Somatic mouse models of gastric cancer reveal genotype-specific features of metastatic disease

Metastatic gastric carcinoma is a highly lethal cancer that responds poorly to conventional and molecularly targeted therapies. Despite its clinical relevance, the mechanisms underlying the behavior and therapeutic response of this disease are poorly understood owing, in part, to a paucity of tractable models. Here we developed methods to somatically introduce different oncogenic lesions directly into the murine gastric epithelium. Genotypic configurations observed in patients produced metastatic gastric cancers that recapitulated the histological, molecular and clinical features of all nonviral molecular subtypes of the human disease. Applying this platform to both wild-type and immunodeficient mice revealed previously unappreciated links between the genotype, organotropism and immune surveillance of metastatic cells, which produced distinct patterns of metastasis that were mirrored in patients. Our results establish a highly portable platform for generating autochthonous cancer models with flexible genotypes and host backgrounds, which can unravel mechanisms of gastric tumorigenesis or test new therapeutic concepts.

A Targetable Myeloid Inflammatory State Governs Disease Recurrence in Clear-Cell Renal Cell Carcinoma

It is poorly understood how the tumor immune microenvironment influences disease recurrence in localized clear-cell renal cell carcinoma (ccRCC). Here we performed whole-transcriptomic profiling of 236 tumors from patients assigned to the placebo-only arm of a randomized, adjuvant clinical trial for high-risk localized ccRCC. Unbiased pathway analysis identified myeloid-derived IL6 as a key mediator. Furthermore, a novel myeloid gene signature strongly correlated with disease recurrence and overall survival on uni- and multivariate analyses and is linked to TP53 inactivation across multiple data sets. Strikingly, effector T-cell gene signatures, infiltration patterns, and exhaustion markers were not associated with disease recurrence. Targeting immunosuppressive myeloid inflammation with an adenosine A2A receptor antagonist in a novel, immunocompetent, Tp53-inactivated mouse model significantly reduced metastatic development. Our findings suggest that myeloid inflammation promotes disease recurrence in ccRCC and is targetable as well as provide a potential biomarker-based framework for the design of future immuno-oncology trials in ccRCC.

SIGNIFICANCE

Improved understanding of factors that influence metastatic development in localized ccRCC is greatly needed to aid accurate prediction of disease recurrence, clinical decision-making, and future adjuvant clinical trial design. Our analysis implicates intratumoral myeloid inflammation as a key driver of metastasis in patients and a novel immunocompetent mouse model. This article is highlighted in the In This Issue feature, p. 2221.

Abstract PO029: Targeting non-canonical Hippo pathway in NRF2-mutant liver cancer

NRF2 is an “undruggable” oncogenic transcription factor recurrently mutated in solid tumors such as liver and lung cancers. Our recent study demonstrates that NRF2 acts by coordinating the redox and metabolic stress responses as well as drug resistance programs in liver and other cancers. Developing an NRF2 inhibitor is a major goal of cancer drug discovery. Here, we identify the non-canonical Hippo pathway protein serine/threonine kinase 38 (STK38/NDR1) as a new NRF2 kinase that is a requirement for its stability and function. STK38 directly phosphorylates NRF2 at two sites (S33 and T559), and both sites contribute to full NRF2 activation. Loss of STK38 disables the redox response in NRF2 driven cancers and leads to tumor regression in vivo. Conversely, STK38 can also activate NRF2 and promote de novo liver cancer development in mice. This oncogenic effect is reflected in low frequency (3%) genomic amplifications in human liver cancers. Importantly, inhibition of the upstream STK38-activating mammalian Hippo kinases STK3/4 (MST2/1) by XMU-MP-1 inactivates STK38-NRF2 regulatory axis in vitroand in vivo. More importantly, XMU-MP-1 produces single agent activity against NRF2-driven liver and lung cancers in vivo in primary and patient derived xenograft (PDX)-based mouse models. In addition, we performed an in-silicoscreen and identified TAE-684 as an STK38 inhibitor that was subsequently confirmed to block STK38 activity in an in vitro kinase assay. TAE-684 treatment resulted in significant growth impairment of NRF2-mutant PDXs in vivo but no activity was observed in NRF2 wildtype counterparts. Together, these results uncover a surprising role of Hippo-related kinase STK38 in NRF2 activation and point to a paradoxical vulnerability in NRF2-driven cancers.

