fibrotic severity in the recovery group. Serum proinflammatory and profibrotic interleukins-1β, -2, -4, -6, and -10, as well as TNF-α and IFN-γ, exhibited dose-dependent induction. An increase in glucose tolerance was observed with a concomitant 3.0-fold decrease in hepatic glycogen linked to increased ascorbic acid biosynthesis and proline metabolism, consistent with increased fibrosis. RNA-Seq identified differential expression of numerous matrisome genes including an 8.8-fold increase in Tgfb2 indicating myofibroblast activation. Further analysis suggests reprogramming of glycogen, ascorbic acid, and amino acid metabolism in support of collagen deposition and the use of proline as a substrate for ATP production via the proline cycle. In summary, we demonstrate that glycogen, ascorbic acid, and amino acid metabolism are also reorganized to support remodeling of the extracellular matrix, progressing to hepatic fibrosis in response to chronic injury from TCDD.

Gene expression profiling indicates an increased capacity for proline, serine, and ATP synthesis and mitochondrial mass by the liver of steers grazing high vs. low endophyte-infected tall fescue

Grazing -infected forages results in a variety of reduced animal performance parameters, collectively known as "fescue toxicosis." The initial, limited evaluations of hepatic mechanisms affected by fescue toxicosis have used transcriptomic expression profiling of experimental phenotypes developed by short-term feeding of concentrated ergot alkaloids or fescue seeds to rodents and steers. To assess the effects of fescue toxicosis in growing cattle using a commercially relevant phenotype, we induced fescue toxicosis in beef steers by summer-long grazing (89 to 105 d) of a single high toxic endophyte-infected tall fescue pasture (HE; 0.746 μg/g ergot alkaloids; 5.7 ha; = 10; BW = 267 ± 14.5 kg) vs. a low toxic endophyte tall fescue-mixed pasture (LE; 0.023 μg/g ergot alkaloids; 5.7 ha; = 9; BW = 266 ± 10.9 kg). High toxic endophyte tall fescue-mixed pasture steers had decreased BW (313 vs. 338 kg) and an increased potential for hepatic gluconeogenesis from AA-derived carbons. To gain a greater perspective into fescue toxicosis-induced hepatic metabolism and identify candidate regulatory mechanisms, the goal of the current research was to examine liver samples for changes in gene (mRNA) expression profiles using a Bovine Affymetrix microarray and selected reverse-transcription PCR and immunoblot analyses. The expression (false discovery rate < 10%; < 0.01) of 147 genes was increased (7 to 268%) and that of 227 was decreased (4 to 87%) in livers of HE vs. LE steers. The top (1) functional gene category was cell-mediated immune response (33 genes; ≤ 0.012), (2) canonical cell signaling pathway was primary immunodeficiency signaling (8 genes; ≤ 0.0003), and (3) canonical metabolic pathways were oxidative phosphorylation (5 genes; ≤ 0.016) and purine metabolism (8 genes; ≤ 0.029). High toxic endophyte tall fescue-mixed pasture steers had increased ( ≤ 0.022) expression of genes critical for increased (1) Pro () and Ser () synthesis, (2) shunting of AA carbons into pyruvate () and ATP synthesis (, , , COX4, , and ), and (3) mitochondrial mass (COX4). Targeted reverse-transcribed PCR or immunoblot assays corroborated ( ≤ 0.035) these latter microarray findings for , , , , and COX4. Moreover, network analysis identified glucocorticoid receptor-mediated signaling as the most probable mechanism to coordinate the above findings. These results greatly extend our knowledge of the consequences of summer-long grazing of endophyte-infected tall fescue to the hepatic metabolism of growing steers.

High resolution metabolomics to discriminate compounds in serum of male lung cancer patients in South Korea

BACKGROUND

The cancer death rate escalated during 20th century. In South Korea, lung cancer is expected to contribute 12,736 deaths in men, the highest amount among all cancers. Several risk factors may increase the chance to acquiring lung cancer, with mostly related to exogenous compounds found in cigarette smoke and synthetic manufacturing materials. As the mortality rate of lung cancer increases, deeper understanding is necessary to explore risk factors that may lead to this malignancy. In this regard, this study aims to apply high resolution metabolomics (HRM) using LC-MS to detect significant compounds that might contribute in inducing lung cancer and find the correlation of these compounds to the subjects' smoking habit.

