Skin sensitization testing needs and data uses by US regulatory and research agencies

United States regulatory and research agencies may rely upon skin sensitization test data to assess the sensitization hazards associated with dermal exposure to chemicals and products. These data are evaluated to ensure that such substances will not cause unreasonable adverse effects to human health when used appropriately. The US Consumer Product Safety Commission, the US Environmental Protection Agency, the US Food and Drug Administration, the Occupational Safety and Health Administration, the National Institute for Occupational Safety and Health, and the US Department of Defense are member agencies of the Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM). ICCVAM seeks to identify opportunities for the use of non-animal replacements to satisfy these testing needs and requirements. This review identifies the standards, test guidelines, or guidance documents that are applicable to satisfy each of these agency's needs; the current use of animal testing and flexibility for using alternative methodologies; information needed from alternative tests to fulfill the needs for skin sensitization data; and whether data from non-animal alternative approaches are accepted by these US federal agencies.

Investigating the susceptibility of mice to a bacterial challenge after intravenous exposure to durable nanoparticles

AIM

The goal of this study was to determine whether bacterial clearance in a rodent model would be impaired upon exposure to gold, silver or silica nanoparticles (NPs).

MATERIALS & METHODS

Mice received weekly injections of NPs followed by a challenge of Listeria monocytogenes (LM). On days 3 and 10 after LM injections, the animals were sacrificed and their tissues were collected for elemental analysis, electron microscopy and LM count determination.

RESULTS

The untreated and NP-treated animals cleared LM at the same rate suggesting that bioaccumulation of NPs did not increase the animals' susceptibility to bacterial infection.

CONCLUSION

The data from this study indicate that the bioaccumulation of NPs does not significantly affect the ability to react to a bacterial challenge.

Evaluating the potential of gold, silver, and silica nanoparticles to saturate mononuclear phagocytic system tissues under repeat dosing conditions

BACKGROUND

As nanoparticles (NPs) become more prevalent in the pharmaceutical industry, questions have arisen from both industry and regulatory stakeholders about the long term effects of these materials. This study was designed to evaluate whether gold (10 nm), silver (50 nm), or silica (10 nm) nanoparticles administered intravenously to mice for up to 8 weeks at doses known to be sub-toxic (non-toxic at single acute or repeat dosing levels) and clinically relevant could produce significant bioaccumulation in liver and spleen macrophages.

RESULTS

Repeated dosing with gold, silver, and silica nanoparticles did not saturate bioaccumulation in liver or spleen macrophages. While no toxicity was observed with gold and silver nanoparticles throughout the 8 week experiment, some effects including histopathological and serum chemistry changes were observed with silica nanoparticles starting at week 3. No major changes in the splenocyte population were observed during the study for any of the nanoparticles tested.

CONCLUSIONS

The clinical impact of these changes is unclear but suggests that the mononuclear phagocytic system is able to handle repeated doses of nanoparticles.

Effect of silica and gold nanoparticles on macrophage proliferation, activation markers, cytokine production, and phagocytosis in vitro

The accumulation of durable nanoparticles (NPs) in macrophages following systemic administration is well described. The ultimate biological impact of this accumulation on macrophage function, however, is not fully understood. In this study, nontoxic doses of two durable NPs, SiO₂ and Au, at particle sizes of ~10 nm and 300 nm were used to evaluate the effect of bioaccumulation on macrophage function in vitro using RAW 264.7 mouse macrophage-like cells as a model system. Cell proliferation, cell cycle, cytokine production, surface marker activation, and phagocytosis responses were evaluated through a panel of assays using flow cytometry and confocal microscopy. The most dramatic change in RAW 264.7 cell function was a reduction in phagocytosis as monitored by the uptake of Escherichia coli. Cells exposed to both 10 nm Au NPs and 10 nm SiO₂ NPs showed ~50% decrease in phagocytosis, while the larger NPs caused a less dramatic reduction. In addition to modifying phagocytosis profiles, 10 nm SiO₂ NPs caused changes in proliferation, cell cycle, and cell morphology. Au NPs had no effect on cell cycle, cytokine production, or surface markers and caused interference in phagocytosis in the form of quenching when the assay was performed via flow cytometry. Confocal microscopy analysis was used to minimize this interference and demonstrated that both sizes of Au NPs decreased the phagocytosis of E. coli. Overall, our results demonstrate that Au and SiO₂ NP uptake by macrophages can influence macrophage phagocytosis in vitro without altering surface markers and cytokine production in vitro. While the biological impact of these findings remains unclear, our results indicate that bioaccumulation of durable NPs within the macrophages may lead to a suppression of bacterial uptake and possibly impair bactericidal activity.

