Association of tenofovir disoproxil fumarate with primary allograft survival in HIV-positive kidney transplant recipients.

BACKGROUND: Tenofovir disoproxil fumarate (TDF) is an antiretroviral agent frequently used to treat human immunodeficiency virus (HIV). There are concerns regarding its potential to cause acute kidney injury, chronic kidney disease, and proximal tubulopathy. Although TDF can effectively suppress HIV after kidney transplantation, it is unknown whether use of TDF-based antiretroviral therapy (ART) after kidney transplantation adversely affects allograft survival. METHODS: We examined 104 HIV+ kidney transplant (KT) recipients at our center between 2001 and 2014. We generated a propensity score for TDF treatment using recipient and donor characteristics. We then fit Cox proportional hazards models to investigate the association between TDF treatment and 3-year, death-censored primary allograft failure, adjusting for the propensity score and delayed graft function (DGF). RESULTS: Of the 104 HIV+ KT candidates who underwent transplantation during the study period, 23 (22%) were maintained on TDF-based ART at the time of transplantation, and 81 (78%) were on non-TDF-based ART. Median age of the cohort was 48 years; 87% were male; 88% were black; and median CD4 count at transplantation was 450 cells/mm3 . Median kidney donor risk index was 1.2. At 3 years post transplantation, primary allograft failure occurred in 26% of patients on TDF-based ART and in 28% of patients on non-TDF-based ART (P=.5). TDF treatment was not associated with primary allograft failure at 3 years post transplant after adjusting for DGF and a propensity score for TDF use (hazard ratio 2.12, 95% confidence interval 0.41-10.9). CONCLUSIONS: In a large single-center experience of HIV+ kidney transplantation, TDF use following kidney transplantation was not significantly associated with primary allograft failure. These results may help inform management for HIV+ KT recipients in need of TDF therapy for adequate viral suppression.

Canine mammary cancer cells direct macrophages toward an intermediate activation state between M1/M2.

BACKGROUND: Canine mammary carcinoma is the most common cancer in female dogs and is often fatal due to the development of distance metastasis. The microenvironment of a tumour often contains abundant infiltrates of macrophages called tumour-associated macrophages (TAMs). TAMs express an activated phenotype, termed M2, which sustains proliferation of cancer cells, and has been correlated with poor clinical outcomes in human cancer patients. Cancer cells themselves have been implicated in stimulating the conversion of macrophages to a TAM with an M2 phenotype. This process has yet to be fully elucidated. Here we investigate the interplay between cancer cells and macrophages in the context of canine mammary carcinoma. RESULTS: We show that cancer cells inhibit lipopolysaccharide (LPS)-induced macrophage activation. Further, we show that macrophage associated proteins, colony-stimulating factor (CSF)-1 and C-C motif ligand (CCL)-2, stimulate macrophages and are responsible for the effects of cancer cells on macrophages. We suggest the existence of a feedback loop between macrophages and cancer cells; while cancer cells influence the phenotype of the TAMs through CSF-1 and CCL2, the macrophages induce canine mammary cancer cells to upregulate their own expression of the receptors for CSF-1 and CCL2 and increase the cancer cellular metabolic activity. However, these cytokines in isolation induce a phenotypic state in macrophages that is between M1 and M2 phenotypes. CONCLUSIONS: Overall, our results demonstrate the extent to which canine mammary carcinoma cells influence the macrophage phenotype and the relevance of a feedback loop between these cells, involving CSF-1 and CCL2 as important mediators.

The long-acting COX-2 inhibitor mavacoxib (Trocoxil™) has anti-proliferative and pro-apoptotic effects on canine cancer cell lines and cancer stem cells in vitro.

BACKGROUND: The NSAID mavacoxib (Trocoxcil™) is a recently described selective COX-2 inhibitor used for the management of inflammatory disease in dogs. It has a long plasma half-life, requiring less frequent dosing and supporting increased owner compliance in treating their dogs. Although the use of NSAIDs has been described in cancer treatment in dogs, there are no studies to date that have examined the utility of mavacoxib specifically. RESULTS: In this study we compared the in vitro activity of a short-acting non-selective COX inhibitor (carprofen) with mavacoxib, on cancer cell and cancer stem cell survival. We demonstrate that mavacoxib has a direct cell killing effect on cancer cells, increases apoptosis in cancer cells in a manner that may be independent of caspase activity, and has an inhibitory effect on cell migration. Importantly, we demonstrate that cancer stem cells derived from osteosarcoma cell lines are sensitive to the cytotoxic effect of mavacoxib. CONCLUSIONS: Both NSAIDs can inhibit cancer cell proliferation and induce apoptosis in vitro. Importantly, cancer stem cells derived from an osteosarcoma cell line are sensitive to the cytotoxic effect of mavacoxib. Our results suggest that mavacoxib has anti-tumour effects and that this in vitro anti-cancer activity warrants further study.

