AI is a viable alternative to high throughput screening: a 318-target study

High throughput screening (HTS) is routinely used to identify bioactive small molecules. This requires physical compounds, which limits coverage of accessible chemical space. Computational approaches combined with vast on-demand chemical libraries can access far greater chemical space, provided that the predictive accuracy is sufficient to identify useful molecules. Through the largest and most diverse virtual HTS campaign reported to date, comprising 318 individual projects, we demonstrate that our AtomNet® convolutional neural network successfully finds novel hits across every major therapeutic area and protein class. We address historical limitations of computational screening by demonstrating success for target proteins without known binders, high-quality X-ray crystal structures, or manual cherry-picking of compounds. We show that the molecules selected by the AtomNet® model are novel drug-like scaffolds rather than minor modifications to known bioactive compounds. Our empirical results suggest that computational methods can substantially replace HTS as the first step of small-molecule drug discovery.

Increased osteoblast GαS level determines bone response to hyperparathyroidism in female mice

GS, the stimulatory heterotrimeric G protein, is an essential regulator of osteogenesis and bone turnover. To determine if increasing GαS in osteoblasts alters bone responses to hyperparathyroidism, we used a transgenic mouse line overexpressing GαS in osteoblasts (GS-Tg mice). Primary osteoblasts from GS-Tg mice showed increased basal and parathyroid hormone (PTH)-stimulated cAMP and greater responses to PTH than cells from WT mice. Skeletal responses to 2-week continuous PTH administration (cPTH) in female mice resulted in trabecular bone loss in WT mice but 74% and 34% increase in trabecular bone mass in long bones and vertebrae, respectively, in GS-Tg mice. Vertebral biomechanical strength was compromised by cPTH treatment in WT mice but not in GS-Tg. Increased peritrabecular fibrosis was greatly increased by cPTH in Gs-Tg compared to WT mice and corresponded with greater increases in Wnt pathway proteins in trabecular bone. Cortical bone responded negatively to cPTH in WT and Gs-Tg mice with large increases in porosity, decreased cortical thickness and compromised biomechanical properties. These results demonstrate that hyperparathyroidism can increase trabecular bone when GS expression and cAMP stimulation in osteoblasts are increased but this is not the case in cortical bone where increased GS expression exacerbates cortical bone loss.

Endostatin as a Mediator Between Endothelial Function and Cognitive Performance in Those at Risk for Vascular Cognitive Impairment

Background:

Patients with coronary artery disease have an increased risk for developing vascular cognitive impairment. Endothelial function is often diminished and has been associated with lower cognitive performance in these patients. The link between endothelial function and cognition in coronary artery disease is not fully understood. Angiogenesis may play a role in mediating the association between endothelial function and cognition since angiogenic processes rely heavily on the endothelium.

Objective:

The aim of this study was to determine if markers of angiogenesis mediate the relationship between endothelial function and cognition in coronary artery disease patients.

Methods:

In 50 participants with coronary artery disease, endothelial function was assessed using peripheral arterial tonometry. Vascular endothelial growth factor (pro-angiogenic) and endostatin (anti-angiogenic) were measured in peripheral serum samples. Cognition was assessed using the Montreal Cognitive Assessment. A mediation analysis, using a bias corrected inferential bootstrapping method with 10,000 permutations, was used to determine if vascular endothelial growth factor or endostatin mediated an association between peripheral arterial tonometry measures and cognitive performance on the Montreal Cognitive Assessment.

Results:

Endostatin, but not vascular endothelial growth factor, mediated a relationship between endothelial function and cognitive performance when controlling for total years of education, body mass index, coronary artery bypass graft, stent, diabetes, and diuretic use. This analysis was also significant when delayed recall was substituted for the overall score on the Montreal Cognitive Assessment.

Conclusion:

These results suggest that endostatin mediates an association between endothelial function and cognitive performance in coronary artery disease.

SAT-402 Investigating Analogues of Parathyroid Hormone (PTH) and PTH-Related Peptide (PTHrP) to Improve Anabolic: Catabolic Response Ratios Using UMR106-01 Osteocytic Cells

Osteoporosis, the most common bone disease in humans, is characterised by decreased bone mass and increased fracture risk¹. Osteoporosis development is associated with an imbalance of bone resorption and deposition mediated by all three bone cell types; osteoblasts, osteocytes and osteoclasts². Current treatments for osteoporosis either prevent further bone resorption (Bisphosphonates and anti-RANKL) or increase bone deposition through anabolism (Teriparatide and Abaloparatide)³. The two anabolic treatments are truncated analogues of endogenous peptides; parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHrP), respectively. Both are administered intermittently, act on PTH1R receptors on osteoblasts and osteocytes leading to an increase in bone mineral density and bone strength. However, both treatments also have side-effects of hypercalcemia and cortical bone porosity caused by osteoclast stimulation. We therefore compared an array of PTH and PTHrP analogues for potential stimulation of catabolic side effects and desired anabolic effects. Screening assays used are UMR106-01 osteocytic cells that have high levels of PTH1R to measure mRNAs involved in anabolic responses (suppression of WNT inhibitors SOST and Dkk1) and catabolic responses (stimulation of RANKL/OPG, IL6). Peptides were also analysed by real-time cellular impedance assays (xCELLigence) to measure unbiased receptor stimulation. xCELLigence assays showed PTH1-34, PTHrP1-34 and Abaloparatide had the highest potencies (3.7 nM,1.4 nM,1.7nM respectively) while Tyr¹PTH1-34 and PTH2-34 had significantly decreased potencies (64nM and 130nM), the β-arrestin biased agonist, D-Trp¹²Tyr³⁴PTH7–34, had no effect. PTH1-34 potently inhibited SOST (IC₅₀ 0.29nM), and catabolic genes (RANK/OPG EC₅₀ 0.5nM, IL6 EC₅₀ 2.5nM). PTHrP1-34 provided higher potencies for anabolic (inhibited SOST IC₅₀ 0.08nM) and catabolic (RANKL/OPG EC₅₀ 0.3nM, IL6 EC₅₀ 1nM) genes. Altering the first amino acid to Tyrosine; Tyr¹PTH1-34 caused potent anabolic responses (SOST IC₅₀ 0.97nM) yet showed decreased potency for catabolic responses (RANKL/OPG EC₅₀ 20nM, IL6 EC₅₀ > 100nM). Removing the first amino acid of PTH to PTH2-34 drastically decreases the effectiveness of the peptide (OPG, SOST and RANKL IC₅₀ – no effect, IL6 EC₅₀ > 100nM). These results indicate the importance of the N-terminal amino acid for PTH affinity and efficacy and suggest that Tyr¹PTH1-34 may offer the best combination of bone stimulation without causing hypercalcemia.

References

¹ A.Gupta, L. March. (2016) Treating osteoporosis, Aust Prescr

² Chen X, et al. (2018) Osteoblast–osteoclast interactions. Connect. Tissue Res

³ Tu KN, et al. (2018) Osteoporosis: A review of treatment options. P T

Lower serum osteocalcin concentrations in patients with type 2 diabetes and relationships with vascular risk factors among patients with coronary artery disease

BACKGROUND

Lower serum concentrations of the osteoblast-derived protein, osteocalcin, have been associated with poorer glycemic control, insulin resistance and atherosclerosis, and with the development of type 2 diabetes (T2DM).

METHODS

This study compares concentrations of two physiological forms of osteocalcin, carboxylated (cOCN) and uncarboxylated (unOCN), between participants with T2DM (n = 20) and age-, gender- and body mass index (BMI)-matched participants without T2DM (n = 40) among patients with coronary artery disease (CAD), and it explores relationships between osteocalcin concentrations and cardiovascular risk factors.