Evaluating the effect of assay preparation on the uptake of gold nanoparticles by RAW264.7 cells

Background

Cell culture conditions can greatly influence the results of nanoparticle (NP) uptake assays. In this study, 10 nm gold nanoparticles (AuNPs) and RAW 264.7 macrophages were used as a model system, while instrumental neutron activation analysis (NAA) was used as the elemental analysis technique to determine AuNP levels produced by the various culturing conditions. Static plate-based and insert-based culture conditions were compared with a dynamic suspension culture to evaluate the conditions’ effect on the rate and extent of AuNP uptake.

Results

The results indicate that a dynamic culturing condition allows for the greatest NP uptake (approximately 3-5 times over the adherent conditions), whereas the plate-based assays have the initial highest rate of NP incorporation.

Conclusions

These data highlight the importance of judiciously choosing the assay conditions prior to evaluating NP uptake.

Lipoxin A₄ modulates adaptive immunity by decreasing memory B-cell responses via an ALX/FPR2-dependent mechanism

Specialized proresolving mediators are endogenous bioactive lipid molecules that play a fundamental role in the regulation of inflammation and its resolution. Lipoxins and other specialized proresolving mediators have been identified in important immunological tissues including bone marrow, spleen, and blood. Lipoxins regulate functions of the innate immune system including the promotion of monocyte recruitment and increase macrophage phagocytosis of apoptotic neutrophils. A major knowledge gap is whether lipoxins influence adaptive immune cells. Here, we analyzed the actions of lipoxin A₄ (LXA₄) and its receptor ALX/FPR2 on human and mouse B cells. LXA₄ decreased IgM and IgG production on activated human B cells through ALX/FPR2-dependent signaling, which downregulated NF-κB p65 nuclear translocation. LXA₄ also inhibited human memory B-cell antibody production and proliferation, but not naïve B-cell function. Lastly, LXA₄ decreased antigen-specific antibody production in an OVA immunization mouse model. To our knowledge, this is the first description of the actions of lipoxins on human B cells, demonstrating a link between resolution signals and adaptive immunity. Regulating antibody production is crucial to prevent unwanted inflammation. Harnessing the ability of lipoxins to decrease memory B-cell antibody production can be beneficial to threat inflammatory and autoimmune disorders.

The role of eNOS phosphorylation in causing drug-induced vascular injury

Previously we found that regulation of eNOS is an important part of the pathogenic process of Drug-induced vascular injury (DIVI) for PDE4i. The aims of the current study were to examine the phosphorylation of eNOS in mesentery versus aorta at known regulatory sites across DIVI-inducing drug classes and to compare changes across species. We found that phosphorylation at S615 in rats was elevated 35-fold 2 hr after the last dose of CI-1044 in mesentery versus 3-fold in aorta. Immunoprecipitation studies revealed that many of the upstream regulators of eNOS activation were associated with eNOS in 1 or more signalosome complexes. Next rats were treated with drugs from 4 other classes known to cause DIVI. Each drug was given alone and in combination with SIN-1 (NO donor) or L-NAME (eNOS inhibitor), and the level of eNOS phosphorylation in mesentery and aorta tissue was correlated with the extent of vascular injury and measured serum nitrite. Drugs or combinations produced altered serum nitrite levels as well as vascular injury score in the mesentery. The results suggested that phosphorylation of S615 may be associated with DIVI activity. Studies with the species-specific A2A adenosine agonist CI-947 in rats versus primates showed a similar pattern.

