Background:A novel flow cytometric assay has been described in an accompanying report (Gombos et al., Methods: The kinetics of the decrease in immunofluorescence intensity was analyzed after the addition of the raft-preserving Triton X-100 or Nonidet P-40, both of which disrupt the entire membrane. Mild treatments by both detergents leave cells attached to only those proteins that are anchored to the cytoskeleton by rafts or independent of rafts. Agents that affect microfilaments and modulate membrane levels of cholesterol by cyclodextrin were used to distinguish between the raft-mediated and non-raft-related associations of the Pgp. Confocal microscopy and flow cytometric fluorescence energy transfer measurements were used to confirm colocalization of Pgp with raft constituents. Results: The assay was proved to be sensitive enough to resolve differences between the resistance of UIC2-labeled cell-surface Pgps to Triton X-100 versus Nonidet P-40. Approximately 34% of the UIC2 Fab-labeled Pgp molecules were associated with the cytoskeleton through detergent-resistant, cholesterol-sensitive microdomains or directly, whereas approximately 15% were found to be directly linked to the cytoskeleton. Accordingly, confocal microscopy showed that Pgps colocalize with raft mark-

P-glycoprotein (Pgp), coded for by the mdr1 gene, is one of the ABC transporters held responsible for the phenomenon of multidrug resistance (mdr), which is reflected by a rapidly escalating failure of chemotherapy with different classes of cytotoxic agents: anthracyclins, vinca alkaloids, taxanes, epipodophylotoxins. Although overcoming resistance conveyed by Pgp alone may not be sufficient for reaching effective treatment, the abundance of observations available for this paradigmatic multidrug transporter at both in vitro and in vivo setting is a tempting ground for an updated assessment of the main currents of mdr research. In this review we attempt to help keep track of the features of Pgp-mediated drug transport that serve as the major starting points for ongoing efforts of mdr reversal. We will analyze the slowly narrowing gaps that prevail between our ever increasing understanding at the protein, cell and organism level, focusing on the molecular interactions involving Pgp.

SUMMARYWe reported earlier that neonatal monocyte-derived macrophages (MDM) could not be fully activated with IFN-g, a finding that could not be attributed to lower expression of IFN-g receptors on the neonatal cells. In this study we explored elements of IFN-gR-mediated signalling in cord monocytes and MDM. Intracellular expression of STAT-1 was analysed by flow cytometry. We have assessed phosphorylation of STAT-1 by using MoAbs that distinguish native and phosphorylated forms of STAT-1 on a discrete cell basis. Using MoAbs against the native form of STAT-1 revealed comparable expression of this protein in cord and adult cells (both monocytes and MDM). However, STAT-1 phosphorylation in response to IFN-g was significantly decreased in neonatal monocytes (P , 0´05) and MDM (P , 0´01) compared to adult cells (n . 5 for each). These data suggest deficient cytokine-receptor signalling in neonatal mononuclear phagocytes exposed to IFN-g. We propose that decreased STAT-1 phosphorylation and activation may represent developmental immaturity and may contribute to the unique susceptibility of neonates to infections by intracellular pathogens.

Background: Lipid rafts are cholesterol-and glycosphingolipid-rich microdomains in the cellular plasma membranes that play critical roles in compartmentalization (concentration, coupling, and isolation) of receptors and signal molecules. Therefore, detecting constitutive or induced raft associations of such proteins is of central interest in cell biology. This has mostly been done with timeand cell-consuming immunobiochemical techniques affected by several sources of artifacts. A flow cytometric analysis of immunocytochemical staining under differential circumstances of detergent treatment offers a new alternative to this method. Methods: Membrane microdomains are resistant to nonionic detergents due to extensive, strong interactions between their molecular constituents. We used this feature to develop a rapid flow cytometric assay of differential detergent resistance based on immunocytochemical labeling of extracellular domain epitopes in membrane proteins. Data evaluation is based on comparative detection of their detergent solubility without and with cholesterol depletion of cell membranes, resolved by moderate concentrations of nonionic detergents. Results: Nonionic detergents Triton X-100 and Nonidet-40 (0.05-0.1%) in cold or Brij-98 (0.1-0.5%) at 37°C efficiently resolved detergent solubility or resistance of

P-glycoprotein (Pgp) is an ABC transporter responsible for the ATP-dependent efflux of chemotherapeutic compounds from multidrug resistant cancer cells. Better understanding of the molecular mechanism of Pgp-mediated transport could promote rational drug design to circumvent multidrug resistance. By measuring drug binding affinity and reactivity to a conformation-sensitive antibody we show here that nucleotide binding drives Pgp from a high to a low substrate-affinity state and this switch coincides with the flip from the inward- to the outward-facing conformation. Furthermore, the outward-facing conformation survives ATP hydrolysis: the post-hydrolytic complex is stabilized by vanadate, and the slow recovery from this state requires two functional catalytic sites. The catalytically inactive double Walker A mutant is stabilized in a high substrate affinity inward-open conformation, but mutants with one intact catalytic center preserve their ability to hydrolyze ATP and to promote drug transport, suggesting that the two catalytic sites are randomly recruited for ATP hydrolysis.