Citation Format: Viraj Sanghvi, Aleksander Rust, Josef Leibold, Scott Lowe, Agnes Viale, John Chodera, Ronald C. Hendrickson, Elisa de Stanchina, Hans-Guido Wendel. Targeting non-canonical Hippo pathway in NRF2-mutant liver cancer [abstract]. In: Proceedings of the AACR Special Conference: Advances in the Pathogenesis and Molecular Therapies of Liver Cancer; 2022 May 5-8; Boston, MA. Philadelphia (PA): AACR; Clin Cancer Res 2022;28(17_Suppl):Abstract nr PO029.

Base editing sensor libraries for high-throughput engineering and functional analysis of cancer-associated single nucleotide variants

Base editing can be applied to characterize single nucleotide variants of unknown function, yet defining effective combinations of single guide RNAs (sgRNAs) and base editors remains challenging. Here, we describe modular base-editing-activity 'sensors' that link sgRNAs and cognate target sites in cis and use them to systematically measure the editing efficiency and precision of thousands of sgRNAs paired with functionally distinct base editors. By quantifying sensor editing across >200,000 editor-sgRNA combinations, we provide a comprehensive resource of sgRNAs for introducing and interrogating cancer-associated single nucleotide variants in multiple model systems. We demonstrate that sensor-validated tools streamline production of in vivo cancer models and that integrating sensor modules in pooled sgRNA libraries can aid interpretation of high-throughput base editing screens. Using this approach, we identify several previously uncharacterized mutant TP53 alleles as drivers of cancer cell proliferation and in vivo tumor development. We anticipate that the framework described here will facilitate the functional interrogation of cancer variants in cell and animal models.

Senolytic CAR T cells reverse senescence-associated pathologies

Cellular senescence is characterized by stable cell-cycle arrest and a secretory program that modulates the tissue microenvironment1,2. Physiologically, senescence serves as a tumour-suppressive mechanism that prevents the expansion of premalignant cells3,4 and has a beneficial role in wound-healing responses5,6. Pathologically, the aberrant accumulation of senescent cells generates an inflammatory milieu that leads to chronic tissue damage and contributes to diseases such as liver and lung fibrosis, atherosclerosis, diabetes and osteoarthritis1,7. Accordingly, eliminating senescent cells from damaged tissues in mice ameliorates the symptoms of these pathologies and even promotes longevity1,2,8-10. Here we test the therapeutic concept that chimeric antigen receptor (CAR) T cells that target senescent cells can be effective senolytic agents. We identify the urokinase-type plasminogen activator receptor (uPAR)11 as a cell-surface protein that is broadly induced during senescence and show that uPAR-specific CAR T cells efficiently ablate senescent cells in vitro and in vivo. CAR T cells that target uPAR extend the survival of mice with lung adenocarcinoma that are treated with a senescence-inducing combination of drugs, and restore tissue homeostasis in mice in which liver fibrosis is induced chemically or by diet. These results establish the therapeutic potential of senolytic CAR T cells for senescence-associated diseases.

Somatic Tissue Engineering in Mouse Models Reveals an Actionable Role for WNT Pathway Alterations in Prostate Cancer Metastasis

To study genetic factors influencing the progression and therapeutic responses of advanced prostate cancer, we developed a fast and flexible system that introduces genetic alterations relevant to human disease directly into the prostate glands of mice using tissue electroporation. These electroporation-based genetically engineered mouse models (EPO-GEMM) recapitulate features of traditional germline models and, by modeling genetic factors linked to late-stage human disease, can produce tumors that are metastatic and castration-resistant. A subset of tumors with Trp53 alterations acquired spontaneous WNT pathway alterations, which are also associated with metastatic prostate cancer in humans. Using the EPO-GEMM approach and an orthogonal organoid-based model, we show that WNT pathway activation drives metastatic disease that is sensitive to pharmacologic WNT pathway inhibition. Thus, by leveraging EPO-GEMMs, we reveal a functional role for WNT signaling in driving prostate cancer metastasis and validate the WNT pathway as therapeutic target in metastatic prostate cancer. SIGNIFICANCE: Our understanding of the factors driving metastatic prostate cancer is limited by the paucity of models of late-stage disease. Here, we develop EPO-GEMMs of prostate cancer and use them to identify and validate the WNT pathway as an actionable driver of aggressive metastatic disease.This article is highlighted in the In This Issue feature, p. 890.