Lipoxin A4 decreases human memory B cell antibody production via an ALX/FPR2-dependent mechanism: A link between resolution signals and adaptive immunity (P5151)

Specialized proresolving mediators (SPMs) are endogenous bioactive lipid molecules that play a fundamental role in the regulation of inflammation and its resolution. SPMs are classified into lipoxins, resolvins, protectins and maresins. Lipoxins and other SPM have been identified in immunological tissues including bone marrow, spleen and blood. Lipoxins regulate functions of the innate immune system such as monocyte recruitment and inhibition of neutrophil infiltration to the site of inflammation. A major knowledge gap is whether lipoxins influence adaptive immune cells. Here, we discovered that B cells express the lipoxin A4 (LXA4) receptor, ALX/FPR2, and upregulate its expression following mitogen activation. Furthermore, LXA4 decreased IgM and IgG production on activated B cells through an ALX/FPR2-dependent mechanism. This caused a reduction in NF-κB p65 nuclear translocation. We found that memory B cells express higher levels of ALX/FPR2 compared to naïve B cells. Interestingly, LXA4 inhibited human memory B cell antibody production and proliferation, but not naïve B cell function. LXA4 also decreased antigen-specific antibody production in vivo. To our knowledge, this is the first description of the effects of lipoxins on B cells, which provides a link between resolution signals and adaptive immunity. The ability of lipoxins to decrease memory B cell antibody production could be particularly beneficial to threat chronic inflammatory diseases and autoimmune disorders.

Evaluation of Viability and Proliferation Profiles on Macrophages Treated with Silica Nanoparticles In Vitro via Plate-Based, Flow Cytometry, and Coulter Counter Assays

Nanoparticles (NPs) are known to interfere with many high-throughput cell viability and cell proliferation assays, which complicates the assessment of their potential toxic effects. The aim of this study was to compare viability and proliferation results for colloidal silica (SiO2 NP; 7 nm) in the RAW 264.7 mouse macrophage cell line using three different techniques: plate-based assays, flow cytometry analysis, and Coulter counter assays. Our data indicate that CellTiter-Blue, XTT, and CyQuant plate-based assays show increased values over control at low SiO2 NPs concentrations (0.001–0.01 g/L). SiO2 NPs show little-to-no interference with flow cytometry and Coulter counter assays, which not only were more reliable in determining cell viability and proliferation at low concentrations in vitro, but also identified changes in cell granularity and size that were not captured by the plate-based assays. At high SiO2 NP concentrations (1 g/L) all techniques indicated cytotoxicity. In conclusion, flow cytometry and Coulter counter identified new cellular features, and flow cytometry offered more flexibility in analyzing the viability and proliferation profile of SiO2 NP-treated RAW 264.7 cells.

Peroxisome proliferator-activated receptor γ B cell-specific-deficient mice have an impaired antibody response

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily. PPARγ, a ligand-activated transcription factor, has important anti-inflammatory and antiproliferative functions, and it has been associated with diseases including diabetes, scarring, and atherosclerosis, among others. PPARγ is expressed in most bone marrow-derived cells and influences their function. PPARγ ligands can stimulate human B cell differentiation and promote Ab production. A knowledge gap is that the role of PPARγ in B cells under physiological conditions is not known. We developed a new B cell-specific PPARγ (B-PPARγ) knockout mouse and explored the role of PPARγ during both the primary and secondary immune response. In this article, we show that PPARγ deficiency in B cells decreases germinal center B cells and plasma cell development, as well as the levels of circulating Ag-specific Abs during a primary challenge. Inability to generate germinal center B cells and plasma cells is correlated to decreased MHC class II expression and decreased Bcl-6 and Blimp-1 levels. Furthermore, B-PPARγ-deficient mice have an impaired memory response, characterized by low titers of Ag-specific Abs and low numbers of Ag-experienced, Ab-secreting cells. However, B-PPARγ-deficient mice have no differences in B cell population distribution within primary or secondary lymphoid organs during development. This is the first report, to our knowledge, to show that, under physiological conditions, PPARγ expression in B cells is required for an efficient B cell-mediated immune response as it regulates B cell differentiation and Ab production.