RESULTS

Concentrations of unOCN (2.71 ± 1.86 vs. 4.70 ± 2.03 ng/mL; t = -3.635, p = 0.001) and cOCN (8.70 ± 2.27 vs. 10.77 ± 3.69 ng/mL; t = -2.30, p = 0.025) were lower in participants with T2DM. In participants without T2DM, concentrations of cOCN were associated with fitness (VO_2Peak rho = 0.317, p = 0.047) and lower body fat (rho = -0.324, p = 0.041). In participants with T2DM, lower unOCN was associated with HbA1c (rho = -0.516, p = 0.020). Higher body mass was associated with higher unOCN (rho = 0.423, p = 0.009) in participants without T2DM, but with lower concentrations of both unOCN (rho = -0.590, p = 0.006) and cOCN (rho = -0.632, p = 0.003) in participants with T2DM.

CONCLUSION

In patients with CAD, lower osteocalcin concentrations were related to type 2 diabetes, and to adverse fitness, metabolic and obesity profiles.

A readout of metabolic efficiency in arylamine N-acetyltransferase-deficient mice reveals minor energy metabolism changes

Recent studies have revealed a possible link between the activities of polymorphic arylamine N-acetyltransferases (NATs) and energy metabolism. We used a Nat1/Nat2 double knockout (KO) mouse model to demonstrate that ablation of the two Nat genes is associated with modest, intermittent alterations in respiratory exchange rate. Pyruvate tolerance tests show that double KO mice have attenuated hepatic gluconeogenesis when maintained on a high-fat/high-sucrose diet. Absence of the two Nat genes also leads to an increase in the hepatic concentration of coenzyme A in mice fed a high-fat/high-sucrose diet. Our results suggest a modest involvement of NAT in energy metabolism in mice, which is consistent with the absence of major phenotypic deregulation of energy metabolism in slow human acetylators.

Effect of 25-HydroxyVitamin D Deficiency and Its Interaction with Prednisone Treatment on Musculoskeletal Health in Growing Mdx Mice

Duchenne muscular dystrophy (DMD) results from genetic mutations of the gene encoding dystrophin, leading to muscle inflammation and degeneration that is typically treated with glucocorticoids. DMD and its treatment with glucocorticoids result in poor bone health and high risk of fractures. Insufficient levels of 25-hydroxyvitamin D (25-hydroxy D) that may contribute to weakened bone are routinely found in DMD patients. To determine the effect of 25-hydroxy D deficiency, this study examined the effects of low vitamin D dietary intake with and without glucocorticoids on the musculoskeletal system of the Mdx mouse model of DMD. At 10 weeks of age, Mdx mice on control diet had low trabecular bone mineral density of distal femurs and lumbar vertebrae with increased osteoclast numbers compared to wild-type mice. Low vitamin D intake resulted in 25-hydroxy D deficiency but had no effect on trabecular or cortical bone. Cortical bone loss and bone weakness were induced by glucocorticoids while they improved muscle grip strength in Mdx mice. 25-hydroxy D deficiency did not result in any significant effects on growing bone or muscle in the Mdx mice. In combination with glucocorticoid treatment, low 25-hydroxy D resulted in no change in cortical bone mineral density but bone ductility was significantly increased suggesting lower bone mineralization.

Deficiency of N-acetyltransferase increases the interactions of isoniazid with endobiotics in mouse liver

Acetylation is the major metabolic pathway of isoniazid (INH) mediated by N-acetyltransferases (NATs). Previous reports suggest that slow acetylators have higher risks of INH hepatotoxicity than rapid acetylators, but the detailed mechanisms remain elusive. The current study used Nat1/2(-/-) mice to mimic NAT slow metabolizers and to investigate INH metabolism in the liver. We found that INH acetylation is abolished in the liver of Nat1/2(-/-) mice, suggesting that INH acetylation is fully dependent on NAT1/2. In addition to the acetylation pathway, INH can be hydrolyzed to form hydrazine (Hz) and isonicotinic acid (INA). We found that INA level was not altered in the liver of Nat1/2(-/-) mice, indicating that deficiency of NAT1/2 has no effect on INH hydrolysis. Because INH acetylation was abolished and INH hydrolysis was not altered in Nat1/2(-/-) mice, we expected an extremely high level of INH in the liver. However, we only observed a modest accumulation of INH in the liver of Nat1/2(-/-) mice, suggesting that there are alternative pathways in INH metabolism in NAT1/2 deficient condition. Our further studies revealed that the conjugated metabolites of INH with endobiotics, including fatty acids and vitamin B6, were significantly increased in the liver of Nat1/2(-/-) mice. In summary, this study illustrated that deficiency of NAT1/2 decreases INH acetylation, but increases the interactions of INH with endobiotics in the liver. These findings can be used to guide future studies on the mechanisms of INH hepatotoxicity in NAT slow metabolizers.

Overexpression of GαS in Murine Osteoblasts In Vivo Leads to Increased Bone Mass and Decreased Bone Quality

Gα_S is a heterotrimeric G protein that transduces signals from activated G protein-coupled receptors on the cell surface to stimulate adenylyl cyclase/cyclic adenosine monophosphate (AMP) signaling. Gα_S plays a central role in mediating numerous growth and maintenance processes including osteogenesis and bone turnover. Decreased Gα_S expression or activating mutations in Gα_S both affect bone, suggesting that modulating Gα_S protein levels may be important for bone health and development. To examine the effects of increased osteoblastic Gα_S expression on bone development in vivo, we generated transgenic mice with Gα_S overexpression in osteoblasts (HOM-Gs mice) driven by the 3.6-kilobase (kb) Col1A1 promoter. Both male and female HOM-Gs mice exhibit increased bone turnover with overactive osteoblasts and osteoclasts, resulting in a high bone mass phenotype with significantly reduced bone quality. At 9 weeks of age, HOM-Gs mice have increased trabecular number, volumetric BMD (vBMD), and bone volume; however, the bone was woven and disorganized. There was also increased cortical bone volume despite an overall reduction in size in HOM-Gs mice along with increased cortical porosity and brittleness. The skeletal phenotype of HOM-Gs mice progressed into maturity at 26 weeks of age with further accrual of trabecular bone, whereas WT mice lost trabecular bone at this age. Although cortical bone volume and geometry were similar between mature HOM-Gs and WT mice, increased porosity persisted and the bone was weaker. At the cellular level, these alterations were mediated by an increase in bone resorption by osteoclasts and an overwhelmingly higher increase in bone formation by osteoblasts. In summary, our findings demonstrate that high osteoblastic Gα_S expression results in aberrant skeletal development in which bone production is favored at the cost of bone quality. © 2017 American Society for Bone and Mineral Research.

Influence of sex and developmental stage on acute hepatotoxic and inflammatory responses to liver procarcinogens in the mouse

The incidence of liver cancer is higher in men than in women. This sex difference is also observed in murine tumor induction models that result in the appearance of liver tumors in adult mice following their exposure on postnatal days 8 and/or 15 to carcinogens such as 4-aminobiphenyl (ABP) or diethylnitrosamine (DEN). Previous studies performed in adult mice showed that acute hepatotoxic and inflammatory responses to high-dose DEN exposure were greater in males than in females, leading to the suggestion that these responses could account for the sex difference in tumor development. We also recently observed that female but not male mice exposed postnatally to ABP had slightly increased expression of the antioxidant defense genes Nqo1 and Ggt1, which are regulated by the oxidative stress response protein nuclear factor erythroid 2-related factor 2 (NRF2), while expression of Hmox1 was increased in both sexes. The goal of the present study was therefore to compare selected acute hepatotoxic, inflammatory and oxidative stress defense responses to ABP, DEN, or the prototype hepatotoxicant carbon tetrachloride (CCl₄), in male and female mice exposed to these chemicals either postnatally or as adults. Exposure of adult mice to ABP, DEN or CCl₄ produced a 2-fold greater acute elevation in serum levels of the hepatotoxicity biomarker alanine aminotransferase (ALT) in males than in females, while levels of the inflammatory biomarker interleukin-6 (IL-6) showed no sex difference. However, treatment of immature mice with either ABP or DEN using standard tumor-inducing postnatal exposure protocols produced no increase in serum ALT or IL-6 levels in either males or females, while CCl₄ produced a 40-fold ALT elevation but with no sex difference. Basal expression of the NRF2-responsive gene Nqo1 was higher in adult females than in males, but there was no sex difference in basal expression of Ggt1 or Hmox1. Sexually immature animals showed no sex difference in basal expression of any of the three genes. Postnatal DEN exposure modestly increased the expression of Ggt1 only in male mice and Nqo1 in both sexes, while CCl₄ slightly increased expression of Ggt1 in both males and females and Nqo1 only in females. Taken together, our results make it unlikely that acute hepatotoxic, inflammatory or NRF2-activated gene responses account for the male predominance in liver tumor growth following postnatal carcinogen exposure in mice. Our findings also suggest that acute toxicity studies performed in adult mice should be interpreted with caution when extrapolating potential mechanisms to liver carcinogenesis models that commonly use postnatally exposed mice.

