Myoseverin, a microtubule-binding molecule with novel cellular effects.

A new microtubule-binding molecule, myoseverin, was identified from a library of 2,6,9-trisubstituted purines in a morphological differentiation screen. Myoseverin induces the reversible fission of multinucleated myotubes into mononucleated fragments. Myotube fission promotes DNA synthesis and cell proliferation after removal of the compound and transfer of the cells to fresh growth medium. Transcriptional profiling and biochemical analysis indicate that myoseverin alone does not reverse the biochemical differentiation process. Instead, myoseverin affects the expression of a variety of growth factor, immunomodulatory, extracellular matrix-remodeling, and stress response genes, consistent with the activation of pathways involved in wound healing and tissue regeneration.

Synthesis and application of functionally diverse 2,6,9-trisubstituted purine libraries as CDK inhibitors.

BACKGROUND: Purines constitute a structural class of protein ligands involved in mediating an astonishing array of metabolic processes and signal pathways in all living organisms. Synthesis of purine derivatives targeting specific purine-binding proteins in vivo could lead to versatile lead compounds for use as biological probes or drug candidates. RESULTS: We synthesized several libraries of 2,6, 9-trisubstituted purines using both solution- and solid-phase chemistry, and screened the compounds for inhibition of cyclin-dependent kinase (CDK) activity and human leukemic cell growth. Lead compounds were optimized by iterative synthesis based on structure-activity relationships (SARs), as well as analysis of several CDK-inhibitor cocrystal structures, to afford several interesting compounds including one of the most potent CDK inhibitors known to date. Unexpectedly, some compounds with similar CDK inhibitory activity arrested cellular proliferation at distinctly different phases of the cell cycle and another inhibitor directly induced apoptosis, bypassing cell-cycle arrest. Some of these compounds selectively inhibited growth of cells derived from specific tumors. CONCLUSIONS: 2,6,9-Trisubstituted purines have various and potent biological activities, despite high concentrations of competing endogenous purine ligands in living cells. Purine libraries constitute a versatile source of small molecules that affect distinct biochemical pathways mediating different cellular functions.

Synthesis and biological evaluation of myoseverin derivatives: microtubule assembly inhibitors.

Myoseverin, a trisubstituted purine, inhibits microtubule assembly in vitro, interferes with normal mitotic spindle assembly, and arrests the cell cycle in mitosis in U937 cells. We synthesized a variety of myoseverin derivatives and screened them for inhibition of spindle assembly in Xenopus egg extracts and for microtubule disassembly in vitro. Selected compounds were tested against 60 cancer cell lines at the National Cancer Institute as possible anticancer drug candidates.

Intracellular drug concentrations and transporters: measurement, modeling, and implications for the liver.

Intracellular concentrations of drugs and metabolites are often important determinants of efficacy, toxicity, and drug interactions. Hepatic drug distribution can be affected by many factors, including physicochemical properties, uptake/efflux transporters, protein binding, organelle sequestration, and metabolism. This white paper highlights determinants of hepatocyte drug/metabolite concentrations and provides an update on model systems, methods, and modeling/simulation approaches used to quantitatively assess hepatocellular concentrations of molecules. The critical scientific gaps and future research directions in this field are discussed.

Effects of macromolecular crowding on nuclear size.

The concentration of macromolecules inside cells is high, and the resultant crowding of cytoplasm can be expected to affect many interactions involving macromolecular assemblies. Here, we have examined the effect of solute size and concentration on nuclear volume in saponin-permeabilized macrophages. Nuclei swelled in the presence of small solutes and shrank reversibly in the presence of larger permeant solutes. Remarkably, the smallest solutes capable of shrinking the nucleus were not excluded by the pores in the nuclear envelope. Indeed, nuclei shrank in the presence of such solutes even after the nuclear envelope had been sheared mechanically or permeabilized with detergent. Nuclei extracted with 1% Triton X-100 shrank in the presence of very high concentrations of small solute molecules (30% w/v) as well as in lower concentrations of larger solutes. Consistent with a macromolecular crowding effect, changes in nuclear volume were dependent on solute size and not simply dependent on the colligative properties of solutes or the exclusion of solutes by the nuclear envelope. Solute size-dependent changes in nuclear volume were independent of the chemical nature of the solutes and of the activity of the ions in the buffer. Together, these observations indicate that high concentrations of macromolecules such as those found inside cells can influence the size of the nucleus by directly affecting nuclear structure.

Microtubules can modulate pseudopod activity from a distance inside macrophages.

Microtubules are thought to influence cell shape as structural components of an integrated cytoskeletal matrix. Here we show that microtubules can affect the dynamics of macrophage pseudopodia without being integrated into their structure. Macrophages landing on glass surfaces spread within 15 min into flattened circular cells with radial symmetry, and the radial distribution of microtubules reflected this symmetry. Depolymerization of microtubules using nocodazole, colchicine, or vinblastine did not inhibit macrophage spreading or the early establishment of radial symmetry. Shortly after spreading, however, macrophages without microtubules gradually became asymmetric, assuming irregular, lobed profiles. The asymmetry resulted from exaggerated protrusion and retraction of pseudopodia, with net retraction overall. This loss of radial symmetry could be inhibited by treatment of initially spread cells with cytochalasin D, indicating that the change in cell shape was mediated by the actin cytoskeleton. Intact microtubules suppressed the exaggerated pseudopod movements, even when they were separated by a distance from the cell margin. In cells treated with taxol, microtubules remained clustered near the cell center after spreading, yet the dynamics of pseudopodia at the cell margin were reduced and cells maintained a circular profile. Similarly, in cells treated with low concentrations of nocodazole, a much reduced microtubule cytoskeleton nonetheless suppressed pseudopod dynamics. We propose that microtubules act to stabilize cell shape at a distance from the cell edge via a biochemical intermediate that affects the structure or function of the microfilament system.

A cyclin-dependent kinase inhibitor inducing cancer cell differentiation: biochemical identification using Xenopus egg extracts.

Cellular differentiation is a complex process involving growth arrest, exit from the cell cycle, and expression of differentiated cell-type-specific functions. To identify small molecules promoting this process, a chemical library was screened by using a myeloid leukemic cell line that retained the potential to differentiate in culture. In the presence of a purine derivative, aminopurvalanol (AP), cells acquired phenotypic characteristics of differentiated macrophages and became arrested in the cell cycle with a 4N DNA content. AP also inhibited mitosis in Xenopus egg extracts, suggesting that it acted on an evolutionarily conserved cell cycle regulatory pathway. Affinity chromatography and biochemical reconstitution experiments with Xenopus egg extracts identified cyclin-dependent kinase (CDK) 1-cyclin B as a target of the compound. Although AP potently inhibited immunoprecipitates of both human CDK1 and CDK2 from human leukemic cell extracts, our results indicate that the compound preferentially targets the G2/M-phase transition in vivo.