Non-steroidal anti-inflammatory drugs reduce optimal antibody production in response to vaccination and vaccinia virus infection (45.18)

Protective humoral immune responses generated by vaccination, depends on optimal production of antibodies. Non-steroidal anti-inflammatory drugs (NSAIDs) are used to curb undesirable effects of vaccination such as fever. We previously demonstrated that B cells express the “proinflammatory” enzyme cyclooxygenase-2 (Cox-2) in response to activation stimuli such as the TLR9 ligand CpG DNA. Using genetic approaches and small molecule Cox-2 selective inhibitors, we demonstrated that Cox-2 is crucial for optimal antibody production. Expression of plasma cell transcriptional regulatory factors, including Blimp-1 and Xbp-1, were dependent on Cox-2 activity. Significantly fewer antibody secreting plasma cell precursors were generated in the presence of Cox-2 inhibitors, indicating that Cox-2 is essential for human B cell terminal differentiation. Mice chronically treated with Cox-2 inhibitors produced less neutralizing IgG following live virus infection with vaccinia virus. In vitro studies using human B cells show that all commonly used NSAIDs, as well as Tylenol (acetaminophen), at pharmacologic doses, blunt antibody production. Interestingly, a recent clinical trial showed that Tylenol use in infants blunted antibody responses to a variety of vaccines. These new data support the concept that the Cox pathway is necessary for optimal antibody production. The use of NSAIDs, may impair antibody production to vaccination and to certain infections.

Memory B cells from older people express normal levels of cyclooxygenase-2 and produce higher levels of IL-6 and IL-10 upon in vitro activation

Worldwide the elderly population is increasing. The elderly show deficiencies in immune function. B lymphocytes are essential elements of the immune system responsible for antibody production. This laboratory previously showed that activated human B cells isolated from young adults express cyclooxygenase-2 (Cox-2) and that Cox-2 is essential for optimal antibody responses. Recent data suggests that Cox-2 expression decreases with age in mouse bone tissue. There is no information regarding Cox-2 expression in B cells from older human subjects. We investigated the expression and activity of Cox-2 in naïve and memory B cells from older people. We show that B cells from older subjects show similar Cox-2 protein expression and activity, antibody production and proliferation compared to younger people. However, we found that activated memory B cells from older people produce higher levels of IL-6 and IL-10 compared to young adults. Therefore, the dysregulated cytokine production could contribute to immune senescence in the elderly.

Non-steroidal anti-inflammatory drugs blunt antibody production in human B lymphocytes from older volunteer (84.10)

Older individuals respond poorly to vaccination. B lymphocytes are responsible for the synthesis of antibodies that are generated in response to infection and vaccination. Our laboratory has shown that activated human B cells isolated from younger subjects (18-35 years old) express cyclooxygenase-2 (Cox-2) and that inhibition of Cox-2 by non-steroidal anti-inflammatory drugs (NSAIDs) blunts antibody production. However, NSAIDs are highly used by people over 60 years old. There is no information regarding Cox-2 expression in B cells in older human subjects (over 60 years old). We hypothesize that Cox-2 activity is reduced in B cells from older individuals. Young (18-35 years old) and older subjects (over 60 years old) were enrolled in the study. B cells were isolated from peripheral blood, stimulated in vitro with anti-Ig plus ODN CpG 2395 in the presence or absence of NSAIDs and Cox-2 expression, prostaglandin production, cytokines and antibody synthesis determined. Cell viability and proliferation were also assayed. Compared to younger subjects, B cells from older volunteers had reduced Cox-2 expression and also produced less antibody. Furthermore, in these individuals, antibody levels were further decreased by NSAIDs. Thus, NSAIDs and low intrinsic Cox-2 expression could be responsible for the poor antibody response to vaccination in the elderly.

