Analysis of ␤-tubulin alleles from nine paclitaxel-resistant Chinese hamster ovary cell lines revealed an unexpected cluster of mutations affecting Leu-215, Leu-217, and Leu-228. Six of the mutant alleles encode a His, Arg, or Phe substitution at Leu-215; another mutant allele has an Arg substitution at Leu-217; and the final two mutant alleles have substitutions of His or Phe at Leu-228. Using plasmids that allow tetracycline regulated expression, the L215H, L217R, and L228F mutations were introduced into a hemagglutinin antigen-tagged ␤-tubulin cDNA and transfected into wild-type Chinese hamster ovary cells. In all three cases, low to moderate expression of the transfected mutant gene conferred paclitaxel resistance. Higher levels of expression caused disruption of microtubule assembly, cell cycle arrest at mitosis, and failure to proliferate. Consistent with reduced microtubule stability, cells expressing mutant hemagglutinin ␤-tubulin had fewer acetylated microtubules than nonexpressing cells in the same population. These data, together with previous studies showing that the paclitaxel-resistant mutant cell lines have less stable microtubules, indicate that the leucine cluster represents an important structural motif for microtubule assembly.Paclitaxel is the prototype for a novel class of agents that inhibit cells in mitosis by promoting and stabilizing microtubule assembly. Early studies with this compound demonstrated that it binds to microtubules in a 1:1 stoichiometry with tubulin heterodimers (1) and inhibits microtubule disassembly. It is also able to induce microtubule assembly both in vitro and in vivo and induces microtubule bundle formation in treated cells (2, 3). Recent interest in this and related compounds has been fueled by clinical studies demonstrating remarkable activity of paclitaxel against a number of malignant diseases (reviewed in Ref. 4). Although still in clinical trials, the demonstrated activity of paclitaxel in phase II studies has led to FDA approval for its use in refractory cases of breast and ovarian cancer. As more patients are treated with this drug, clinical resistance is expected to become an increasingly significant problem.The mechanisms by which tumor cells acquire resistance to paclitaxel are not fully understood. Cell culture studies have shown that paclitaxel is a substrate for the multidrug resistance pump (gP170), 1 and cells selected for high levels of resistance to the drug have increased gP170 (reviewed in Ref. 5). Nevertheless, it has yet to be demonstrated that this mechanism is significant in paclitaxel refractory tumors. Indeed, the remarkable efficacy of paclitaxel in early clinical studies of patients who were pretreated with Adriamycin, a well known substrate for gP170, argues that the multidrug resistance (mdr) phenotype may not be as clinically prevalent as had initially been anticipated (4).Additional mechanisms of resistance to paclitaxel have been reported. For example, several laboratories have provided evidence that changes in the expression of sp...

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SummaryIn the absence of oxygen, many bacteria preferentially use nitrate as a terminal electron acceptor for anaerobic respiration. In Escherichia coli, there are two membrane-bound, differentially regulated nitrate reductases. While the physiological basis for this metabolic redundancy is not completely understood, during exponential growth, synthesis of NRA is greatly induced by anaerobiosis plus nitrate, whereas NRZ is expressed at a low level that is not in¯uenced by anaerobiosis or nitrate. In the course of identifying genes controlled by the stationary phase regulatory factor RpoS (s s ),we found that the expression of NRZ is induced during entry into stationary phase and highly dependent on this alternative sigma factor. Expression studies, using operon fusions and nitrate reductase assays, revealed that the NRZ operon is controlled mainly at the level of transcription and is induced 10-fold at the onset of stationary phase in rich media. Consistent with previous reports of RpoS expression, the RpoS dependency of NRZ in minimal media was very high (several hundredfold). We also observed a ®ve-fold stationary phase induction of NRZ in an rpoS background, indicating that other regulatory factors, besides RpoS, are probably involved in transcriptional control of NRZ. The RpoS dependence of NRZ expression was con®rmed by Northern analyses using RNA extracted from wild-type and rpoS À strains sampled in exponential and stationary phase. In toto, these data indicate that RpoS-mediated regulation of NRZ may be an important physiological adaptation that allows the cell to use nitrate under stress-associated conditions.

The expression and function of inducible nitric oxide synthase (iNOS) in the stomach is unclear. This study assessed the effects of endotoxin on rat gastric iNOS expression and its role in gastric injury from luminal irritants. In conscious rats, a 5-h treatment with intraperitoneal lipopolysaccharide (LPS; 1–20 mg/kg) dose dependently increased gastric mucosal iNOS immunoreactivity and increased gastric luminal nitrate and nitrite accumulation (Griess reaction). LPS also increased gastric luminal fluid accumulation and reduced macroscopic gastric injury from orogastric acidified ethanol. Aminoguanidine (45 mg/kg) did not prevent LPS-induced gastroprotection or gastric fluid accumulation. N G-nitro-l-arginine methyl ester increased gastric luminal fluid and caused macroscopic gastric injury when given with LPS. Using an anesthetized preparation followed by removal of luminal fluid, LPS reduced gastric mucosal blood flow and exacerbated gastric injury from either acidified ethanol or acidified taurocholate, an effect that was negated by aminoguanidine. These data indicate that in conscious rats, the gastroprotective effect of endotoxin is dependent on constitutive NOS but not iNOS activity. However, the inducible isoform participates in the ability of endotoxin to exacerbate gastric injury from luminal irritants in the anesthetized rat.