Senescence-Induced Vascular Remodeling Creates Therapeutic Vulnerabilities in Pancreas Cancer

KRAS mutant pancreatic ductal adenocarcinoma (PDAC) is characterized by a desmoplastic response that promotes hypovascularity, immunosuppression, and resistance to chemo- and immunotherapies. We show that a combination of MEK and CDK4/6 inhibitors that target KRAS-directed oncogenic signaling can suppress PDAC proliferation through induction of retinoblastoma (RB) protein-mediated senescence. In preclinical mouse models of PDAC, this senescence-inducing therapy produces a senescence-associated secretory phenotype (SASP) that includes pro-angiogenic factors that promote tumor vascularization, which in turn enhances drug delivery and efficacy of cytotoxic gemcitabine chemotherapy. In addition, SASP-mediated endothelial cell activation stimulates the accumulation of CD8⁺ T cells into otherwise immunologically "cold" tumors, sensitizing tumors to PD-1 checkpoint blockade. Therefore, in PDAC models, therapy-induced senescence can establish emergent susceptibilities to otherwise ineffective chemo- and immunotherapies through SASP-dependent effects on the tumor vasculature and immune system.

Abstract PR01: Senescence induction triggers vascular remodeling and new vulnerabilities to chemo- and immunotherapy in pancreas cancer

Background: KRAS mutant pancreatic ductal adenocarcinoma (PDAC) is characterized by a desmoplastic response that promotes hypovascularity, poor drug delivery, immunosuppression, and de novo resistance to chemo- and immunotherapies. Recently, we demonstrated that a combination of MEK and CDK4/6 inhibitors can potently suppress PDAC tumor cell proliferation through induction of RB-mediated senescence and trigger a senescence-associated secretory phenotype (SASP) capable of remodeling the tumor microenvironment (TME) (Ruscetti et al., Science 2018). Here, we set out to explore how senescence induction could remodel the PDAC TME and alter the treatment landscape of this disease.

Methods: The Pdx1-Cre;LSL-KRASG12D;Trp53fl/wt (KPC) genetically engineered mouse model (GEMM) of PDAC, as well as immunocompetent C57BL/6 mice transplanted with PDAC organoids derived from this model, were treated for 2 weeks with the MEK inhibitor trametinib and CDK4/6 inhibitor palbociclib. Induction of senescence was determined by SA-β-gal staining, and secretion of SASP factors was determined by qPCR and cytokine array. The impact on vascularization and vascular function, as well as the immune system, was determined by immunohistochemistry and flow cytometry analysis. shRNAs targeting the p65 subunit of NF-KB were used to assess the effect of SASP knockdown on treatment responses, and high doses of a VEGFR2 blocking antibody were used to assess the effects of inhibiting neovascularization on these SASP-dependent phenotypes. Trametinib and palbociclib treatment was combined with the chemotherapeutic agent gemcitabine or PD-1 checkpoint blockade immunotherapy to study the impact on tumor responses and long-term survival of PDAC tumor-bearing animals.

Results: We find that therapy-induced senescence following trametinib and palbociclib treatment produces a SASP rich in proangiogenic factors, culminating in increased vascular density and perfusion in hypovascular PDAC tumors. This SASP-dependent vascular remodeling leads to enhanced drug uptake of the chemotherapeutic agent gemcitabine, and combining our senescence-inducing therapy with gemcitabine drives tumor regressions and prolonged survival in gemcitabine-refractory PDAC GEMMs and PDXs. In addition, increased antigen presentation and SASP-mediated vascular remodeling upon treatment mediates CD8+ T cell accumulation and activation within the PDAC TME, sensitizing these tumors to PD-1 checkpoint blockade.

Conclusions: These results demonstrate that therapy-induced senescence can establish emergent susceptibilities to otherwise ineffective chemo- and immunotherapies in PDAC through SASP-dependent, non-cell autonomous effects on the tumor vasculature and immune system.

This abstract is also being presented as Poster A46.

Citation Format: Marcus Ruscetti, John P. Morris, IV, Riccardo Mezzadra, James Russell, Josef Leibold, Paul B. Romesser, Janelle Simon, Amanda Kulick, Yu-jui Ho, Myles Fennell, Jinyang Li, Robert J. Norgard, John E. Wilkinson, Direna Alonso-Curbelo, Ramya Sridharan, Xiang Li, Daniel Heller, Elisa de Stanchina, Ben Z. Stanger, Charles J. Sherr, Scott W. Lowe. Senescence induction triggers vascular remodeling and new vulnerabilities to chemo- and immunotherapy in pancreas cancer [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr PR01.