Inhibition of cyclooxygenase-2 activity impairs antibody production in human B cells and in mice infected with vaccinia virus (84.8)

Generation of protective humoral immune responses against vaccines depends on optimal production of antibodies by B lymphocytes. Non-steroidal anti-inflammatory drugs (NSAIDs) are used to curb undesirable symptoms of vaccination, including pain, fever and swelling. We previously demonstrated in B cells that cyclooxygenase-2 (Cox-2) is induced in response to both T-dependent and T-independent stimuli, including the TLR9 ligand CpG DNA, and that Cox-2 is crucial for antibody production. Expression of plasma cell transcriptional regulatory factors, including Blimp-1 and Xbp-1, were dependent on Cox-2 activity. Significantly fewer antibody secreting plasma cell precursors were generated in the presence of Cox-2 selective inhibitors, indicating that Cox-2 is essential for human B cell terminal differentiation. Cox-2 selective inhibitors were administered in vivo to determine the role of Cox-2 in a humoral immune response to live infection with vaccinia virus. Mice chronically treated with Cox-2 selective inhibitors produced less viral neutralizing IgG following live virus infection. Inhibition of Cox-2 resulted in a reduced frequency of IFN-γ producing T cells, indicating that T cell priming of B cells was attenuated. These data support the idea that impaired humoral immunity in the absence of Cox-2 activity can be dependent on both B cells and T cells and that the use of Cox-2 selective inhibitors have the potential to dampen humoral immunity to live virus infection and vaccines.

Chronic inhibition of cyclooxygenase-2 attenuates antibody responses against vaccinia infection

Generation of optimal humoral immunity to vaccination is essential to protect against devastating infectious agents such as the variola virus that causes smallpox. Vaccinia virus (VV), employed as a vaccine against smallpox, provides an important model of infection. Herein, we evaluated the importance cyclooxygenase-2 (Cox-2) in immunity to VV using Cox-2 deficient mice and Cox-2 selective inhibitory drugs. The effects of Cox-2 inhibition on antibody responses to live viruses such as vaccinia have not been previously described. Here, we used VV infection in Cox-2 deficient mice and in mice chronically treated with Cox-2 selective inhibitors and show that the frequency of VV-specific B cells was reduced, as well as the production of neutralizing IgG. VV titers were approximately 70 times higher in mice treated with a Cox-2 selective inhibitor. Interestingly, Cox-2 inhibition also reduced the frequency of IFN-gamma producing CD4(+) T helper cells, important for class switching. The significance of these results is that the chronic use of non-steroidal anti-inflammatory drugs (NSAIDs), and other drugs that inhibit Cox-2 activity or expression, blunt the ability of B cells to produce anti-viral antibodies, thereby making vaccines less effective and possibly increasing susceptibility to viral infection. These new findings support an essential role for Cox-2 in regulating humoral immunity.

Dysfunctional memory CD8+ T cells after priming in the absence of the cell cycle regulator E2F4

The transcriptional repressor E2F4 is important for cell cycle exit and terminal differentiation in epithelial cells, neuronal cells and adipocytes but its role in T lymphocytes proliferation and memory formation is not known. Herein, we investigated the function of E2F4 protein for the formation of functional murine memory T cells. Murine transgenic CD8+ T cells were infected in vitro with lentivirus vector expressing a shRNA targeted against E2F4 followed by in vitro stimulation with SIINFEKL antigenic peptide. For in vivo assays, transduced cells were injected into congenic mice which were then infected with HSV-OVA. The primary response, memory formation and secondary stimulation were determined for CD8+ lentivirus transduced cells. In the absence of E2F4 cell cycle repressor, activated CD8+ T cells underwent intensive proliferation in vitro and in vivo. These cells had the ability to differentiate into memory cells in vivo, but they were defective in recall proliferation. We show that transient suppression of E2F4 during CD8+ T cell priming enhances primary proliferation and has a negative effect on secondary stimulation. These findings demonstrate that the cell cycle repressor E2F4 is essential for the formation of functional memory T cells. A decrease in CD8+ T-lymphocyte compartment would diminish our capacity to control viral infections.

