Fostriecin, an inhibitor of protein phosphatase 2A, limits myocardial infarct size even when administered after onset of ischemia.

BACKGROUND: The role of protein phosphatases (PPs) during ischemic preconditioning in the rabbit heart was examined. METHODS AND RESULTS: Fostriecin, a potent inhibitor of PP2A, was administered to isolated rabbit hearts starting either 15 minutes before or 10 minutes after the onset of a 30-minute period of regional ischemia and continuing until the onset of reperfusion. After 2 hours of reperfusion, infarct size was measured with triphenyltetrazolium chloride. In a second study with isolated rabbit cardiomyocytes, the effect of fostriecin pretreatment was assessed by measuring changes in cell osmotic fragility during simulated ischemia. PP1 and PP2A activities of isolated control and ischemically preconditioned cells were also measured. In a third series of experiments, left ventricular biopsies of isolated rabbit hearts were obtained before and at selected times during 60 minutes of global ischemia, and the tissue was assayed for PP1 and PP2A activities. In isolated hearts pretreated with fostriecin, only 8% of the ischemic zone infarcted, significantly less than that in untreated control hearts (33%; P<0.001) but comparable to that in ischemically preconditioned hearts (9%; P<0.001 versus control). Significant protection was also observed in the hearts treated only after the onset of ischemia (18% infarction; P<0.05 versus control). In isolated myocytes, fostriecin also provided protection comparable to that produced by metabolic preconditioning. Preconditioning had no apparent effect on the activity of either PP1 or PP2A in isolated ventricular myocytes or ventricular tissue obtained from heart biopsies. CONCLUSIONS: Fostriecin, a potent inhibitor of PP2A, can protect the rabbit heart from infarction even when administered after the onset of ischemia. But inhibition of either PP1 or PP2A does not appear to be the mechanism of protection from ischemic preconditioning.

Fostriecin-mediated G2-M-phase growth arrest correlates with abnormal centrosome replication, the formation of aberrant mitotic spindles, and the inhibition of serine/threonine protein phosphatase activity.

Fostriecin, a structurally unique phosphate ester, is presently under evaluation in clinical trials to determine its potential use as an antitumor drug in humans. Fostriecin has been reported as having inhibitory activity against DNA topoisomerase type II and protein phosphatases implicated in cell-cycle control. However, the relative contribution of these mechanisms to the antitumor activity of fostriecin has not yet been elucidated. In this study, after confirming that fostriecin is a potent inhibitor of serine/threonine protein phosphatase type 2A and a weak inhibitor of serine/threonine protein phosphatase type 1, we show that fostriecin inhibits approximately 50% of the divalent cation independent serine/threonine protein phosphatase (PPase) activity contained in whole cell homogenates of Chinese hamster ovary cells at concentrations associated with antitumor activity (1-20 microM). Investigations into the cellular effects produced by fostriecin treatment reveal that 1-20 microM fostriecin induces a dose-dependent arrest of cell growth during the G2-M phase of the cell cycle. Immunostaining of treated cells indicates that growth arrest occurs before the completion of mitosis and that fostriecin-induced growth arrest is associated with the aberrant amplification of centrosomes, which results in the formation of abnormal mitotic spindles. The "mitotic block" induced by fostriecin is reversible if treatment is discontinued in <24 h. However, after approximately 24-30 h of continuous treatment, growth arrest is not reversible, and treated cells die even when placed in fostriecin-free media. Correlative studies conducted with established PPase inhibitors reveal that, when applied at concentrations that inhibit PPase activity to a comparable extent, both okadaic acid and cantharidin also induce aberrant centrosome replication, the appearance of multiple aberrant mitotic spindles, and G2-M-phase growth arrest. These studies add additional support to the concept that PPase inhibition underlies the antitumor activity of fostriecin and suggest that other type-selective PPase inhibitors should be evaluated for potential antitumor activity.

Fostriecin, an antitumor antibiotic with inhibitory activity against serine/threonine protein phosphatases types 1 (PP1) and 2A (PP2A), is highly selective for PP2A.

Fostriecin, an antitumor antibiotic produced by Streptomyces pulveraceus, is a strong inhibitor of type 2A (PP2A; IC50 3.2 nM) and a weak inhibitor of type 1 (PP1; IC50 131 microM) serine/threonine protein phosphatases. Fostriecin has no apparent effect on the activity of PP2B, and dose-inhibition studies conducted with whole cell homogenates indicate that fostriecin also inhibits the native forms of PP1 and PP2A. Studies with recombinant PP1/PP2A chimeras indicate that okadaic acid and fostriecin have different binding sites.

Model development and analysis of tenidap-induced proteinuria in the rat.

Tenidap is a novel antirheumatic agent that causes a mild, reversible proteinuria in human clinical trials. In order to achieve a mechanistic understanding and safety perspective of the proteinuric effects of tenidap observed in clinical trials, female Sprague-Dawley rats were treated with up to 100 mg/kg/day of tenidap in the diet for 4 to 6 weeks followed by a 1- to 6-week reversal period. Pharmacokinetics and measurements of renal function and histology were assessed during the study. Sustained high plasma concentrations of tenidap [area under the plasma concentration curve (0-24 hr) of 941-1021 micrograms. hr/ml and peak plasma concentration of 61-67 micrograms/ml] increased urinary protein, albumin and phosphate excretion (2- to 8-fold) in rats. These renal effects were reversible within 9 days after removal of the drug. These effects preceded later occurring changes in renal morphology (papillary degeneration and necrosis). There was no evidence of glomerular damage, proximal tubule degeneration or necrosis or tubulointerstitial nephritis at the light microscopic level. Other indices of overall renal function (glomerular filtration rate, electrolyte and glucose excretion) were unaffected. Examination in situ of microperfused proximal tubules from treated rats revealed a 68% decrease in the rate of proximal tubule albumin absorption compared to controls (19 +/- 4 vs. 59 +/- 7 pg/min/mm, respectively). Fluid absorption rate and bicarbonate handling by the proximal tubule, along with blood bicarbonate concentrations, pH, PCO2 and PO2, were unaffected by treatment. It was concluded that tenidap caused a rapid, stable and reversible phosphaturia, microalbuminuria and proteinuria in the rat. The proteinuric effects were due to impaired proximal tubule albumin reabsorption that were not associated with other signs of impaired renal function or histological evidence of tubulointerstitial nephritis or proximal tubule/glomerular damage.

Skeletal lesions and deformities in large sharks.

Four blunt-snouted sandbar sharks (Carcharhinus plumbeus) were noted among 555 examined over a 13-yr period. Radiographs of one specimen revealed that the three rostral cartilage rods were abnormally short and failed to join at the anterior tip. The deformity appeared to be congenital. Four cases of vertebral lesions were noted in three species of shark (Carcharhinus plumbeus, Negaprion brevirostris and Odontaspis taurus). The vertebral columns had fused centra, ribs and neural arches, extra deposition and erosion of calcified material in the centra, and in one case, compression of centra. The causes of the vertebral lesions are unknown.