The Oncogenic Action of NRF2 Depends on De-glycation by Fructosamine-3-Kinase

The NRF2 transcription factor controls a cell stress program that is implicated in cancer and there is great interest in targeting NRF2 for therapy. We show that NRF2 activity depends on Fructosamine-3-kinase (FN3K)-a kinase that triggers protein de-glycation. In its absence, NRF2 is extensively glycated, unstable, and defective at binding to small MAF proteins and transcriptional activation. Moreover, the development of hepatocellular carcinoma triggered by MYC and Keap1 inactivation depends on FN3K in vivo. N-acetyl cysteine treatment partially rescues the effects of FN3K loss on NRF2 driven tumor phenotypes indicating a key role for NRF2-mediated redox balance. Mass spectrometry reveals that other proteins undergo FN3K-sensitive glycation, including translation factors, heat shock proteins, and histones. How glycation affects their functions remains to be defined. In summary, our study reveals a surprising role for the glycation of cellular proteins and implicates FN3K as targetable modulator of NRF2 activity in cancer.

Induction of Oxidative Stress Through Inhibition of Thioredoxin Reductase 1 Is an Effective Therapeutic Approach for Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is one of the most prevalent and lethal cancers worldwide which lacks effective treatment. Cancer cells experience high levels of oxidative stress due to increased generation of reactive oxygen species (ROS). Increased antioxidant-producing capacity is therefore found in cancer cells to counteract oxidative stress. The thioredoxin system is a ubiquitous mammalian antioxidant system which scavenges ROS, and we demonstrate that it is vital for HCC growth as it maintains intracellular reduction-oxidation (redox) homeostasis. Transcriptome sequencing in human HCC samples revealed significant overexpression of thioredoxin reductase 1 (TXNRD1), the cytosolic subunit and key enzyme of the thioredoxin system, with significant correlations to poorer clinicopathological features and patient survival. Driven by the transcriptional activation of nuclear factor (erythroid-derived 2)-like 2, the master protector against oxidative stress, TXNRD1 counteracts intracellular ROS produced in human HCC. Inhibition of TXNRD1 through genetic inhibition hindered the proliferation of HCC cells and induced apoptosis in vitro. Administration of the pharmacological TXNRD1 inhibitor auranofin (AUR) effectively suppressed the growth of HCC tumors induced using the hydrodynamic tail vein injection and orthotopic implantation models in vivo. Furthermore, AUR sensitized HCC cells toward the conventional therapeutic sorafenib. Conclusion: Our study highlights the reliance of HCC cells on antioxidants for redox homeostasis and growth advantage; targeting TXNRD1 resulted in dramatic accumulation of ROS, which was found to be an effective approach for the suppression of HCC tumor growth.

NK cell-mediated cytotoxicity contributes to tumor control by a cytostatic drug combination

Molecularly targeted therapies aim to obstruct cell autonomous programs required for tumor growth. We show that mitogen-activated protein kinase (MAPK) and cyclin-dependent kinase 4/6 inhibitors act in combination to suppress the proliferation of KRAS-mutant lung cancer cells while simultaneously provoking a natural killer (NK) cell surveillance program leading to tumor cell death. The drug combination, but neither agent alone, promotes retinoblastoma (RB) protein-mediated cellular senescence and activation of the immunomodulatory senescence-associated secretory phenotype (SASP). SASP components tumor necrosis factor-α and intercellular adhesion molecule-1 are required for NK cell surveillance of drug-treated tumor cells, which contributes to tumor regressions and prolonged survival in a KRAS-mutant lung cancer mouse model. Therefore, molecularly targeted agents capable of inducing senescence can produce tumor control through non-cell autonomous mechanisms involving NK cell surveillance.