Ibuprofen and other widely used non-steroidal anti-inflammatory drugs inhibit antibody production in human cells

The widely used non-steroidal anti-inflammatory drugs (NSAIDs) function mainly through inhibition of cyclooxygenases 1 and 2 (Cox-1 and Cox-2). Unlike Cox-1, Cox-2 is considered an inducible and pro-inflammatory enzyme. We previously reported that Cox-2 is upregulated in activated human B lymphocytes and using Cox-2 selective inhibitors that Cox-2 is required for optimal antibody synthesis. It is not known whether commonly used non-prescription and non-Cox-2 selective drugs also influence antibody synthesis. Herein, we tested a variety of Cox-1/Cox-2 non-selective NSAIDs, namely ibuprofen, tylenol, aspirin and naproxen and report that they blunt IgM and IgG synthesis in stimulated human peripheral blood mononuclear cells (PBMC). Ibuprofen had its most profound effects in inhibiting human PBMCs and purified B lymphocyte IgM and IgG synthesis when administered in the first few days after activation. As shown by viability assays, ibuprofen did not kill B cells. The implications of this research are that the use of widely available NSAIDs after infection or vaccination may lower host defense. This may be especially true for the elderly who respond poorly to vaccines and heavily use NSAIDs.

Ibuprofen, the popular over-the counter non-steroidal anti-inflammatory drug, blunts human antibody synthesis in vitro (134.41)

Background: Non-steroidal anti-inflammatory drugs (NSAIDs) are effective in reducing fever, relieving pain and inhibiting inflammation. NSAIDs function through inhibition of cyclooxygenases 1 and 2 (Cox-1 and Cox-2). Cox-1 is ubiquitously expressed, while Cox-2 expression is increased during inflammation and in malignant cells. We recently showed that Cox-2 is up-regulated in normal human B lymphocytes after activation and is required for optimal antibody synthesis. We hypothesized that ibuprofen (dual Cox-1/Cox-2 inhibitor) blunts antibody synthesis in human peripheral blood mononuclear cells (PBMC) and B lymphocytes.

Methods: IgM and IgG production in anti-IgM + CpG 2395 stimulated human PBMC and B cells was detected by enzyme-linked immunosorbent assay (ELISA). B cell proliferation and viability in the presence of ibuprofen was calculated using mitochondrial dehydrogenase activity (MTT) and 3H-Thymidine incorporation assays.

Results: Pharmacological concentrations of ibuprofen significantly reduced IgM and IgG synthesis in human PBMC and B lymphocytes in a dose-dependent manner. Ibuprofen had only a modest effect on B cell viability, but significantly inhibited B cell proliferation.

Conclusion: Ibuprofen reduces optimal antibody synthesis in part, by decreasing B cell proliferation.

Research supported by DE011390, AI071064, ES01247 grants.

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Targeting cyclooxygenase-2 in hematological malignancies: rationale and promise

There is much interest in the potential use of Cox-2 selective inhibitors in combination with other cancer therapeutics. Malignancies of hematopoietic and non-hematopoietic origin often have increased expression of cyclooxygenase-2 (Cox-2), a key modulator of inflammation. For example, hematological malignancies such as chronic lymphocytic leukemia, chronic myeloid leukemia, Hodgkin's lymphoma, non-Hodgkin's lymphoma and multiple myeloma often highly express Cox-2, which correlates with poor patient prognosis. Expression of Cox-2 enhances survival and proliferation of malignant cells, while negatively influencing anti-tumor immunity. Hematological malignancies expressing elevated levels of Cox-2 potentially avoid immune responses by producing factors that enhance angiogenesis and metastasis. Cellular immune responses regulated by natural killer cells, cytotoxic T lymphocytes, and T regulatory cells are also influenced by Cox-2 expression. Therefore, Cox-2 selective inhibitors have promising therapeutic potential in patients suffering from certain hematological malignancies.