The class I PI3K/Akt pathway is critical for cancer cell survival in dogs and offers an opportunity for therapeutic intervention.

BACKGROUND: Using novel small-molecular inhibitors, we explored the feasibility of the class I PI3K/Akt/mTORC1 signaling pathway as a therapeutic target in canine oncology either by using pathway inhibitors alone, in combination or combined with conventional chemotherapeutic drugs in vitro. RESULTS: We demonstrate that growth and survival of the cell lines tested are predominantly dependent on class I PI3K/Akt signaling rather than mTORC1 signaling. In addition, the newly developed inhibitors ZSTK474 and KP372-1 which selectively target pan-class I PI3K and Akt, respectively, and Rapamycin which has been well-established as highly specific mTOR inhibitor, decrease viability of canine cancer cell lines. All inhibitors demonstrated inhibition of phosphorylation of pathway members. Annexin V staining demonstrated that KP372-1 is a potent inducer of apoptosis whereas ZSTK474 and Rapamycin are weaker inducers of apoptosis. Simultaneous inhibition of class I PI3K and mTORC1 by ZSTK474 combined with Rapamycin additively or synergistically reduced cell viability whereas responses to the PI3K pathway inhibitors in combination with conventional drug Doxorubicin were cell line-dependent. CONCLUSION: This study highlighted the importance of class I PI3K/Akt axis signaling in canine tumour cells and identifies it as a promising therapeutic target.

Correction: Pang et al. Canine Mammary Cancer Stem Cells Are Radio- and Chemo- Resistant and Exhibit an Epithelial-Mesenchymal Transition Phenotype. Cancers 2011, 3, 1744-1762.

In the original article [...].

Effective Penetration of a Liposomal Formulation of Bleomycin through Ex-Vivo Skin Explants from Two Different Species.

Bleomycin is a chemotherapy agent that, when administered systemically, can cause severe pulmonary toxicity. Bleosome is a novel formulation of bleomycin encapsulated in ultra-deformable (UD) liposomes that may be applicable as a topical chemotherapy for diseases such as non-melanoma skin cancer. To date, the ability of Bleosome to effectively penetrate through the skin has not been evaluated. In this study, we investigated the ability of Bleosome to penetrate through ex vivo skin explants from dogs and horses. We visualized the penetration of UD liposomes through the skin by transmission electron microscopy. However, to effectively image the drug itself we fluorescently labeled bleomycin prior to encapsulation within liposomes and utilized multiphoton microscopy. We showed that UD liposomes do not penetrate beyond the stratum corneum, whereas bleomycin is released from UD liposomes and can penetrate to the deeper layers of the epidermis. This is the first study to show that Bleosome can effectively penetrate through the skin. We speculate that UD liposomes are penetration enhancers in that UD liposomes carry bleomycin through the outer skin to the stratum corneum and then release the drug, allowing diffusion into the deeper layers. Our results are comparative in dogs and horses and warrant further studies on the efficacy of Bleosome as topical treatment.

A modular phase transfer and ligand exchange protocol for quantum dots.

In this paper, we describe a quantum dot (qdot) phase transfer protocol using ligand exchange and the amino acid histidine. The phase transfer from nonpolar solvents to aqueous buffers is homogeneous, and no appreciable precipitation occurs. The molecule histidine was chosen in order to first displace the organic encapsulation and second to provide a weakly chemisorbing intermediate at the qdot ionic interface. This allows the histidine to act as an intermediate shell upon which further direct ligand exchange can occur. Since this intermediate encapsulation is easily displaced by an assortment of different molecules while in aqueous buffers, we refer to this approach as modular. Characterization via FTIR and NMR revealed the extent of ligand exchange, and provides insights into the interfacial binding mechanism. The colloidal stability and photostability of the qdots was probed via UV-vis and steady state fluorescence, which revealed promising quantum yield stability of greater than 1 year. The qdots have hydrodynamic diameters of <12 nm and surface charges dependent upon ligand type and coverage. The modularity of this approach is shown by tailoring the qdot surface charge via sequential ligand exchange using mixed monolayers of carboxylic acid and poly(ethylene glycol)-terminated thiols.