The effect of phosphorylation on arrestin-rhodopsin interaction in the squid visual system

Invertebrate visual opsins are G protein-coupled receptors coupled to retinoid chromophores that isomerize reversibly between inactive rhodopsin and active metarhodopsin upon absorption of photons of light. The squid visual system has an arrestin protein that binds to metarhodopsin to block signaling to Gq and activation of phospholipase C. Squid rhodopsin kinase (SQRK) can phosphorylate both metarhodopsin and arrestin, a dual role that is unique among the G protein-coupled receptor kinases. The sites and role of arrestin phosphorylation by SQRK were investigated here using recombinant proteins. Arrestin was phosphorylated on serine 392 and serine 397 in the C-terminus. Unphosphorylated arrestin bound to metarhodopsin and phosphorylated metarhodopsin with similar high affinities (Kd 33 and 21 nM respectively), while phosphorylation of arrestin reduced the affinity 3- to 5-fold (Kd 104 nM). Phosphorylation of metarhodopsin slightly increased the dissociation of arrestin observed during a 1 hour incubation. Together these studies suggest a unique role for SQRK in phosphorylating both receptor and arrestin and inhibiting the binding of these two proteins in the squid visual system. Invertebrate visual systems are inactivated by arrestin binding to metarhodopsin that does not require receptor phosphorylation. Here we show that squid rhodopsin kinase phosphorylates arrestin on two serines (S392,S397) in the C-terminus and phosphorylation decreases the affinity of arrestin for squid metarhodopsin. Metarhodopsin phosphorylation has very little effect on arrestin binding but does increase arrestin dissociation.

Positive effects of bisphosphonates on bone and muscle in a mouse model of Duchenne muscular dystrophy

Patients with Duchenne muscular dystrophy are at increased risk of decreased bone mineral density and bone fracture as a result of inactivity. To determine if antiresorptive bisphosphonates could improve bone quality and their effects on muscle we studied the Mdx mouse, treated with pamidronate during peak bone growth at 5 and 6 weeks of age, and examined the outcome at 13 weeks of age. Pamidronate increased cortical bone architecture and strength in femurs with increased resistance to fracture. While overall long bone growth was not affected by pamidronate, there was significant inhibition of remodeling in metaphyseal trabecular bone with evidence of residual calcified cartilage. Pamidronate treatment had positive effects on skeletal muscle in the Mdx mice with decreased serum and muscle creatine kinase and evidence of improved muscle histology and grip strength.

Loss of the Mono-ADP-ribosyltransferase, Tiparp, Increases Sensitivity to Dioxin-induced Steatohepatitis and Lethality

The aryl hydrocarbon receptor (AHR) mediates the toxic effects of the environmental contaminant dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin; TCDD). Dioxin causes a range of toxic responses, including hepatic damage, steatohepatitis, and a lethal wasting syndrome; however, the mechanisms are still unknown. Here, we show that the loss of TCDD-inducible poly(ADP-ribose) polymerase (Tiparp), an ADP-ribosyltransferase and AHR repressor, increases sensitivity to dioxin-induced toxicity, steatohepatitis, and lethality. Tiparp(-/-) mice given a single injection of 100 μg/kg dioxin did not survive beyond day 5; all Tiparp(+/+) mice survived the 30-day treatment. Dioxin-treated Tiparp(-/-) mice exhibited increased liver steatosis and hepatotoxicity. Tiparp ADP-ribosylated AHR but not its dimerization partner, the AHR nuclear translocator, and the repressive effects of TIPARP on AHR were reversed by the macrodomain containing mono-ADP-ribosylase MACROD1 but not MACROD2. These results reveal previously unidentified roles for Tiparp, MacroD1, and ADP-ribosylation in AHR-mediated steatohepatitis and lethality in response to dioxin.

Relative Contributions of CYP1A2 and CYP2E1 to the Bioactivation and Clearance of 4-Aminobiphenyl in Adult Mice

4-Aminobiphenyl (ABP), a prototypical aromatic amine carcinogen in rodents and humans, requires bioactivation to manifest its toxic effects. A traditional model of ABP bioactivation, based on in vitro enzyme kinetic evidence, had postulated initial N-hydroxylation by the cytochrome P450 isoform CYP1A2. This is followed by phase 2 O-conjugation and hydrolysis to form a reactive nitrenium ion that covalently binds to DNA and produces tumor-initiating mutations. However, Cyp1a2(-/-) mice still possess significant liver ABP N-hydroxylation activity, DNA damage, and incidence of ABP-induced liver tumors, and in vivo induction of CYP1A2 paradoxically reduces levels of ABP-induced DNA damage. Competing ABP detoxification pathways can include N-acetylation by arylamine N-acetyltransferase 1 (NAT1) and/or NAT2; however, wild-type and Nat1/2(-/-) mice have similar in vivo ABP clearance rates. Together, these studies suggest the existence of novel ABP bioactivating and clearance/detoxification enzymes. In the present study, we detected similar reductions in Vmax for ABP N-hydroxylation by liver microsomes from Cyp1a2(-/-) and Cyp2e1(-/-) mice when compared with wild-type mice. In addition, recombinant mouse CYP1A2 and CYP2E1 were both able to N-hydroxylate ABP in mouse hepatoma cells. However, the in vivo clearance of ABP was significantly reduced in Cyp1a2(-/-) but not in Cyp2e1(-/-) mice. Our results support a significant role for CYP2E1 as a novel ABP N-oxidizing enzyme in adult mice, and suggest a more important contribution of CYP1A2 to the in vivo plasma clearance and thus detoxification of ABP.

N-hydroxylation of 4-aminobiphenyl by CYP2E1 produces oxidative stress in a mouse model of chemically induced liver cancer

4-Aminobiphenyl (ABP) is a trace component of cigarette smoke and hair dyes, a suspected human carcinogen and a potent rodent liver carcinogen. Postnatal exposure of mice to ABP results in a higher incidence of liver tumors in males than in females, paralleling the sex difference in human liver cancer incidence. A traditional model of ABP tumorigenesis involves initial CYP1A2-mediated N-hydroxylation, which eventually leads to production of mutagenic ABP-DNA adducts that initiate tumor growth. However, several studies have found no correlation between sex or CYP1A2 function and the DNA-damaging, mutagenic, or tumorigenic effects of ABP. Oxidative stress may be an important etiological factor for liver cancer, and it has also been linked to ABP exposure. The goals of this study were to identify novel enzyme(s) that contribute to ABP N-oxidation, and to investigate a potential role for oxidative stress in ABP liver tumorigenicity. Isozyme-selective inhibition experiments using liver microsomes from wild-type and genetically modified mice identified CYP2E1 as a major ABP N-hydroxylating enzyme. The N-hydroxylation of ABP by transiently expressed CYP2E1 produced oxidative stress in cultured mouse hepatoma cells. In vivo postnatal exposure of mice to a tumorigenic dose of ABP also produced oxidative stress in male wild-type mice, but not in male Cyp2e1(-/-) mice or in female mice. However, a stronger NRF2-associated antioxidant response was observed in females. Our results identify CYP2E1 as a novel ABP-N-oxidizing enzyme, and suggest that sex differences in CYP2E1-dependent oxidative stress and antioxidant responses to ABP may contribute to the observed sex difference in tumor incidence.