METHODS

The comparison was made between healthy control and lung cancer groups for metabolic differences. Further analyses to determine if these differences are related to tobacco-induced lung cancer (past-smoker control vs. past-smoker lung cancer patients (LCPs) and non-smoker control vs. current-smoker LCPs) were selected. The univariate analysis was performed, including a false discovery rate (FDR) of q = 0.05, to determine the significant metabolites between the analyses. Hierarchical clustering analysis (HCA) was done to discriminate metabolites between the control and case subjects. Selected compounds based on significant m/z features of human serum then experienced MS/MS examination, showing that for many m/z, the patterns of ion dissociation matched with standards. Then, the significant metabolites were identified using Metlin database and features were mapped on the human metabolic pathway mapping tool of the Kyoto Encyclopedia of Genes and Genomes (KEGG).

RESULTS

Using metabolomics-wide association studies, metabolic changes were observed among control group and lung cancer patients. Bisphenol A (211.11, [M + H-H2O](+)), retinol (287.23, [M + H](+)) and L-proline (116.07, [M + H](+)) were among the significant compounds found to have contributed in the discrimination between these groups, suggesting that these compounds might be related in the development of lung cancer. Retinol has been seen to have a correlation with smoking while both bisphenol A and L-proline were found to be unrelated.

CONCLUSIONS

Two potential biomarkers, retinol and L-proline, were identified and these findings may create opportunities for the development of new lung cancer diagnostic tools.

Arginine and proline applied as food additives stimulate high freeze tolerance in larvae of Drosophila melanogaster

The fruit fly Drosophila melanogaster is an insect of tropical origin. Its larval stage is evolutionarily adapted for rapid growth and development under warm conditions and shows high sensitivity to cold. In this study, we further developed an optimal acclimation and freezing protocol that significantly improves larval freeze tolerance (an ability to survive at −5°C when most of the freezable fraction of water is converted to ice). Using the optimal protocol, freeze survival to adult stage increased from 0.7% to 12.6% in the larvae fed standard diet (agar, sugar, yeast, cornmeal). Next, we fed the larvae diets augmented with 31 different amino compounds, administered in different concentrations, and observed their effects on larval metabolomic composition, viability, rate of development and freeze tolerance. While some diet additives were toxic, others showed positive effects on freeze tolerance. Statistical correlation revealed tight association between high freeze tolerance and high levels of amino compounds involved in arginine and proline metabolism. Proline- and arginine-augmented diets showed the highest potential, improving freeze survival to 42.1% and 50.6%, respectively. Two plausible mechanisms by which high concentrations of proline and arginine might stimulate high freeze tolerance are discussed: (i) proline, probably in combination with trehalose, could reduce partial unfolding of proteins and prevent membrane fusions in the larvae exposed to thermal stress (prior to freezing) or during freeze dehydration; (ii) both arginine and proline are exceptional among amino compounds in their ability to form supramolecular aggregates which probably bind partially unfolded proteins and inhibit their aggregation under increasing freeze dehydration.