Exosomes Derived from Radioresistant Breast Cancer Cells Promote Therapeutic Resistance in Naïve Recipient Cells.

Radiation resistance is a significant challenge in the treatment of breast cancer in humans. Human breast cancer is commonly treated with surgery and adjuvant chemotherapy/radiotherapy, but recurrence and metastasis upon the development of therapy resistance results in treatment failure. Exosomes are extracellular vesicles secreted by most cell types and contain biologically active cargo that, when transferred to recipient cells, can influence the cells' genome and proteome. We propose that exosomes secreted by radioresistant (RR) cells may be able to disseminate the RR phenotype throughout the tumour. Here, we isolated exosomes from the human breast cancer cell line, MDA-MB-231, and the canine mammary carcinoma cell line, REM134, and their RR counterparts to investigate the effects of exosomes derived from RR cells on non-RR recipient cells. Canine mammary cancer cells lines have previously been shown to be excellent translational models of human breast cancer. This is consistent with our current data showing that exosomes derived from RR cells can increase cell viability and colony formation in naïve recipient cells and increase chemotherapy and radiotherapy resistance, in both species. These results are consistent in cancer stem cell and non-cancer stem cell populations. Significantly, exosomes derived from RR cells increased the tumoursphere-forming ability of recipient cells compared to exosomes derived from non-RR cells. Our results show that exosomes are potential mediators of radiation resistance that could be therapeutically targeted.

Corrigendum: Comparative Analysis of the Development of Acquired Radioresistance in Canine and Human Mammary Cancer Cell Lines.

[This corrects the article DOI: 10.3389/fvets.2020.00439.].

COX-2 Silencing in Canine Malignant Melanoma Inhibits Malignant Behaviour.

Metastatic melanoma is a very aggressive form of cancer in both humans and dogs. Dogs primarily develop oral melanoma of mucosal origin. Although oral melanoma in humans is rare, both diseases are highly aggressive with frequent metastases. This disease represents a "One Health" opportunity to improve molecular and mechanistic understanding of melanoma progression. Accumulating evidence suggests that cyclooxygenase-2 (COX-2) may play a critical role in the malignant behaviour of melanoma. In this study we analysed 85 histologically confirmed melanomas from canine patients and showed that COX-2 is overexpressed in both oral and cutaneous melanomas and that COX-2 expression correlates with established markers of poor prognosis. To determine the role of COX-2 in melanoma we developed two melanoma cell lines with stable integration of an inducible doxycycline-regulated expression vector containing a COX-2 targeted micro-RNA (miRNA). Using this system, we showed that cellular proliferation, migration and invasion are COX-2 dependent, establishing a direct relationship between COX-2 expression and malignant behaviour in canine melanoma. We have also developed a powerful molecular tool to aid further dissection of the mechanisms by which COX-2 regulates melanoma progression.

Using naturally occurring tumours in dogs and cats to study telomerase and cancer stem cell biology.

The recently described cancer stem cell theory opens up many new challenges and opportunities to identify targets for therapeutic intervention. However, the majority of cancer related therapeutic studies rely upon rodent models of human cancer that rarely translate into clinical success in human patients. Naturally occurring cancers in dogs, cats and humans share biological features, including molecular targets, telomerase biology and tumour genetics. Studying cancer stem cell biology and telomere/telomerase dynamics in the cancer bearing pet population may offer the opportunity to develop a greater understanding of cancer biology in the natural setting and evaluate the development of novel therapies targeted at these systems.

The Importance of the Tumor Microenvironment and Hypoxia in Delivering a Precision Medicine Approach to Veterinary Oncology.

