Chemotherapeutic agents and gene expression in cardiac myocytes

It is becoming apparent that anti-cancer chemotherapies are increasingly associated with cardiac dysfunction or even congestive heart failure (Minotti et al., 2004; Eliott, 2006; Suter et al., 2004; Ren, 2005). Our data suggest that one of the contributing factors to the cardiotoxicitiy of these drugs may be the activation of the AhR-response (including the increased expression of Cyp1a1) and/or other detoxification program in cardiac myocytes themselves. The induction of such responses may have secondary effects (e.g. to increase the level of intracellular oxidative stress), which may influence the contractility or even survival of cardiac myocytes. Furthermore, the specific response of cardiac myocytes, both with respect to the metabolizing enzymes and the export channels, potentially differs from other cells (e.g. we failed to detect any increase in expression of other “classical” AhR-responsive genes, Ugt1a1 and Ugt1a6). This could account for, for example, the observation that doxoribicinol (the 13-hydroxy form of doxorubicin) accumulates in cardiac myocytes but not in hepatocytes (Del Tacca et al., 1985; Olson et al., 1988). Given the vulnerability of the heart and the almost irreparable damage that can be done by severe oxidative stress, further studies would seem to be merited specifically on the effects of chemotherapeutic agents on cardiac myocytes.     

The significance of PTEN's protein phosphatase activity

Many hundreds of research papers over the last ten years have established the significance of PTEN's lipid phosphatase activity in mediating many of its effects on specific cellular processes in many different cell types, including cell growth, proliferation, survival, and migration ([Backman et al., 2002], [Iijima et al., 2002], [Leslie and Downes, 2002] and [Salmena et al., 2008]). In some cases, detailed signalling mechanisms have been identified by which these PtdInsP₃-dependent effects are manifest ([Kolsch et al., 2008], [Manning and Cantley, 2007] and [Tee and Blenis, 2005]). Further, in some settings, in vivo data from, for example genetic deletion of PTEN, relates closely with independent manipulation of the PI3K/Akt signalling pathway ([Bayascas et al., 2005], [Chen et al., 2006], [Crackower et al., 2002] and [Ma et al., 2005]). Together these studies indicate that the dominant effects of PTEN function are mediated through its regulation of PtdInsP₃-dependent signalling, but that its protein phosphatase activity also contributes in some settings. These conclusions are of great importance given the intense efforts underway to develop PI3K (EC 2.7.1.153) inhibitors as cancer therapeutics. The experiments reviewed here have firmly established that the protein phosphatase activity of PTEN plays a role in the regulation of cellular processes including migration. On the other hand, it has not been established beyond doubt that PTEN acts on substrates other than itself; no such substrates have been confidently identified and effector mechanisms for PTEN's protein phosphatase activity are currently unclear. The goal for future research must be firstly to understand the signalling mechanisms by which PTEN protein phosphatase activity acts: whether this is through identifying substrates, or working out how autodephosphorylation mediates its effects. Secondly, and critically, the significance of PTEN's protein phosphatase activity must be established in vivo. This can be achieved through relating the phenotypes intervening with both PTEN and with protein phosphatase effector pathways when they are identified, and through the generation of mouse models expressing substrate selective PTEN mutants. We should then be able to answer the important question of whether PTEN's protein phosphatase activity contributes to tumour suppression.     

Phosphatidic acid phosphohydrolase in the regulation of inflammatory signaling

Since the cloning and characterization of PAP-1 by Carman's group, a preponderance of new information has emerged pertaining to the molecular function of the enzyme and its physiological function in lipid metabolism ([Carman and Han, 2006] and [Donkor et al., 2007]). As a consequence of this development, PAP-1 and lipin research have informed us further regarding lipid metabolism and adipose tissue development ([Carman and Han, 2006] and [Reue and Zhang, 2008]). In recent years, the field of inflammatory lipid signaling has undergone a great expansion and has come to identify a wide variety of protein and metabolic inflammatory mediators, including PAP-1 as well as PLD, PKC, PLC and DAG. The focus of this review was to summarize experimental evidence supporting a role for PAP-1 in inflammatory signaling, which is summarized in Scheme 2 (Grkovich et al., in press). Further clarification is needed to identify the precise signaling functions and downstream targets of DAG forming enzymes, such as PLD/PAP-1 and PLC, as evidence supports both as participants in inflammatory signaling in different systems. Clarification of these regulatory questions should lead to a more complete understanding of inflammatory signaling while helping to identify future pharmacological targets.     