Comparative genomics and evolution of transcriptional regulons in Proteobacteria

Comparative genomics approaches are broadly used for analysis of transcriptional regulation in bacterial genomes. In this work, we identified binding sites and reconstructed regulons for 33 orthologous groups of transcription factors (TFs) in 196 reference genomes from 21 taxonomic groups of Proteobacteria. Overall, we predict over 10 600 TF binding sites and identified more than 15 600 target genes for 1896 TFs constituting the studied orthologous groups of regulators. These include a set of orthologues for 21 metabolism-associated TFs from Escherichia coli and/or Shewanella that are conserved in five or more taxonomic groups and several additional TFs that represent non-orthologous substitutions of the metabolic regulators in some lineages of Proteobacteria. By comparing gene contents of the reconstructed regulons, we identified the core, taxonomy-specific and genome-specific TF regulon members and classified them by their metabolic functions. Detailed analysis of ArgR, TyrR, TrpR, HutC, HypR and other amino-acid-specific regulons demonstrated remarkable differences in regulatory strategies used by various lineages of Proteobacteria. The obtained genomic collection of in silico reconstructed TF regulons contains a large number of new regulatory interactions that await future experimental validation. The collection provides a framework for future evolutionary studies of transcriptional regulatory networks in Bacteria. It can be also used for functional annotation of putative metabolic transporters and enzymes that are abundant in the reconstructed regulons.

Fluorinated methacrylamide chitosan hydrogels enhance collagen synthesis in wound healing through increased oxygen availability

In this study, methacrylamide chitosan modified with perfluorocarbon chains (MACF) is used as the base material to construct hydrogel dressings for treating dermal wounds. MACF hydrogels saturated with oxygen (+O2) are examined for their ability to deliver and sustain oxygen, degrade in a biological environment, and promote wound healing in an animal model. The emerging technique of metabolomics is used to understand how MACF+O2 hydrogel dressings improve wound healing. Results indicate that MACF treatment facilitates oxygen transport rate that is two orders of magnitude greater than base MAC hydrogels. MACF hydrogel dressings are next tested in an in vivo splinted rat excisional wound healing model. Histological analysis reveals that MACF+O2 dressings improve re-epithelialization (p<0.0001) and synthesis of collagen over controls (p<0.01). Analysis of endogenous metabolites in the wounds using global metabolomics demonstrates that MACF+O2 dressings promotes a regenerative metabolic process directed toward hydroxyproline and collagen synthesis, with confirmation of metabolite levels within this pathway. The results of this study confirm that increased oxygen delivery through the application of MACF+O2 hydrogels enhances wound healing and metabolomics analyses provides a powerful tool to assess wound healing physiology.

STATEMENT OF SIGNIFICANCE

This work presents the first application of a novel class of oxygen delivering biomaterials (methacrylamide chitosan modified with perfluorocarbon chains (MACF)) as a hydrogel wound dressing. This manuscript also contains strong focus on the biochemical benefits of MACF dressings on underlying mechanisms vital to successful wound healing. In this vein, this manuscript presents the application of applied metabolomics (tandem mass spectroscopy) to uncover biomaterial interactions with wound healing mechanisms. We believe the approaches described in this manuscript will be of great interest to biomedical scientists and particularly to researchers studying wound healing, metabolomics, applied biomaterials and regenerative medicine.

Deletion of Amino Acid Transporter ASCT2 (SLC1A5) Reveals an Essential Role for Transporters SNAT1 (SLC38A1) and SNAT2 (SLC38A2) to Sustain Glutaminolysis in Cancer Cells

Many cancer cells depend on glutamine as they use the glutaminolysis pathway to generate building blocks and energy for anabolic purposes. As a result, glutamine transporters are essential for cancer growth and are potential targets for cancer chemotherapy with ASCT2 (SLC1A5) being investigated most intensively. Here we show that HeLa epithelial cervical cancer cells and 143B osteosarcoma cells express a set of glutamine transporters including SNAT1 (SLC38A1), SNAT2 (SLC38A2), SNAT4 (SLC38A4), LAT1 (SLC7A5), and ASCT2 (SLC1A5). Net glutamine uptake did not depend on ASCT2 but required expression of SNAT1 and SNAT2. Deletion of ASCT2 did not reduce cell growth but caused an amino acid starvation response and up-regulation of SNAT1 to replace ASCT2 functionally. Silencing of GCN2 in the ASCT2(-/-) background reduced cell growth, showing that a combined targeted approach would inhibit growth of glutamine-dependent cancer cells.