Treating individual patients on the basis of specific factors, such as biomarkers, molecular signatures, phenotypes, environment, and lifestyle is what differentiates the precision medicine initiative from standard treatment regimens. Although precision medicine can be applied to almost any branch of medicine, it is perhaps most easily applied to the field of oncology. Cancer is a heterogeneous disease, meaning that even though patients may be histologically diagnosed with the same cancer type, their tumors may have different molecular characteristics, genetic mutations or tumor microenvironments that can influence prognosis or treatment response. In this review, we describe what methods are currently available to clinicians that allow them to monitor key tumor microenvironmental parameters in a way that could be used to achieve precision medicine for cancer patients. We further describe exciting novel research involving the use of implantable medical devices for precision medicine, including those developed for mapping tumor microenvironment parameters (e.g., O2, pH, and cancer biomarkers), delivering local drug treatments, assessing treatment responses, and monitoring for recurrence and metastasis. Although these research studies have predominantly focused on and were tailored to humans, the results and concepts are equally applicable to veterinary patients. While veterinary clinical studies that have adopted a precision medicine approach are still in their infancy, there have been some exciting success stories. These have included the development of a receptor tyrosine kinase inhibitor for canine mast cell tumors and the production of a PCR assay to monitor the chemotherapeutic response of canine high-grade B-cell lymphomas. Although precision medicine is an exciting area of research, it currently has failed to gain significant translation into human and veterinary healthcare practices. In order to begin to address this issue, there is increasing awareness that cross-disciplinary approaches involving human and veterinary clinicians, engineers and chemists may be needed to help advance precision medicine toward its full integration into human and veterinary clinical practices.

Naturally-Occurring Canine Mammary Tumors as a Translational Model for Human Breast Cancer.

Despite extensive research over many decades, human breast cancer remains a major worldwide health concern. Advances in pre-clinical and clinical research has led to significant improvements in recent years in how we manage breast cancer patients. Although survival rates of patients suffering from localized disease has improved significantly, the prognosis for patients diagnosed with metastatic disease remains poor with 5-year survival rates at only 25%. In vitro studies using immortalized cell lines and in vivo mouse models, typically using xenografted cell lines or patient derived material, are commonly used to study breast cancer. Although these techniques have undoubtedly increased our molecular understanding of breast cancer, these research models have significant limitations and have contributed to the high attrition rates seen in cancer drug discovery. It is estimated that only 3-6% of drugs that show promise in these pre-clinical models will reach clinical use. Models that can reproduce human breast cancer more accurately are needed if significant advances are to be achieved in improving cancer drug research, treatment outcomes, and prognosis. Canine mammary tumors are a naturally-occurring heterogenous group of cancers that have several features in common with human breast cancer. These similarities include etiology, signaling pathway activation and histological classification. In this review article we discuss the use of naturally-occurring canine mammary tumors as a translational animal model for human breast cancer research.

Comparative Analysis of the Development of Acquired Radioresistance in Canine and Human Mammary Cancer Cell Lines.

Research using in vitro canine mammary cancer cell lines and naturally-occurring canine mammary tumors are not only fundamental models used to advance the understanding of cancer in veterinary patients, but are also regarded as excellent translational models of human breast cancer. Human breast cancer is commonly treated with radiotherapy; however, tumor response depends on both innate radiosensitivity and on tumor repopulation by cells that develop radioresistance. Comparative canine and human studies investigating the mechanisms of radioresistance may lead to novel cancer treatments that benefit both species. In this study, we developed a canine mammary cancer (REM-134) radioresistant (RR) cell line and investigated the cellular mechanisms related to the development of acquired radioresistance. We performed a comparative analysis of this resistant model with our previously developed human breast cancer radioresistant cell lines (MCF-7 RR, ZR-751 RR, and MDA-MB-231 RR), characterizing inherent differences through genetic, molecular, and cell biology approaches. RR cells demonstrated enhanced invasion/migration capabilities, with phenotypic evidence suggestive of epithelial-to-mesenchymal transition. Similarities were identified between the REM-134 RR, MCF-7 RR, and ZR-751 RR cell lines in relation to the pattern of expression of both epithelial and mesenchymal genes, in addition to WNT, PI3K, and MAPK pathway activation. Following the development of radioresistance, transcriptomic data indicated that parental MCF-7 and ZR-751 cell lines changed from a luminal A classification to basal/HER2-overexpressing (MCF-7 RR) and normal-like/HER2-overexpressing (ZR-751 RR). These radioresistant subtypes were similar to the REM-134 and REM-134 RR cell lines, which were classified as HER2-overexpressing. To our knowledge, our study is the first to generate a canine mammary cancer RR cell line model and provide a comparative genetic and phenotypic analysis of the mechanisms of acquired radioresistance between canine and human cancer cell lines. We demonstrate that the cellular processes that occur with the development of acquired radioresistance are similar between the human and canine cell lines; our results therefore suggest that the canine model is appropriate to study both human and canine radioresistant mammary cancers, and that treatment strategies used in human medicine may also be applicable to veterinary patients.