Abscisic acid is required for somatic embryo initiation through mediating spatial auxin response in Arabidopsis

Abscisic acid (ABA) regulates many aspects of plant development, including somatic embryo (SE) initiation. However, mechanisms of ABA functions on SE initiation have remained to be investigated. In this study, we examined the endogenous ABA contents of calli in Arabidopsis during the SE inductive process. We further found that the capacity for SE initiation was strongly impaired by treatment of fluridone, a potent inhibitor of ABA biosynthesis, as well as by mutation of ABA biosynthetic gene ABA2, suggesting that ABA is required for SE initiation. Furthermore, treatment of fluridone inhibited local auxin biosynthesis and auxin polar transport in the embryonic calli, resulting in the disturbance of auxin response pattern and the decreased regeneration frequency of SEs. However, application of exogenous ABA in the medium almost recovered patterns of auxin response and SE initiation. Thus, the results suggest that ABA functions on SE initiation through mediating both auxin biosynthesis and polar transport for establishment of auxin response pattern in callus. Our study provides new information for understanding mechanisms of SE initiation.     

Embryogenic potential and expression of embryogenesis-related genes in conifers are affected by treatment with a histone deacetylase inhibitor

Somatic embryogenesis is used for vegetative propagation of conifers. Embryogenic cultures can be established from zygotic embryos; however, the embryogenic potential decreases during germination. In Arabidopsis, LEAFY COTYLEDON (LEC) genes are expressed during the embryonic stage, and must be repressed to allow germination. Treatment with the histone deacetylase inhibitor trichostatin A (TSA) causes de-repression of LEC genes. ABSCISIC ACID3 (ABI3) and its Zea mays ortholog VIVIPAROUS1 (VP1) act together with the LEC genes to promote embryo maturation. In this study, we have asked the question whether TSA treatment in a conifer affects the embryogenic potential and the expression of embryogenesis-related genes. We isolated two conifer LEC1-type HAP3 genes, HAP3A and HAP3B, from Picea abies and Pinus sylvestris. A comparative phylogenetic analysis of plant HAP3 genes suggests that HAP3A and HAP3B are paralogous genes originating from a duplication event in the conifer lineage. The expression of HAP3A is high, in both somatic and zygotic embryos, during early embryo development, but decreases during late embryogeny. In contrast, the expression of VP1 is initially low but increases during late embryogeny. After exposure to TSA, germinating somatic embryos of P. abies maintain the competence to differentiate embryogenic tissue, and simultaneously the germination progression is partially inhibited. Furthermore, when embryogenic cultures of P. abies are exposed to TSA during embryo maturation, the maturation process is arrested and the expression levels of PaHAP3A and PaVP1 are maintained, suggesting a possible link between chromatin structure and expression of embryogenesis-related genes in conifers.     

Identification of novel VHL regulated genes by transcriptomic analysis of RCC10 renal carcinoma cells

Previous microarray studies have revealed a broad range of genes which are regulated by VHL and have provided much insight into how VHL may function as a tumour suppressor gene ([Wykoff et al., 2000b] and [Zatyka et al., 2002]). The current study has highlighted several genes of interest which are not currently recognised as being regulated by VHL. Of the candidate VHL regulated genes that we identified ASS was selected for further study due to its therapeutic implications. Tumours with low ASS levels display a reduced capacity to synthesise arginine, and as such are reliant on extracellular arginine for normal cellular function. Promising results in mouse xenograft models have shown that arginine deprivation may be a useful treatment strategy for these tumours. Understanding how ASS expression levels are regulated should provide insight into which tumour types would be most sensitive to treatment with arginine degrading enzymes. In this study we provide strong evidence that VHL status regulates ASS expression levels in three independent CCRCC cell backgrounds. Regulation of ASS by VHL/HIF suggests that arginine deprivation may be useful in the treatment of VHL defective CCRCCs and non-renal hypoxic tumours.     