Reduced trabecular bone mass and strength in mice overexpressing Gα11 protein in cells of the osteoblast lineage

G protein-coupled receptors (GPCRs) require G proteins for intracellular signaling to regulate a variety of growth and maintenance processes, including osteogenesis and bone turnover. Bone maintenance events may be altered by changes in the activity or level of G proteins, which then modify signaling in bone cells such as osteoblasts. We have previously reported increased levels of Gα11 protein and signaling to phospholipase C/protein kinase C pathways in response to dexamethasone in osteoblastic UMR 106-01 cells. Here we generated pOBCol3.6-GNA11 transgenic mice that overexpress Gα11 protein in cells of the osteoblast lineage (G11-Tg mice). G11-Tg mice exhibit an osteopenic phenotype characterized by significant reductions in trabecular bone mineral density, thickness, number and strength. The numbers of osteoblasts and osteocytes were unchanged in G11-Tg bone, but early markers of osteoblast differentiation, Alp and Bsp, were increased while the late stage differentiation marker Ocn was not changed suggesting reduced osteoblast maturation in G11-Tg trabecular bone which was accompanied by a decreased bone formation rate. Furthermore, in vitro cultures of G11-Tg primary osteoblasts show delayed osteoblast differentiation and mineralization. Histological analyses also revealed increased osteoclast parameters, accompanied by elevated mRNA expression of Trap and Ctsk. mRNA levels of Rankl and M-csf were elevated in vitro in bone marrow stromal cells undergoing osteogenesis and in trabecular bone in vivo. Together, these findings demonstrate that increasing Gα11 protein expression in osteoblasts can alter gene expression and result in a dual mechanism of trabecular bone loss.

5-methyl-tetrahydrofolate and the S-adenosylmethionine cycle in C57BL/6J mouse tissues: gender differences and effects of arylamine N-acetyltransferase-1 deletion

Folate catabolism involves cleavage of the C⁹-N¹⁰ bond to form p-aminobenzoylgluamate (PABG) and pterin. PABG is then acetylated by human arylamine N-acetyltransferase 1 (NAT1) before excretion in the urine. Mice null for the murine NAT1 homolog (Nat2) show several phenotypes consistent with altered folate homeostasis. However, the exact role of Nat2 in the folate pathway in vivo has not been reported. Here, we examined the effects of Nat2 deletion in male and female mice on the tissue levels of 5-methyl-tetrahydrofolate and the methionine-S-adenosylmethionine cycle. We found significant gender differences in hepatic and renal homocysteine, S-adenosylmethionine and methionine levels consistent with a more active methionine-S-adenosylmethionine cycle in female tissues. In addition, methionine levels were significantly higher in female liver and kidney. PABG was higher in female liver tissue but lower in kidney compared to male tissues. In addition, qPCR of mRNA extracted from liver tissue suggested a significantly lower level of Nat2 expression in female animals. Deletion of Nat2 affected liver 5- methyl-tetrahydrofolate in female mice but had little effect on other components of the methionine-S-adenosylmethionine cycle. No N-acetyl-PABG was observed in any tissues in Nat2 null mice, consistent with the role of Nat2 in PABG acetylation. Surprisingly, tissue PABG levels were similar between wild type and Nat2 null mice. These results show that Nat2 is not required to maintain tissue PABG homeostasis in vivo under normal conditions.

Three-dimensional culture and cAMP signaling promote the maturation of human pluripotent stem cell-derived hepatocytes

Human pluripotent stem cells (hPSCs) represent a novel source of hepatocytes for drug metabolism studies and cell-based therapy for the treatment of liver diseases. These applications are, however, dependent on the ability to generate mature metabolically functional cells from the hPSCs. Reproducible and efficient generation of such cells has been challenging to date, owing to the fact that the regulatory pathways that control hepatocyte maturation are poorly understood. Here, we show that the combination of three-dimensional cell aggregation and cAMP signaling enhance the maturation of hPSC-derived hepatoblasts to a hepatocyte-like population that displays expression profiles and metabolic enzyme levels comparable to those of primary human hepatocytes. Importantly, we also demonstrate that generation of the hepatoblast population capable of responding to cAMP is dependent on appropriate activin/nodal signaling in the definitive endoderm at early stages of differentiation. Together, these findings provide new insights into the pathways that regulate maturation of hPSC-derived hepatocytes and in doing so provide a simple and reproducible approach for generating metabolically functional cell populations.

Abstract 4795: N-hydroxylation of 4-aminobiphenyl (ABP) and associated oxidative stress may influence ABP carcinogenicity in the mouse liver

Liver cancer is the 3rd most common cause of death from cancer worldwide due to poor prognosis and lack of treatment options. A prominent sex difference is observed in human liver cancer such that men have a 3 to 5 fold higher incidence than women even after accounting for known etiological factors. In a tumor study carried out previously in our laboratory, two doses of the human carcinogen 4-aminobiphenyl (ABP) given to mice on postnatal days 8 and 15 resulted in the formation of liver tumors at 1 year. Moreover, female mice were dramatically protected from liver tumors compared to males, which parallels the sex difference found in human liver cancer. Our goal is to elucidate the molecular mechanisms behind the sex differences in ABP carcinogenesis in mouse liver with the hope of extrapolating to the human condition.

Oxidative stress may play an important role in human liver carcinogenesis since all major etiological factors for human liver cancer, including viral hepatitis, alcohol, obesity and chemical carcinogens have been shown to be associated with oxidative stress. To determine whether oxidative stress is involved in ABP-mediated carcinogenesis in the mouse, we first assessed the ability of ABP to generate oxidative stress in the mouse Hepa1c1c7 hepatoma cell line. ABP did not produce reactive oxygen species (ROS) or oxidative DNA damage in Hepa1c1c7 cells. However, N-hydroxy-ABP (HOABP), an in vivo metabolite of ABP, was a potent inducer of both ROS and oxidative DNA damage. Furthermore, HOABP-induced oxidative stress was dose-dependent and could be blocked by co-treatment with the antioxidant N-acetylcysteine. These results suggest that HOABP may be a potential source of oxidative stress in mouse liver. We subsequently characterized the kinetics of ABP N-hydroxylation by mouse liver microsomes, and found that more than one cytochrome P450 (CYP) appears to be involved in the reaction. As predicted by the traditional model of ABP bioactivation, the high affinity ABP N-hydroxylation reaction is mediated by CYP1A2, while at least one additional low affinity site belonging to an as yet unidentified CYP is responsible for significant ABP N-hydroxylation at higher concentrations of ABP that would be expected following the doses of ABP used in our tumor study. However, no sex difference was found in either high or low affinity ABP N-hydroxylation activities between liver microsomes from male and female postnatal (day 15) or adult mice. Future studies will focus on quantifying both acute and chronic changes in oxidative stress and antioxidant levels in the mouse liver following carcinogenic doses of ABP, and correlating such changes to tumor formation.