C-23 hydroxylation by Arabidopsis CYP90C1 and CYP90D1 reveals a novel shortcut in brassinosteroid biosynthesis

Brassinosteroids (BRs) are biosynthesized from campesterol via several cytochrome P450 (P450)-catalyzed oxidative reactions. We report the functional characterization of two BR-biosynthetic P450s from Arabidopsis thaliana: CYP90C1/ROTUNDIFOLIA3 and CYP90D1. The cyp90c1 cyp90d1 double mutant exhibits the characteristic BR-deficient dwarf phenotype, although the individual mutants do not display this phenotype. These data suggest redundant roles for these P450s. In vitro biochemical assays using insect cell-expressed proteins revealed that both CYP90C1 and CYP90D1 catalyze C-23 hydroxylation of various 22-hydroxylated BRs with markedly different catalytic efficiencies. Both enzymes preferentially convert 3-epi-6-deoxocathasterone, (22S,24R)-22-hydroxy-5alpha-ergostan-3-one, and (22S,24R)-22-hydroxyergost-4-en-3-one to 23-hydroxylated products, whereas they are less active on 6-deoxocathasterone. Likewise, cyp90c1 cyp90d1 plants were deficient in 23-hydroxylated BRs, and in feeding experiments using exogenously supplied intermediates, only 23-hydroxylated BRs rescued the growth deficiency of the cyp90c1 cyp90d1 mutant. Thus, CYP90C1 and CYP90D1 are redundant BR C-23 hydroxylases. Moreover, their preferential substrates are present in the endogenous Arabidopsis BR pool. Based on these results, we propose C-23 hydroxylation shortcuts that bypass campestanol, 6-deoxocathasterone, and 6-deoxoteasterone and lead directly from (22S,24R)-22-hydroxy-5alpha-ergostan-3-one and 3-epi-6-deoxocathasterone to 3-dehydro-6-deoxoteasterone and 6-deoxotyphasterol.

Dwarfism and cytochrome P450-mediated C-6 oxidation of plant steroid hormones

BRs (brassinosteroids) are plant steroid hormones that are essential for normal plant development. The dramatic dwarfism exhibited by mutants in the CYP (cytochrome P450) enzymes involved in BR biosynthesis indicates a role for these hormones in plant growth and development. Since the mid-1990s, collaborative research has been geared towards developing a better understanding of the CYP85 class of CYPs involved in BR biosynthesis in both Arabidopsis and tomato. Some of the most recent observations include the fact that certain CYP85 CYPs catalyse the synthesis of the most bioactive BR, BL (brassinolide). Current evidence suggests that evolution of this function may have occurred independently in different dicotyledonous species. Interestingly, BL accumulates in tomato fruits, highlighting a key role for this hormone in fruit development. At the same time as developing a better understanding of the enzymatic function of these CYPs, we have also carried out experiments towards characterizing where and when these genes are expressed and mechanisms of their regulation. As expected for a hormone involved in growth and development, biosynthetic gene promoter activity is associated with young rapidly growing cells and with fruit development.

Diurnal regulation of the brassinosteroid-biosynthetic CPD gene in Arabidopsis

Plant steroid hormones, brassinosteroids (BRs), are essential for normal photomorphogenesis. However, the mechanism by which light controls physiological functions via BRs is not well understood. Using transgenic plants carrying promoter-luciferase reporter gene fusions, we show that in Arabidopsis (Arabidopsis thaliana) the BR-biosynthetic CPD and CYP85A2 genes are under diurnal regulation. The complex diurnal expression profile of CPD is determined by dual, light-dependent, and circadian control. The severely decreased expression level of CPD in phytochrome-deficient background and the red light-specific induction in wild-type plants suggest that light regulation of CPD is primarily mediated by phytochrome signaling. The diurnal rhythmicity of CPD expression is maintained in brassinosteroid insensitive 1 transgenic seedlings, indicating that its transcriptional control is independent of hormonal feedback regulation. Diurnal changes in the expression of CPD and CYP85A2 are accompanied by changes of the endogenous BR content during the day, leading to brassinolide accumulation at the middle of the light phase. We also show that CPD expression is repressed in extended darkness in a BR feedback-dependent manner. In the dark the level of the bioactive hormone did not increase; therefore, our data strongly suggest that light also influences the sensitivity of plants to BRs.