<Emphasis Type="Italic">LEAFY COTYLEDON1</Emphasis>, <Emphasis Type="Italic">FUSCA3</Emphasis> expression and auxin treatment in relation to somatic embryogenesis induction in Arabidopsis

The expression pattern of the LEC1 and FUS3 genes during somatic embryogenesis in Arabidopsis explants (immature zygotic embryos) induced in vitro was analysed, using Real-time quantitative PCR (qRT-PCR). The analysis revealed differential expression of LEC1 but not FUS3 within a 30 day time course of somatic embryo development, and a significant auxin-dependent upregulation of LEC1 was found over the time course. In contrast to embryogenic culture, the level of LEC1 and FUS3 expression was noticeably lower in non-embryogenic callus of Col-0 and hormonal mutants (cbp20 and axr4-1) with low SE-efficiency. In addition, the expression profile of LEC1 and FUS3 was followed in the embryogenic culture derived from 35S::LEC2-GR explants. A significant increase of LEC1 but not FUS3 activity was observed under LEC2 overexpression induced in auxin-treated explants. The work provides further experimental evidence on LEC gene involvement in the embryogenic response in Arabidopsis somatic cells, and also implicates LEC1 function in more advanced stages of SE culture in relation to somatic embryo differentiation and development.     

Gibberellins and seed development in maize. II. Gibberellin synthesis inhibition enhances abscisic acid signaling in cultured embryos

Abscisic acid (ABA) is required for seed maturation in maize (Zea mays L.) and other plants. Gibberellins (GAs) are also present in developing maize embryos, and mutual antagonism of GAs and ABA appears to govern the choice between precocious germination or quiescence and maturation. Exogenous ABA can also induce quiescence and maturation in immature maize embryos in culture. To examine the role of GAs versus ABA in regulating maize embryo maturation, the effects of modulating GA levels were compared with those of ABA in embryos cultured at successive stages of development. The effects of GA synthesis inhibition or exogenous GA application differed markedly in embryos at different stages of development, indicating changes in both endogenous GA levels and in the capacity for GA synthesis as embryogenesis and maturation progress. In immature embryos, the inhibition of GA synthesis mimicked the effects of exogenous ABA, as shown by the suppression of germination, the acquisition of anthocyanin pigments, and the accumulation of a variety of maturation-phase mRNAs. We suggest that GA antagonizes ABA signaling in developing maize embryos, and that the changing hormone balance provides temporal control over the maturation phase.     

Polarity proteins regulate mammalian cell–cell junctions and cancer pathogenesis

Polarity pathways regulate important functions during the formation and maintenance of cell–cell junctions and during morphogenesis. In addition, cell polarity pathways are emerging as critical regulators of initiation and progression of carcinoma by functioning as tumor suppressors, downstream of oncogenes, or promoters of the metastatic process (Figure 2). It is highly likely that further analysis of cell polarity proteins and the pathways they control will identify novel biomarkers and potential drug targets for managing and treating patients with carcinoma.

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

• of special interest

•• of outstanding interest

Influence of culture conditions on embryo formation and maturation in auxin-induced embryogenic cultures of Digitalis obscura

Somatic embryogenesis was induced in hypocotyls of Digitalis obscura using indoleacetic acid or 2,4-dichlorophenoxyacetic acid with different culture and subculture conditions. Indoleacetic acid-induced embryogenic cultures were used to investigate the effects of amino acids, polyamines and growth regulators on embryo differentiation and maturation. Supplementation of the media with amino acids, polyamines or abscisic acid did not influence or had an adverse effect on embryogenic response. Gibberellic acid at 1.4 M in either culture (30 days) or subculture medium was effective in promoting both differentiation and normal embryo development. The efficiency of somatic embryogenesis was greatly enhanced when isolated indoleacetic acid-induced proembryogenic masses were subcultured in liquid medium with reduced auxin content.Abbreviations ABA abscisic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - GA gibberellic acid - IAA indoleacetic acid - Ptr putrescine - Spd spermidine - Spn spermine