Citation Format: Shuang Wang, Kim S. Sugamori, Denis M. Grant. N-hydroxylation of 4-aminobiphenyl (ABP) and associated oxidative stress may influence ABP carcinogenicity in the mouse liver. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4795. doi:10.1158/1538-7445.AM2013-4795

Influence of arylamine N-acetyltransferase, sex, and age on 4-aminobiphenyl-induced in vivo mutant frequencies and spectra in mouse liver

One model for cancer initiation by 4-aminobiphenyl (ABP) involves N-oxidation by cytochrome P450 CYP1A2 followed by O-conjugation by N-acetyltransferase(s) NAT1 and/or NAT2 and decomposition to a DNA-binding nitrenium ion. We recently observed that neonatal ABP exposure produced liver tumors in male but not in female mice, and that NAT deficiency reduced liver tumor incidence. However, ABP-induced liver tumor incidence did not correlate with liver levels of N-(deoxyguanosin-8-yl)-ABP adducts 24 hr after exposure. In this study, we compared in vivo ABP-induced DNA mutant frequencies and spectra between male and female wild-type and NAT-deficient Muta™Mouse using both the tumor-inducing neonatal exposure protocol and a 28-day repetitive dosing adult exposure protocol. ABP produced an increase in liver DNA mutant frequencies in both neonates and adults. However, we observed no sex or strain differences in mutant frequencies in neonatally exposed mice, and higher frequencies in adult females than males. Neonatal ABP exposure of wild-type mice increased the proportion of G-T transversions in both males and females, while exposure of Nat1/2(-/-) mice produced increased G-T transversions in males and a decrease in females, even though females had higher levels of N-(deoxyguanosin-8-yl)-4-ABP adducts. There was no correlation of mutant frequencies or spectra between mice dosed as neonates or as adults. These results suggest that observed sex- and NAT-dependent differences in ABP-induced liver tumor incidence in mice are not due to differences in either mutation rates or mutational spectra, and that mechanisms independent of carcinogen bioactivation, covalent DNA binding and mutation may be responsible for these differences.

Abstract 4423: Transgenic liver expression of human arylamine N-acetyltransferase NAT2 restores sensitivity of male Nat1/2 null mice to 4-aminobiphenyl-induced liver tumorigenesis

We recently observed that male Nat1/2(-/-) mice, which are deficient in the arylamine N-acetyltransferases Nat1 and Nat2, have a significantly lower incidence and multiplicity of liver tumors than wild-type C57BL/6 mice after their neonatal exposure to the aromatic amine carcinogen 4-aminobiphenyl (ABP). Female mice showed low or no liver tumors from ABP exposure regardless of Nat1/2 status. The goal of the present study was to determine the effect of liver-selective transgenic expression of human NAT2 on ABP-induced liver tumorigenicity in male and female Nat1/2(-/-) mice. We generated a hNAT2tgNat1/2(-/-) strain of mice that express human NAT2 selectively in liver on a Nat1/2 null background, exposed male and female mice to either DMSO vehicle, a low dose (600 nmoles) or a high dose (1200 nmoles) of ABP during the second postnatal week, and assessed mice for tumor growth at one year of age. Male hNAT2tgNat1/2(-/-) mice had liver tumor incidences of 29% (low dose) and 61% (high dose), which more closely resembled those of C57BL/6 mice (60% and 69%, respectively) than those of Nat1/2(-/-) mice (0% and 35%, respectively). This suggests that human NAT2 is capable of substantially restoring the sensitivity of mice to ABP-induced liver tumorigenicity that is lost by the absence of mouse Nat1 and Nat2. On the other hand, female hNAT2tgNat1/2(-/-) mice were still as resistant to liver tumors as C57BL/6 or Nat1/2(-/-) mice. No tumors were observed in any other organs. These results suggest that the human and mouse NAT enzymes play similar roles in enhancing liver tumor growth, and that the mechanism of female protection against ABP-induced tumor growth is unrelated to NAT status. However, neither the sex nor the NAT status of the mice appear to be correlated with the ability of ABP to produce acute DNA-damaging or mutation-inducing effects in liver. This suggests that the role of the human and mouse NAT enzymes in promoting liver tumor growth in mice may be due to a mechanism that is independent of their ability to influence the process of ABP metabolic activation and subsequent DNA damage leading to tumor initiation.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4423. doi:1538-7445.AM2012-4423

Reduced 4-aminobiphenyl-induced liver tumorigenicity but not DNA damage in arylamine N-acetyltransferase null mice

The aromatic amine 4-aminobiphenyl (ABP) is a liver procarcinogen in mice, requiring enzymatic bioactivation to exert its tumorigenic effect. To assess the role of arylamine N-acetyltransferase (NAT)-dependent acetylation capacity in the risk for ABP-induced liver tumors, we compared 1-year liver tumor incidence following the postnatal exposure of wild-type and NAT-deficient Nat1/2(-/-) mice to ABP. At an ABP exposure of 1200 nmol, male Nat1/2(-/-) mice had a liver tumor incidence of 36% compared to 69% in wild-type males, and at 600 nmol there was a complete absence of tumors compared to 60% in wild-type mice. Only one female wild-type mouse had a tumor using this exposure protocol. However, levels of N-deoxyguanosin-8-yl-ABP-DNA adducts did not correlate with either the strain or sex differences in tumor incidence. These results suggest that female sex and NAT deficiency reduce risk for ABP-induced liver tumors, but by mechanisms unrelated to differences in DNA-damaging events.

Liver-selective expression of human arylamine N-acetyltransferase NAT2 in transgenic mice

Human arylamine N-acetyltransferase 2 (NAT2) mediates the biotransformation of arylamine drugs and procarcinogens into either innocuous or reactive DNA-damaging metabolites and is expressed predominantly in liver. Interspecies differences and incongruous results between in vitro, in vivo, and epidemiological studies make it difficult to extrapolate animal results to human risk. We have generated human NAT2 transgenic mice on both C57BL/6 (hNAT2(tg)) and Nat1/2 null backgrounds [hNAT2(tg)Nat1/2(-/-)], in which liver-selective expression of human NAT2 is driven by the mouse albumin promoter. We detected expression of the human NAT2 transcript and protein in mouse liver by real-time PCR and Western blot analysis. NAT2 enzyme activity, measured using the human NAT2-selective substrate sulfamethazine (SMZ), was 40- to 80-fold higher in liver cytosols from hNAT2(tg)Nat1/2(-/-) mice than in wild-type mice. An unexpected gender difference was observed, with males displaying 2-fold higher activity than females. Transgenic mice also had an increased in vivo plasma clearance of SMZ and higher levels of N-acetylated SMZ than wild-type mice. Liver expression of human NAT2 did not affect the disposition of the human NAT1-selective substrate p-aminosalicylic acid (PAS), because hNAT2(tg)Nat1/2(-/-) mice displayed in vivo PAS pharmacokinetic profiles similar to those of Nat1/2(-/-) mice. The metabolism of 4-aminobiphenyl was similar between hNAT2(tg)Nat1/2(-/-) and wild-type mice with the exception of a more liver-restricted pattern in hNAT2(tg)Nat1/2(-/-) mice and lower activity in females. Overall, the hNAT2(tg)Nat1/2(-/-) mouse mimics human expression of NAT2 and may thus be of value in clarifying the role of human NAT2 in arylamine clearance, detoxification, and bioactivation.

Effect of Arylamine AcetyltransferaseNat3Gene Knockout onN-Acetylation in the Mouse

Arylamine N-acetyltransferases (NAT) catalyze the biotransformation of many important arylamine drugs and procarcinogens. NAT can either detoxify or activate procarcinogens, complicating the manner in which these enzymes may participate in enhancing or preventing toxic responses to particular agents. Mice possess three NAT isoenzymes: Nat1, Nat2, and Nat3. Whereas Nat1 and Nat2 can efficiently acetylate many arylamines, few substrates appear to be appreciably metabolized by Nat3. We generated a Nat3 knockout mouse strain and used it along with our double Nat1/2(-/-) knockout strain to further investigate the functional role of Nat3. Nat3(-/-) mice showed normal viability and reproductive capacity. Nat3 expression was very low in wild-type animals and completely undetectable in Nat3(-/-) mice. In contrast, greatly elevated expression of Nat3 transcript was observed in Nat1/2(-/-) mice. We used a transcribed marker polymorphism approach to establish that the increased expression of Nat3 in Nat1/2(-/-) mice is a positional artifact of insertion of the phosphoglycerate kinase-neomycin resistance cassette in place of the Nat1/Nat2 gene region and upstream of the intact Nat3 gene, rather than a biological compensatory mechanism. Despite the increase in Nat3 transcript, the N-acetylation of p-aminosalicylate, sulfamethazine, 2-aminofluorene, and 4-aminobiphenyl was undetectable either in vivo or in vitro in Nat1/2(-/-) animals. In parallel, no difference was observed in the in vivo clearance or in vitro metabolism of any of these substrates between wild-type and Nat3(-/-) mice. Thus, Nat3 is unlikely to play a significant role in the N-acetylation of arylamines either in wild-type mice or in mice lacking Nat1 and Nat2 activities.

