Identification and characterization of a phenyl-thiazolyl-benzoic acid derivative as a novel RAR/RXR agonist

Objective To identify an agonist of RXRα and RARα with reduced undesired profiles of all-trans retinoic acid for differentiation-inducing therapy of acute promyelocytic leukemia (APL), such as its susceptibility to P450 enzyme, induction of P450 enzyme, increased sequestration by cellular retinoic acid binding protein and increased expression of P-glycoprotein, a virtual screening was performed. Results and conclusion In this study, a phenyl-thiazolyl-benzoic acid derivative (PTB) was identified as a potent agonist of RXRα and RARα. PTB was characterized in nuclear receptor binding, reporter gene, cell differentiation and cell growth assays. PTB bound directly to RXRα and RARα, but not to PPARα, δ(β) or γ. PTB fully activated reporter genes with enhancer elements for RXRα/RXRα, and partially activated reporter genes with enhancer elements for RARα/RXRα, PPARδ(β) and PPARγ. Furthermore, PTB induced differentiation and inhibited the growth of human APL cells. Thus, PTB is a novel dual agonist of RXRα and RARα and works as both a differentiation inducer and a proliferation inhibitor to leukemic cells

A new caffeine biosynthetic pathway in tea leaves: utilisation of adenosine released from the <i>S</i>-adenosyl-L-methionine cycle

The four-step caffeine biosynthetic pathway includes three methylation steps that utilise &lt;i&gt;S&lt;/i&gt;-adenosyl-L-methionine (SAM) as the methyl donor. In the process SAM is converted to &lt;i&gt;S&lt;/i&gt;-adenosyl-L-homocysteine (SAH) which in turn is hydrolysed to L-homocysteine and adenosine. Significant amounts of radioactivity from [methyl-&lt;sup&gt;14&lt;/sup&gt;C]methionine and [methyl-&lt;sup&gt;14&lt;/sup&gt;C]SAM were incorporated into theobromine and caffeine in young tea leaf segments, and very high SAH hydrolase activity was found in cell-free extracts from young tea leaves. Substantial amounts of radioactivity from [adenosyl-&lt;sup&gt;14&lt;/sup&gt;C]SAH were also recovered as theobromine and caffeine in tea leaf segments, indicating that adenosine derived from SAH is utilised for the synthesis of the purine ring of caffeine. From the profiles of activity of related enzymes in tea leaf extracts, it is proposed that the major route from SAM to caffeine is a SAM→SAH→adenosine→adenine→AMP→IMP→XMP→xanthosine→7-methylxanthosine→7-methylxanthine→theobromine→caffeine pathway. In addition, direct adenosine kinase-catalysed formation of AMP from adenosine may participate as an alternative minor route. The activity of two of the three &lt;i&gt;N&lt;/i&gt;-methyltransferase activities involved in caffeine biosynthesis and part of the activities of SAH hydrolase, adenosine nucleosidase, adenine phosphoribosyltransferase and adenosine kinase were located in tea chloroplasts. In contrast, no detectable activity of SAM synthetase was associated with the purified chloroplast fraction. This is a first demonstration that the purine skeleton of caffeine is synthesised from adenosine released from the SAM cycle