Calibration of CAR activation potential directs alternative T cell fates and therapeutic potency

Chimeric antigen receptors (CARs) are synthetic receptors that target and reprogram T cells to acquire augmented antitumor properties¹. CD19-specific CARs that comprise CD28 and CD3ζ signaling motifs² have induced remarkable responses in patients with refractory leukemia^3-5 and lymphoma⁶ and were recently approved by the US Food and Drug Administration⁷. These CARs program highly performing effector functions that mediate potent tumor elimination^4,8 despite the limited persistence they confer on T cells^3-6,8. Extending their functional persistence without compromising their potency should improve current CAR therapies. Strong T cell activation drives exhaustion^9,10, which may be accentuated by the redundancy of CD28 and CD3ζ signaling^11,12 as well as the spatiotemporal constraints imparted by the structure of second-generation CARs². Thus, we hypothesized that calibrating the activation potential of CD28-based CARs would differentially reprogram T cell function and differentiation. Here, we show that CARs encoding a single immunoreceptor tyrosine-based activation motif direct T cells to different fates by balancing effector and memory programs, thereby yielding CAR designs with enhanced therapeutic profiles.

Quantitative self-assembly prediction yields targeted nanomedicines

Development of targeted nanoparticle drug carriers often requires complex synthetic schemes involving both supramolecular self-assembly and chemical modification. These processes are generally difficult to predict, execute, and control. We describe herein a targeted drug delivery system that is accurately and quantitatively predicted to self-assemble into nanoparticles based on the molecular structures of precursor molecules, which are the drugs themselves. The drugs assemble with the aid of sulfated indocyanines into particles with ultrahigh drug loadings of up to 90%. We devised quantitative structure-nanoparticle assembly prediction (QSNAP) models to identify and validate electrotopological molecular descriptors as highly predictive indicators of nano-assembly and nanoparticle size. The resulting nanoparticles selectively targeted kinase inhibitors to caveolin-1-expressing human colon cancer and autochthonous liver cancer models to yield striking therapeutic effects while avoiding pERK inhibition in healthy skin. This finding enables the computational design of nanomedicines based on quantitative models for drug payload selection.

Differential expression of THELPER 1 cytokines upon antigen stimulation predicts ex vivo proliferative potential and cytokine production of virus-specific T cells following re-stimulation

INTRODUCTION

Cytomegalovirus (CMV) and human adenovirus (ADV) infections are causes of morbidity after stem cell transplantation. Antigen (Ag)-specific T cells are essential for the control of viral infections. However, in vivo expansion potential of T-cell subpopulations is hardly predictable in humans. Furthermore, ex vivo identification of human T cells with repopulating capacity for adoptive T-cell transfer has been difficult.

METHODS

We analyzed Ag-specific T-cell populations, subdivided according to the expression of different THELPER- 1 (Th1) cytokines. Isolation by flow cytometry was based on interferon-gamma (IFN)-γ, interleukin (IL)-2, or tumor necrosis factor-alpha (TNF-α) secretion of T cells after ex vivo stimulation with the Ags hexon (for ADV) and pp65 (for CMV). Isolated T cells were expanded and examined for functional characteristics, expansion/differentiation potential, and naïve, effector memory, central memory, and late effector phenotypes.

RESULTS

Isolation based on IFN-γ production provides a T-cell population with a mixture of early, central memory, and effector memory T cells, high expansion potential, and effective cytokine production. Selection of T cells with Ag-specific expression of IL-2 or TNF-α, however, results in a T-cell population with reduced proliferation and lower effector potential after expansion.

CONCLUSION

We conclude that the exclusive secretion of IFN-γ in the human antiviral T-cell responses preferentially leads to higher repopulation capacities of antiviral T cells, compared to IL-2 or TNF-α secreting T-cell populations.

[42-year-old patient with portal hypertension - case 6/2012]

HISTORY AND ADMISSION FINDINGS

A 42-year-old woman was referred with a bleeding in the upper gastrointestinal tract, varices in the fundus of the stomach and portal hypertension of unknown primary.

INVESTIGATIONS

Ultrasound examinations showed splenomegaly as well as portal hypertension. Blood examinations revealed low levels of haemoglobin. CT imaging showed multiple arteriovenous malformations with arterioportal shunts within the liver which led to a volume-induced portal hypertension. The genetic analysis revealed no mutations in the activin receptor-like kinase (ALK) 1 or endoglin genes.

DIAGNOSIS, TREATMENT AND COURSE

The patient was clinically diagnosed with hereditary hemorrhagic teleangiectasia, also known as Osler-Weber-Rendu disease. Because of the multiple arterioportal shunts within the liver and the resulting portal hypertension with live-threatening gastrointestinal bleeding, the only therapeutic option for the patient is liver transplantation. Therefore, an application for a standard exception was made at Eurotransplant and the patient is going to be liver transplantated within the next months.