In Vivo and in Vitro Metabolism of Arylamine Procarcinogens in Acetyltransferase-Deficient Mice

Arylamine N-acetyltransferases (NATs) catalyze the biotransformation of a number of aromatic and heterocyclic amines, many of which are procarcinogenic agents. Interestingly, these enzymes are binary in nature, participating in both detoxification and activation reactions, and thus it is unclear what role NATs actually play in either preventing or enhancing toxic responses. The ultimate direction may be substrate-specific and dependent on its tissue-specific metabolism by competing, but genetically variable, drug-metabolizing enzymes. To investigate the effect of N-acetylation on the metabolism of some classical procarcinogenic arylamines, we have used our double knockout Nat1/2(-/-) mouse model to test both in vitro activity and the in vivo clearance of some of these agents. As expected, N-acetylation activity was undetectable in tissue cytosol preparations from Nat1/2(-/-) mice for 4-aminobiphenyl (ABP) and 2-aminofluorene (AF), whereas significant levels were measured in all wild-type tissue cytosols tested, indicating the widespread metabolism of these agents. Nat1/2(-/-) mice displayed a variable response with respect to in vivo pharmacokinetics. AF appeared to be most severely compromised, with a 3- to 4-fold increased area under the curve (AUC), whereas the clearance of ABP was found to be less dependent on N-acetylation, with no difference in ABP-AUC between wild-type and knockout animals. 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine was neither N-acetylated nor was its clearance affected by NAT genotype, signifying a dependence on other drug-metabolizing enzymes. The elucidation of the role that N-acetylation plays in the clearance of procarcinogenic agents is the first step in attempting to correlate metabolism by NATs to toxic outcome prevention or augmentation.

Delineation of the conserved functional properties of D1A, D1B and D1C dopamine receptor subtypes in vertebrates

The three main subtypes of dopamine D(1) receptor (D(1A), D(1B) and D(1C)) subtypes found in most vertebrate groups were generated by two major steps of gene duplications, early in evolution. To identify the functional characteristics contributing to conservation of these paralogous D(1) receptors in vertebrates, the pharmacological and functional properties of fish (Anguilla anguilla), amphibian (Xenopus laevis) and human receptors were systematically analysed in transfected cells. The ligand-binding parameters appeared essentially similar for orthologous receptors, but differed significantly among the subtypes. The D(1A) receptors from the three species displayed low intrinsic activity and a fast rate of agonist-induced desensitization. All the orthologous D(1B) receptors exhibited a similar desensitization time-course, but with smaller amplitude of decrease than D(1A) receptors, in agreement with their higher basal activity. In contrast, D(1C) receptors, which do not exist in mammals, have low intrinsic activity and exhibit only weak, but rapid, agonist-induced desensitization, without any changes upon longer treatment with agonist. Thus, each of the three D(1) receptor subtypes are characterized by activation and desensitization properties, in a sequence-specific manner, which has been probably acquired early after gene duplications, and constrained their conservation during vertebrate evolution. These properties have been instrumental to adapt dopamine system to the physiology of the numerous neuronal networks and functions they control in the large and complex brains of vertebrates.

Generation and functional characterization of arylamine N-acetyltransferase Nat1/Nat2 double-knockout mice

Arylamine N-acetyltransferases (NATs) catalyze the biotransformation of a variety of arylamine drugs and carcinogens and may play diametrically opposing roles in enhancing either the detoxification of these chemicals or their metabolic activation into DNA-binding electrophiles. To facilitate the study of these processes, we have generated a Nat1/Nat2 double-knockout mouse model by gene targeting in embryonic stem cells. Nat1/2(-/-) mice were born at the expected frequency and seemed normal and viable with no overt phenotype, indicating that these genes are not critical for development or physiological homeostasis. In wild-type mice, NAT1 and NAT2 transcripts were detectable by RT-PCR in all tissues assayed including liver, kidney, colon, brain, bladder, and spleen. NAT1 and NAT2 transcripts were completely undetectable in the Nat1/2(-/-) mice. The in vitro N-acetylation of p-aminosalicylate was detected at significant levels in liver and kidney cytosols from either wild-type inbred 'rapid acetylator' C57BL/6 mice or from outbred CD-1 mice possessing homozygous rapid, heterozygous, or homozygous 'slow acetylator' Nat2 genotypes. Activity was undetectable in cytosol preparations from Nat1/2(-/-) mice. Nat1/2(-/-) mice also displayed severely compromised in vivo pharmacokinetics of p-aminosalicylate (PAS) and sulfamethazine (SMZ), with a drastically increased plasma area under the curve for PAS and a complete absence of their acetylated metabolites (AcPAS or AcSMZ) from plasma, confirming the functional absence of these enzymes and impaired drug metabolism capacity. This knockout mouse model should be helpful in delineating the role that variation in acetylating enzymes plays in mediating interindividual differences in susceptibility to arylamine-induced chemical toxicity and/or carcinogenesis.

A cognate dopamine transporter-like activity endogenously expressed in a COS-7 kidney-derived cell line

The activity of the dopamine transporter is an important mechanism for the maintenance of normal dopaminergic homeostasis by rapidly removing dopamine from the synaptic cleft. In kidney-derived COS-7, COS-1 and HEK-293 but not in other mammalian cell lines (CHO, Y1, Ltk-), we have characterized a putative functional dopamine transporter displaying a high affinity (Km approximately 250 nM) and a low capacity (approximately 0.1 pmol/10(5) cells/min) for [3H]dopamine uptake. Uptake displayed a pharmacological profile clearly indicative of the neuronal dopamine transporter. Estimated Ki values of numerous substrates and inhibitors for the COS-dopamine transporter and the cloned human neuronal transporter (human dopamine transporter) correlate well with the exception of a few notable compounds, including the endogenous neurotransmitter dopamine, the dopamine transporter inhibitor GBR 12,909 and the dopaminergic agonist apomorphine. As with native neuronal and cloned dopamine transporters, the uptake velocity was sodium-sensitive and reduced by phorbol ester pre-treatment. Two mRNA species of 3.8 and 4.0 kb in COS-7 cells were revealed by Northern blot analysis similar in size to that seen in native neuronal tissue. A reverse-transcribed PCR analysis confirmed the existence of a processed dopamine transporter. However, no immunoreactive proteins of expected dopamine transporter molecular size or [3H]WIN 35,428 binding activity were detected. A partial cDNA of 1.3 kb, isolated from a COS-1 cDNA library and encoding transmembrane domains 1-6, displayed a deduced amino acid sequence homology of approximately 96% to the human dopamine transporter. Taken together, the data suggest the existence of a non-neuronal endogenous high affinity dopamine uptake system sharing strong functional and molecular homology to that of the cloned neuronal dopamine transporter.