Cyclooxygenase-2 expression is associated with infiltration of inflammatory cells in oral and skin canine melanomas.

Melanoma is a fast-growing tumour in dogs and represents 7% of the total malignant neoplasms from the skin and is the most common tumour found in the oral cavity. In these tumours, high expression of cyclooxygenase-2 (COX-2) is associated with a poor prognosis. The aim of this study was to verify if the overexpression of COX-2 is related to the modulation of lymphocytes and if it is associated with the angiogenic and proliferative capacity of the melanoma. Canine melanoma samples (n = 85) were analysed by immunohistochemistry to detect the expression of S-100, Melan-A, PNL-2, COX-2, Factor VIII, Ki-67 and immune cells markers (CD3, CD4, FOXP3 and MAC387); and expression levels of MAC387, NOS and CD206 were determined by immunofluorescence. Our study showed a concurrent difference between the expression of COX-2 and inflammatory cell infiltration: Oral melanomas showed positivity for COX-2 in 34% of the cases and this expression was associated with CD3 positivity in the inflammatory infiltrate and angiogenesis; whereas cutaneous melanomas presented positivity for COX-2 in 42% of the cases and this expression was associated with positive staining for CD3, CD4, FOXP3 and MAC387. These markers are associated with inflammatory cells, angiogenesis and proliferation. Interestingly, melanomas were highly infiltrated by FOXP3+ cells, this is related to angiogenesis, whereas CD3, CD4 and MAC387 expression was only associated with cutaneous melanomas. The macrophage profile analysis showed that both oral and cutaneous melanomas with low COX-2 expression have an M1 phenoptype, whereas the cases with high COX-2 expression demonstrate a hybrid M1/M2 profile pattern. We concluded that the COX-2 is overexpressed in 42% of cutaneous melanomas and in 34% of oral melanomas, with a direct association with angiogenesis, proliferation, and intratumoral lymphocyte infiltration. We propose that COX-2 is a key regulator of immune cell infiltration and may drive tumour associated macrophage activation.

Precision Medicine and the Role of Biomarkers of Radiotherapy Response in Breast Cancer.

Radiotherapy remains an important treatment modality in nearly two thirds of all cancers, including the primary curative or palliative treatment of breast cancer. Unfortunately, largely due to tumor heterogeneity, tumor radiotherapy response rates can vary significantly, even between patients diagnosed with the same tumor type. Although in recent years significant technological advances have been made in the way radiation can be precisely delivered to tumors, it is proving more difficult to personalize radiotherapy regimens based on cancer biology. Biomarkers that provide prognostic or predictive information regarding a tumor's intrinsic radiosensitivity or its response to treatment could prove valuable in helping to personalize radiation dosing, enabling clinicians to make decisions between different treatment options whilst avoiding radiation-induced toxicity in patients unlikely to gain therapeutic benefit. Studies have investigated numerous ways in which both patient and tumor radiosensitivities can be assessed. Tumor molecular profiling has been used to develop radiosensitivity gene signatures, while the assessment of specific intracellular or secreted proteins, including circulating tumor cells, exosomes and DNA, has been performed to identify prognostic or predictive biomarkers of radiation response. Finally, the investigation of biomarkers related to radiation-induced toxicity could provide another means by which radiotherapy could become personalized. In this review, we discuss studies that have used these methods to identify or develop prognostic/predictive signatures of radiosensitivity, and how such assays could be used in the future as a means of providing personalized radiotherapy.

The selective cyclooxygenase-2 inhibitor mavacoxib (Trocoxil) exerts anti-tumour effects in vitro independent of cyclooxygenase-2 expression levels.

The inducible inflammatory enzyme cyclooxygenase-2 (COX-2) and its product prostaglandin E2 (PGE2 ) are prominent tumour promoters, and expression of COX-2 is elevated in a number of tumours of both humans and canines. Targeting COX-2 in cancer is an attractive option because of readily available non-steroidal anti-inflammatory drugs (NSAIDs), and there is a clear epidemiological link between NSAID use and cancer risk. In this study, we aim to establish the anti-tumourigenic effects of the selective, long-acting COX-2 inhibitor mavacoxib. We show here that mavacoxib is cytotoxic to a panel of human and canine osteosarcoma, mammary and bladder carcinoma cancer cell lines; that it can induce apoptosis and inhibit the migration of these cells. Interestingly, we establish that mavacoxib can exert these effects independently of elevated COX-2 expression. This study highlights the potential novel use of mavacoxib as a cancer therapeutic, suggesting that mavacoxib may be an effective anti-cancer agent independent of tumour COX-2 expression.