CONCLUSIONS

Osler-Weber-Rendu disease is an autosomal dominant hereditary disease which leads to arteriovenous malformations and which can affect different organ systems. The course of the disease can be rather benign, but it can also lead to live-threatening complications requiring fast interventions.

Hypoxia regulates the cAMP- and Ca2+/calmodulin signaling systems in PC12 cells

Hypoxic/ischemic trauma is a primary factor in the pathology of various disease states. Yet, very little is known about the molecular mechanisms involved in cellular responses and adaptations to hypoxia. As a means of identifying intracellular signaling systems that are regulated in response to hypoxia, the effects of acute and chronic hypoxia on the activity of protein kinase A (PKA) and Ca2+/CaM-dependent protein kinase II (CaMK-II) were evaluated in rat pheochromocytoma (PC12) cells. Chronic (> 6 hr), but not acute exposure to hypoxia (5% O2) significantly decreased both PKA enzyme activity and immunoreactivity compared to control levels. This effect was not due to hypoxia-induced alterations in cell number or viability. Similarly, chronic hypoxia significantly decreased CaMK-II enzyme activity and protein levels in PC12 cells. These data demonstrate that down-regulation of the cAMP and Ca2+/CaM-signaling systems is a mechanism by which PC12 cells adapt to long-term hypoxia.

Evidence for the role of proteinases in uremic catabolism

Enhanced muscle protein breakdown has been demonstrated in acutely uremic rats by numerous authors. In order to investigate the pathogenetic role of skeletal muscle proteinases leupeptin, a low-molecular weight proteinase inhibitor, was administered intraperitoneally to acutely uremic rats. Twenty-four hours after bilateral nephrectomy, leupeptin-treated animals displayed significantly lowered serum urea levels (-32%), as compared to untreated uremic rats. As a sign of muscle protein breakdown, plasma levels of Nt-methylhistidine, an indicator of myofibrillar protein degradation, were also decreased (-35%) in the uremic animals treated with leupeptin as compared to untreated uremic rats. Finally, leupeptin treatment resulted in a significant inhibition of the myofibrillar alkaline proteinase activity, a proteinase which has been related to various catabolic conditions. These findings suggest that the increased muscle protein breakdown in uremia is caused by enhanced activity of muscular proteinases and that anti-proteolytic agents display favourable effects on the enhanced protein degradation observed in acute uremia.

Evidence for reduced catabolism by the antiglucocorticoid RU 38486 in acutely uremic rats

Previous studies suggested that increased blood levels of, or increased tissue sensitivity to, glucocorticoids may contribute to catabolism in acute uremia. To examine this possibility we determined urea nitrogen (urea-N) appearance, plasma levels of Nt-methylhistidine and the activity of the alkaline myofibrillar proteinase in acutely uremic rats with and without treatment with RU 38486, a selective antiglucocorticoid. Forty-eight hours after bilateral nephrectomy, the rats had markedly elevated serum levels of urea-N, creatinine, potassium and phosphorus. In uremic rats receiving RU 38486, comparable levels of serum creatinine were found, but the serum levels of urea-N (221 +/- 4 vs. 259 +/- 5 mg/dl) and phosphorus (6.5 +/- 0.3 vs. 8.5 +/- 0.4 mmol/l) were significantly decreased as compared to uremic animals without RU 38486. In comparison to sham-operated rats, urea-N appearance (net urea production) was increased by 56% 48 h after bilateral nephrectomy. This increment was almost completely reversed in uremic animals receiving the antiglucocorticoid. In untreated uremic rats, plasma levels of Nt-methylhistidine were 10.3 +/- 0.9 microgram/dl, whereas the administration of RU 38486 caused a significant decline in the levels of this amino acid (7.6 +/- 0.5 microgram/dl). This reduction in Nt-methylhistidine was associated with a concomitant decrease of myofibrillar proteinase activity in muscle tissue homogenates. Compared to sham-operated animals, this proteinase activity was increased by 30% in uremic rats, but was normal in those given RU 38486. Taken together, these data support the view that in acute uremia accelerated ureagenesis occurs, while enhanced muscle protein breakdown, owing to an increment in myofibrillar proteinase activity, provides the necessary amino acid precursors.(ABSTRACT TRUNCATED AT 250 WORDS)