Dopamine D1B receptor chimeras reveal modulation of partial agonist activity by carboxyl-terminal tail sequences

NNC 01-0012, a second-generation benzazepine compound, pharmacologically differentiates multiple vertebrate D1 receptor subtypes (D1A, D1B, D1C, and D1D) and displays high selectivity and affinity for dopamine D1C receptors. Functionally, whereas NNC 01-0012 acts as a full or poor antagonist at D1C and D1A receptor-mediated cyclic AMP production, respectively, it exhibits partial agonist activity at the D1B receptor. To define some of the structural motifs that regulate the pharmacological and functional differentiation of vertebrate dopamine D1 receptors by NNC 01-0012, a series of receptor chimeras were constructed in which the divergent carboxyl-terminal (CT) receptor tails were replaced with the corresponding sequences of D1A, D1B, or D1C receptors. Substitution of the vertebrate D1B carboxyl-terminal-tail at position Tyr345 with carboxyl-terminal-tail sequences of the D1A receptor abolished the partial agonist activity of NNC 01-0012 without affecting dopamine-stimulated cyclic AMP accumulation. At vertebrate D1B/D1CcT-tail receptor mutants, however, the intrinsic activity of the partial agonist NNC 01-0012 (10 microM) was markedly enhanced (approximately 60% relative to 10 microM dopamine) with no concomitant alteration in the molecule's ligand binding affinity or constitutive activity of the chimeric receptor. Similar results were obtained with other benzazepines such as SKF-38393 and SCH-23390, which act as partial agonists at vertebrate D1B receptors. Substitution of D1A and D1C receptor carboxyl-terminal tails with sequences encoded by the D1B receptor carboxyl-terminal tail did not, however, produce receptors with functional characteristics significantly different from wild type. Taken together, these data clearly suggest that in addition to well-characterized domains and amino acid residues in the third cytoplasmic loop, partial agonist activity at the D1B receptor is modulated by sequence-specific motifs within the carboxyl-terminal tail, a region that may underlie the possible structural basis for functionally divergent roles of multiple dopamine D1-like receptors.

Functional differentiation of multiple dopamine D1-like receptors by NNC 01-0012

Although members of the multiple vertebrate/mammalian dopamine D1 receptor gene family can be selectively classified on the basis of their molecular/phylogenetic, structural, and tissue distribution profiles, no subtype-specific discriminating agents have yet been identified that can functionally differentiate these receptors. To define distinct pharmacological/functional attributes of multiple D1-like receptors, we analyzed the ligand binding profiles, affinity, and functional activity of 12 novel NNC compounds at mammalian/vertebrate D1/D1A and D5/D1B, as well as vertebrate D1C/D1D, dopamine receptors transiently expressed in COS-7 cells. Of all the compounds tested, only NNC 01-0012 displayed preferential selectivity for vertebrate D1C receptors, inhibiting [3H]SCH-23390 binding with an estimated affinity (approximately 0.6 nM) 20-fold higher than either mammalian/vertebrate D1/D1A or D5/D1B receptors or the D1D receptor. Functionally, NNC 01-0012 is a potent antagonist at D1C receptors, inhibiting to basal levels dopamine (10 microM)-stimulated adenylyl cyclase activity. In contrast, NNC 01-0012 (10 microM) exhibits weak antagonist activity at D1A receptors, inhibiting only 60% of maximal cyclic AMP production by dopamine, while acting as a partial agonist at vertebrate D1B and D1D receptors, stimulating adenylyl cyclase activity by approximately 33% relative to the full agonist dopamine (10 microM), an effect that was blocked by the selective D1 receptor antagonist NNC 22-0010. These data clearly suggest that the benzazepine NNC 01-0012, despite lacking the N-methyl residue in the R3 position, is a selective and potent D1C receptor antagonist. Moreover, the differential signal transduction properties exhibited by NNC 01-0012 at these receptor subtypes provide further evidence, at least in vertebrates, for the classification of the D1C receptor as a distinct D1 receptor subtype.

Early emergence of three dopamine D1 receptor subtypes in vertebrates. Molecular phylogenetic, pharmacological, and functional criteria defining D1A, D1B, and D1C receptors in European eel Anguilla anguilla

The existence of dopamine D1C and D1D receptors in Xenopus and chicken, respectively, challenged the established duality (D1A and D1B) of the dopamine D1 receptor class in vertebrates. To ascertain the molecular diversity of this gene family in early diverging vertebrates, we isolated four receptor-encoding sequences from the European eel Anguilla anguilla. Molecular phylogeny assigned two receptor sequences (D1A1 and D1A2) to the D1A subtype, and a third receptor to the D1B subtype. Additional sequence was orthologous to the Xenopus D1C receptor and to several other previously unclassified fish D1-like receptors. When expressed in COS-7 cells, eel D1A and D1B receptors display affinity profiles for dopaminergic ligands similar to those of other known vertebrate homologues. The D1C receptor exhibits pharmacological characteristics virtually identical to its Xenopus homologue. Functionally, while all eel D1 receptors stimulate adenylate cyclase, the eel D1B receptor exhibits greater constitutive activity than either D1A or D1C receptors. Semiquantitative reverse transcription-polymerase chain reaction reveals the differential distribution of D1A1, D1A2, D1B, and D1C receptor mRNA within the hypothalamic-pituitary axis of the eel brain. Taken together, these data suggest that the D1A, D1B, and D1C receptors arose prior to the evolutionary divergence of fish and tetrapods and exhibit molecular, pharmacological, and functional attributes that unambiguously allow for their classification as distinct D1 receptor subtypes in the vertebrate phylum.

Molecular and functional characterization of a partial cDNA encoding a novel chicken brain melatonin receptor

An approach based on homology probing was used to clone a partial cDNA encoding a novel melatonin (ML) receptor (MLR) from chicken (Gallus domesticus) brain. Based on available deduced amino-acid sequence, the chicken MLR (cMLR) displayed greater sequence homology to the frog (Xenopus) MLR than cloned human/mammalian receptors, with overall identities of 73% and 66%, respectively. In order to gain functional expression, a chimeric frog/chicken (flc)MLR was constructed in which the 5' end of the cMLR, including the N-terminus, TM1 and part of the first intracellular loop was substituted by fMLR sequence. [125I]Iodo-ML bound with high affinity (Kd of approximately 35 pM) to COS-7 cells transiently expressing the flcMLR in a saturable and guanine nucleotide-sensitive manner with the following rank order of potency: 2-iodo-ML > ML > 6-Cl-ML > S20750 > 6-OH-ML > S20642 > S20753 > N-acetyl-5HT >> 5-HT. Estimated Ki values for these compounds at the flcMLR correlated well to those obtained in native chicken brain membranes. In line with the observed structural similarity to the fMLR, the flcMLR exhibited affinities for ML, 6-Cl-ML and 6-OH-ML approximately 10-fold lower than mammalian receptors. Functionally, opposing interactions between ML and dopamine receptor signal transduction pathways were observed with ML potently inhibiting dopamine D1A-receptor-mediated cAMP accumulation in cells (HEK-293) transiently co-expressing these receptors. cMLR mRNAs were found expressed in chicken brain and kidney with trace levels observed in the lung. The availability of cloned vertebrate MLRs distinct at both the amino acid and pharmacological level from their mammalian counterparts may now allow for the identification of those amino-acid residues and structural motifs that regulate ML-binding specificity and affinity.

A primordial dopamine D1-like adenylyl cyclase-linked receptor from Drosophila melanogaster displaying poor affinity for benzazepines

We report here the isolation from Drosophila melanogaster of a 2.0 kb cDNA clone encoding a 385 amino acid protein (dDA1) displaying, within putative transmembrane domains, highest amino acid sequence homology (49-53%) to members of the vertebrate dopamine D1-like receptor family. When expressed in either Sf9 or COS-7 cells, dDA1 did not bind the specific D1-like receptor antagonist [3H]SCH-23390 or numerous other dopaminergic, adrenergic or serotoninergic ligands with high affinity. However, like vertebrate dopamine D1-like receptors, dDA1 stimulated the accumulation of cAMP in response to DA (EC50 approximately 300 nM) and 6,7-ADTN (EC50 approximately 500 nM). The dopaminergic rank order of potency (DA > NE >> 5-HT) and the lack of stimulation by other possible neurotransmitters (octopamine, tyramine, tryptamine) or DA metabolites (e.g. N-acetyl dopamine) found in Drosophila suggests that this receptor functionally belongs to the dopamine D1-like subfamily. Benzazepines, which characteristically bind to vertebrate dopamine D1-like receptors with high affinity, were relatively poor in stimulating (SKF-38393, SKF-82526; EC50 > 10 microM) dDA1-mediated accumulation of cAMP. Of the numerous compounds tested, a few dopaminergic antagonists inhibited DA-stimulated production of cAMP in a concentration-dependent manner, albeit with considerably reduced affinity, and with the rank order of potency: (+)-butaclamol(Kb approximately 125nM) > SCH-23390(Kb approximately 230nM) > alpha-flupenthixol (Kb approximately 400 nM) > chlorpromazine > or = spiperone (Kb approximately 680 nM) > or = clozapine. In situ hybridization revealed that dDA1 receptor mRNA is expressed as a maternal transcript, and at later blastoderm stages is restricted to apical regions of the cortical peripheral cytoplasm. The generation of inter-species D1 receptor chimeras may help to identify those particular sequence-specific motifs or amino acid residues conferring high affinity benzaepine receptor interactions.