Development and characterisation of acquired radioresistant breast cancer cell lines.

BACKGROUND: Radiotherapy plays an important role in the multimodal treatment of breast cancer. The response of a breast tumour to radiation depends not only on its innate radiosensitivity but also on tumour repopulation by cells that have developed radioresistance. Development of effective cancer treatments will require further molecular dissection of the processes that contribute to resistance. METHODS: Radioresistant cell lines were established by exposing MDA-MB-231, MCF-7 and ZR-751 parental cells to increasing weekly doses of radiation. The development of radioresistance was evaluated through proliferation and colony formation assays. Phenotypic characterisation included migration and invasion assays and immunohistochemistry. Transcriptomic data were also generated for preliminary hypothesis generation involving pathway-focused analyses. RESULTS: Proliferation and colony formation assays confirmed radioresistance. Radioresistant cells exhibited enhanced migration and invasion, with evidence of epithelial-to-mesenchymal-transition. Significantly, acquisition of radioresistance in MCF-7 and ZR-751 cell lines resulted in a loss of expression of both ERα and PgR and an increase in EGFR expression; based on transcriptomic data they changed subtype classification from their parental luminal A to HER2-overexpressing (MCF-7 RR) and normal-like (ZR-751 RR) subtypes, indicating the extent of phenotypic changes and cellular plasticity involved in this process. Radioresistant cell lines derived from ER+ cells also showed a shift from ER to EGFR signalling pathways with increased MAPK and PI3K activity. CONCLUSIONS: This is the first study to date that extensively describes the development and characterisation of three novel radioresistant breast cancer cell lines through both genetic and phenotypic analysis. More changes were identified between parental cells and their radioresistant derivatives in the ER+ (MCF-7 and ZR-751) compared with the ER- cell line (MDA-MB-231) model; however, multiple and likely interrelated mechanisms were identified that may contribute to the development of acquired resistance to radiotherapy.

Epidermal growth factor receptor activity is elevated in glioma cancer stem cells and is required to maintain chemotherapy and radiation resistance.

Glioblastoma remains among the most aggressive of all human and canine malignancies, displaying high mortality rates and limited treatment options. We propose that given the similarities between canine and human gliomas, such as incidence of occurrence, histopathology, molecular characteristics, and response to therapy, that canine gliomas are a natural model of the human disease. A range of human and canine tumours have been shown to harbor specific subpopulations of cells with stem cell-like properties that initiate and maintain neoplasticity while resisting conventional therapies. Here, we show that both canine and human glioma cell lines contain a small population of cancer stem cells (CSCs), and by molecular profiling highlight the important role of the epidermal growth factor receptor (EGFR) pathway in canine CSCs. EGFR signaling is crucial in the regulation of cancer cell proliferation, migration and survival. To date EGFR-targeted interventions alone have been largely ineffective. Our findings confirm that specifically inhibiting EGFR signaling alone has no significant effect on the viability of CSCs. However inhibition of EGFR did enhance the chemo- and radio-sensitivity of both canine and human glioma CSCs, enabling this resistant, tumourigenic population of cells to be effectively targeted by conventional therapies.

A Molecular and Cellular Context-Dependent Role for Ir76b in Detection of Amino Acid Taste.

Amino acid taste is expected to be a universal property among animals. Although sweet, bitter, salt, and water tastes have been well characterized in insects, the mechanisms underlying amino acid taste remain elusive. From a Drosophila RNAi screen, we identify an ionotropic receptor, Ir76b, as necessary for yeast preference. Using calcium imaging, we identify Ir76b+ amino acid taste neurons in legs, overlapping partially with sweet neurons but not those that sense other tastants. Ir76b mutants have reduced responses to amino acids, which are rescued by transgenic expression of Ir76b and a mosquito ortholog AgIr76b. Co-expression of Ir20a with Ir76b is sufficient for conferring amino acid responses in sweet-taste neurons. Notably, Ir20a also serves to block salt response of Ir76b. Our study establishes the role of a highly conserved receptor in amino acid taste and suggests a mechanism for mutually exclusive roles of Ir76b in salt- and amino-acid-sensing neurons.