Contrasting dynamics of water and mineral nutrients in stems shown by stable isotope tracers and cryo‐SIMS

Lateral exchange of water and nutrients between xylem and surrounding tissues helps to de‐couple uptake from utilization in all parts of a plant. We studied the dynamics of these exchanges, using stable isotope tracers for water (H₂¹⁸O), magnesium (²⁶Mg), potassium (⁴¹K) and calcium (⁴⁴Ca) delivered via a cut stem for various periods to the transpiration stream of bean shoots (Phaseolus vulgaris cv. Fardenlosa Shiny). Tracers were subsequently mapped in stem cross‐sections with cryo‐secondary ion mass spectrometry. The water tracer equilibrated within minutes across the entire cross‐section. In contrast, the nutrient tracers showed a very heterogeneous exchange between xylem vessels and the different stem tissues, even after 4 h. Dynamics of nutrients in the tissues revealed a fast and extensive exchange of nutrients in the xylem parenchyma, with, for example, calcium being completely replaced by tracer in less than 5 min. Dilution of potassium tracer during its 30 s transit in xylem sap through the stem showed that potassium concentration was up‐regulated over many hours, to the extent that some of it was probably supplied by phloem recirculation from the shoot.     

Phospholipases. I. Effect ofn-alkanols on the rate of enzymatic hydrolysis of egg phosphatidylcholine

Summary The rate of hydrolysis of unsonicated liposomes of egg lecithin by phospholipase A (from bee venom and Russell viper venom) and phospholipase C (fromBacillus cereus andClostridium welchii) is markedly dependent on the nature and concentration of a variety of added alcohols. Typical plots of rate against alcohol concentration are bell-shaped. The maximum rate and the alcohol concentration at which it is achieved are alcohol-specific. In a homologous series ofn-alkanols, the maximal rates increase and the optimal concentrations decrease as the chain length is increased from C₄ to C₈. For longer alcohols (C₉ to C₁₂), progressively higher concentrations are required to elicit maximal activation. The optimal activating concentrationsC for C₄ to C₈ n-alkanols obey the relationshipp C=a logP _octanol+constant [cf. Hansch & Dunn,J. Pharm. Sci. 61:1 (1972)], suggesting that the alcohol-activating effect is a consequence of their incorporation into the liposomes with resultant modification of liposomal structure.     

Expression of splice variants of 1alpha-hydroxylase in mcf-7 breast cancer cells

1,25-Dihydroxyvitamin D(3) (calcitriol) is the most active natural metabolite of Vitamin D(3). It has strong antiproliferative and differentiating effects on various cell types including breast cancer cells. 25-Hydroxyvitamin D(3)-1alpha-hydroxylase (1alpha-hydroxylase, CYP27B1) is one of the key enzymes in the formation of calcitriol. It has been found in breast cancer cells suggesting an autocrine regulation of formation of calcitriol in these cells. Alternative splicing of the encoding genes for this enzyme can possibly play a role in regulating the enzyme level and can explain tissue specific variations of 1alpha-hydroxylase activity. Splice variants containing intron 1 may encode for truncated proteins with deletion of protein domains which are essential for its enzymatic activity. In order to obtain more information on the abundance of 1alpha-hydroxylase splice variants, we performed a highly specific nested touchdown PCR in MCF-7 cells. The full-length sequence of 1alpha-hydroxylase and two different splice variants of this enzyme containing intron 1 were isolated. By Western blot technique we then confirmed the protein products of the full-length enzyme and its splice variants. We hypothesize that that the expression of splice variants can lead to a quantitatively lower expression of the mRNA of the full-length enzyme. The abundance of less active 1alpha-hydroxylase protein variants can alter the local synthesis of calcitriol in the cells and may explain variations of enzymatic activity in different cells and tissues.     

Signalling via integrins: implications for cell survival and anticancer strategies

Integrin-associated signalling renders cells more resistant to genotoxic anti-cancer agents like ionizing radiation and chemotherapeutic substances, a phenomenon termed cell adhesion-mediated radioresistance/drug resistance (CAM-RR, CAM-DR). Integrins are heterodimeric cell-surface molecules that on one side link the actin cytoskeleton to the cell membrane and on the other side mediate cell-matrix interactions. In addition to their structural functions, integrins mediate signalling from the extracellular space into the cell through integrin-associated signalling and adaptor molecules such as FAK (focal adhesion kinase), ILK (integrin-linked kinase), PINCH (particularly interesting new cysteine-histidine rich protein) and Nck2 (non-catalytic (region of) tyrosine kinase adaptor protein 2). Via these molecules, integrin signalling tightly and cooperatively interacts with receptor tyrosine kinase signalling to regulate survival, proliferation and cell shape as well as polarity, adhesion, migration and differentiation. In tumour cells of diverse origin like breast, colon or skin, the function and regulation of these molecules is partly disturbed and thus might contribute to the malignant phenotype and pre-existent and acquired multidrug resistance. These issues as well as a variety of therapeutic options envisioned to influence tumour cell growth, metastasis and resistance, including kinase inhibitors, anti-integrin antibodies or RNA interference, will be summarized and discussed in this review.