The dopamine D1D receptor. Cloning and characterization of three pharmacologically distinct D1-like receptors from Gallus domesticus

Three genomic clones encoding dopamine D1-like receptors were isolated from the avian species Gallus domesticus. Two of these genes encode proteins of 451 and 488 amino acids, which, based on deduced amino acid sequence identity and homology of exhibited pharmacological profiles, appear to be species homologs of mammalian and vertebrate D1/D1A and D5/D1B receptors, respectively. The third genomic clone, termed D1D, encodes a protein of 445 amino acids displaying a deduced amino acid sequence identity within putative transmembrane domains of 75% to mammalian D1/D1A and 77% to D5/D1B receptors with overall sequence homologies of only 49% and 46%, respectively. Membranes from COS-7 cells transfected with D1D DNA bound [3H]SCH-23390 in a saturable manner with high affinity (approximately 300 pM) and with a pharmacological profile clearly indicative of a dopamine D1-like receptor. The D1D receptor exhibited affinities for 6,7-dihydroxy-2-aminotetralin and dopamine 10-fold higher than D1/D1A receptors, characteristic of the D5/D1B receptor subfamily. In contrast, the D1D receptor bound dopaminergic agents, such as SKF-38393, apomorphine, pergolide, and lisuride, with affinities 10-fold higher than other cloned mammalian or vertebrate D1A/D1B receptor subtypes, while both clozapine and haloperidol displayed considerably lower affinity for the D1D receptor. Based on the low overall amino acid sequence identity (54%) and unique pharmacological profile, the avian dopamine D1D receptor does not appear to be a species homolog of the recently cloned vertebrate D1C receptor (Sugamori, K.S., Demchyshyn, L. L., Chung, M., and Niznik, H. B. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 10536-10540). As with all cloned mammalian and vertebrate D1-like receptors, the D1D receptor stimulates adenylate cyclase activity in the presence of dopamine or SKF-82526. Northern blot analysis reveals the selective expression of both avian D1D and D1A receptor mRNAs only in brain with the D1B receptor more widely distributed and localized in tissues such as brain, kidney, and spleen. The isolation of four distinct vertebrate dopamine D1 receptor subtypes suggests the existence of additional mammalian D1 like receptor genes that may account for the observed pharmacological and biochemical multiplicity of dopamine D1-like receptor mediated events.

D1A, D1B, and D1C dopamine receptors from Xenopus laevis

Three distinct genes encoding members of the D1 dopamine receptor family were isolated from Xenopus laevis. Based on the deduced amino acid sequence, two of the receptors (Xen D1A and Xen D1B) appear to be homologues of mammalian D1/D1A and D5/D1B receptors. The third receptor, termed Xen D1C, displays equal overall amino acid and nucleotide sequence identity (approximately 55%) with mammalian D1A and D1B/D5 receptors. In agreement with their structural similarities, Xen D1A and D1B receptors, when expressed in COS-7 cells, displayed pharmacological profiles that paralleled those of their mammalian counterparts, with dopamine and 2-amino-6,7-dihydroxytetralin exhibiting 10-fold higher affinity for D1B than for D1A. The Xen D1C receptor displayed an overall rank order of potency and pharmacological profile clearly indicative of a D1-like receptor, with individual affinities for most agonists higher than those for either Xen or mammalian D1/D1A and D5/D1B receptors, whereas antagonist Ki values were intermediate to those for the D1/D1A and D5/D1B receptors. All three receptors stimulated adenylate cyclase activity in response to dopamine or SKF-82526. Xen D1A, D1B, and D1C receptor mRNAs were differentially distributed, with all three receptors expressed in brain and only D1B and D1C receptors expressed in kidney. The existence of a receptor which lacks appreciable overall sequence similarity to, but displays pharmacological homology with, mammalian D1-like receptors lends strong support to the contention that additional mammalian D1-like receptor gene products may exist to allow for the expression of the full spectrum of D1-like dopamine receptor-mediated events.

Cloning, expression, and localization of a chloride-facilitated, cocaine-sensitive serotonin transporter from Drosophila melanogaster

We report here on the isolation and characterization of a serotonin (5HT) transporter from Drosophila melanogaster. A 3.1-kb complementary DNA clone (dSERT) was found to encode a protein of 622 amino acid residues with a predicted molecular mass of approximately 69 kDa and a putative transmembrane topology characteristic of cloned members of the mammalian Na+/Cl- neurotransmitter cotransporter gene family. dSERT displays highest overall amino acid sequence identity with the mammalian 5HT (51%), norepinephrine (47%), and dopamine (47%) transporters and shares with all transporters 104 absolutely conserved amino acid residues. Upon transient expression in HeLa cells, dSERT exhibited saturable, high-affinity, and sodium-dependent [3H]5HT uptake with estimated Km and Vmax values of approximately 500 nM and 5.2 x 10(-18) mol per cell per min, respectively. In marked contrast to the human SERT (hSERT), 5HT-mediated transport by dSERT was not absolutely dependent on extracellular Cl-, while the sodium-dependent uptake of 5HT was facilitated by increased extracellular Cl- concentrations. dSERT displays a pharmacological profile and rank order of potency consistent with, but not identical to, mammalian 5HT transporters. Comparison of the affinities of various compounds for the inhibition of 5HT transport by both dSERT and hSERT revealed that antidepressants were 3- to 300-fold less potent on dSERT than on hSERT, while mazindol displayed approximately 30-fold greater potency for dSERT. Both cocaine and RTI-55 inhibited 5HT uptake by dSERT with estimated inhibition constants of approximately 500 nM, while high concentrations (> 10 microM) of dopamine, norepinephrine, octopamine, tyramine, and histamine failed to inhibit transport. In situ hybridization reveals the selective expression of dSERT mRNA to specific cell bodies in the ventral ganglion of the embryonic and larval Drosophila nervous system with a distribution pattern virtually identical to that of 5HT-containing neurons. The dSERT gene was mapped to position 60C on chromosome 2. The availability of the gene encoding the unique ion dependence and pharmacological characteristics of dSERT may allow for identification of those amino acid residues and structural motifs that confer the pharmacologic specificity and genetic regulation of the 5HT transport process.

Serotonin receptor cDNA cloned from Lymnaea stagnalis

Serotonin (5-HT) is a major neurotransmitter that influences various behaviors, neuronal plasticity, learning, and memory in molluscs. Although the physiology of 5-HT transmission in molluscs is well studied, little is known about the pharmacology and diversity of the 5-HT receptor system. Based on the high homology of genes coding for guanine nucleotide-binding protein (G protein)-coupled receptors, we have cloned a gene for the Lymnaea stagnalis 5-HT (5HTlym) receptor. The putative receptor protein, 509 amino acids long, has highest homology with the Drosophila 5-HT receptors and mammalian 5HT1 receptors. As revealed by RNA blot-hybridization analysis, two mRNA species of 2.3 and 3.2 kb are detected in the central nervous system of Lymnaea. Transient expression of 5HTlym in COS-7 cells showed saturable [3H]lysergic acid diethylamide binding with an estimated dissociation constant of 0.9 nM. The 5HTlym receptor exhibited a mixed 5HT-like pharmacology that cannot be precisely categorized with existing mammalian classification nomenclature. However, the 5HTlym receptor does display some characteristics that have been attributed to the putative mammalian vascular 5HT1-like receptor.