Arginase 1 is an innate lymphoid-cell-intrinsic metabolic checkpoint controlling type 2 inflammation

Group 2 innate lymphoid cells (ILC2s) regulate tissue inflammation and repair following activation by cell-extrinsic factors including host-derived cytokines. However, the cell-intrinsic metabolic pathways that control ILC2 function are undefined. Here we demonstrate that expression of the enzyme Arginase 1 (Arg1) is a conserved trait of murine and human ILC2s during acute or chronic lung inflammation. Deletion of murine ILC-intrinsic Arg1 abrogated type 2 lung inflammation by restraining ILC2 proliferation and dampening cytokine production. Mechanistically, inhibition of Arg1 enzymatic activity disrupted multiple components of ILC2 metabolic programming by altering arginine catabolism, impairing polyamine biosynthesis and reducing aerobic glycolysis. These data identify Arg1 as a key regulator of ILC2 bioenergetics, controlling proliferative capacity and pro-inflammatory functions that promote type 2 inflammation.

Reprogramming of glucose, fatty acid and amino acid metabolism for cancer progression

Metabolic reprogramming is widely observed during cancer development to confer cancer cells the ability to survive and proliferate, even under the stressed, such as nutrient-limiting, conditions. It is famously known that cancer cells favor the "Warburg effect", i.e., the enhanced glycolysis or aerobic glycolysis, even when the ambient oxygen supply is sufficient. In addition, deregulated anabolism/catabolism of fatty acids and amino acids, especially glutamine, serine and glycine, have been identified to function as metabolic regulators in supporting cancer cell growth. Furthermore, extensive crosstalks are being revealed between the deregulated metabolic network and cancer cell signaling. These exciting advancements have inspired new strategies for treating various malignancies by targeting cancer metabolism. Here we review recent findings related to the regulation of glucose, fatty acid and amino acid metabolism, their crosstalk, and relevant cancer therapy strategy.

Proline biosynthesis augments tumor cell growth and aerobic glycolysis: involvement of pyridine nucleotides

The metabolism of the nonessential amino acid proline contributes to tumor metabolic reprogramming. Previously we showed that MYC increases proline biosynthesis (PB) from glutamine. Here we show MYC increases the expression of the enzymes in PB at both protein and mRNA levels. Blockade of PB decreases tumor cell growth and energy production. Addition of Δ¹-pyrroline-5-carboxylate (P5C) or proline reverses the effects of P5C synthase knockdown but not P5C reductases knockdown. Importantly, the reversal effect of proline was blocked by concomitant proline dehydrogenase/oxidase (PRODH/POX) knockdown. These findings suggest that the important regulatory contribution of PB to tumor growth derives from metabolic cycling between proline and P5C rather than product proline or intermediate P5C. We further document the critical role of PB in maintaining pyridine nucleotide levels by connecting the proline cycle to glycolysis and to the oxidative arm of the pentose phosphate pathway. These findings establish a novel function of PB in tumorigenesis, linking the reprogramming of glucose, glutamine and pyridine nucleotides, and may provide a novel target for antitumor therapy.

Amino Acid transporters in cancer and their relevance to "glutamine addiction": novel targets for the design of a new class of anticancer drugs

Tumor cells have an increased demand for amino acids because of their rapid proliferation rate. In addition to their need in protein synthesis, several amino acids have other roles in supporting cancer growth. There are approximately two-dozen amino acid transporters in humans, and tumor cells must upregulate one or more of these transporters to satisfy their demand for amino acids. If the transporters that specifically serve this purpose in tumor cells are identified, they can be targeted for the development of a brand new class of anticancer drugs; the logical basis of such a strategy would be to starve the tumor cells of an important class of nutrients. To date, four amino acid transporters have been found to be expressed at high levels in cancer: SLC1A5, SLC7A5, SLC7A11, and SLC6A14. Their induction occurs in a cancer type-specific manner with a direct or indirect involvement of the oncogene c-Myc. Further, these transporters are functionally coupled, thus maximizing their ability to promote cancer growth and chemoresistance. Progress has been made in preclinical studies, exploiting these transporters as drug targets in cancer therapy. These transporters also show promise in development of new tumor-imaging probes and in tumor-specific delivery of appropriately designed chemotherapeutic agents.