fumonisin-b1 and Precursor-Cell-Lymphoblastic-Leukemia-Lymphoma

Recent studies are beginning to implicate sphingolipids in the heat stress response. In the yeast Saccharomyces cerevisiae, heat stress has been shown to activate de novo biosynthesis of sphingolipids, whereas in mammalian cells the sphingolipid ceramide has been implicated in the heat shock responses. In the current study, we found an increase in the ceramide mass of Molt-4 cells in response to heat shock, corroborating findings in HL-60 cells. Increased ceramide was determined to be from de novo biosynthesis by two major lines of evidence. First, the accumulation of ceramide was dependent upon the activities of both ceramide synthase and serine palmitoyltransferase. Second, pulse labeling studies demonstrated increased production of ceramide through the de novo biosynthetic pathway. Significantly, the de novo sphingolipid biosynthetic pathway was acutely induced upon heat shock, which resulted in a 2-fold increased flux in newly made ceramides within 1-2 min of exposure to 42.5 degrees C. Functionally, heat shock induced the dephosphorylation of the SR proteins, and this effect was demonstrated to be dependent upon the accumulation of de novo-produced ceramides. Thus, these studies disclose an evolutionary conserved activation of the de novo pathway in response to heat shock. Moreover, SR dephosphorylation is emerging as a specific downstream target of accumulation of newly made ceramides in mammalian cells.

Topics: Biological Transport; Cell Survival; Ceramides; Fatty Acids, Monounsaturated; Fumonisins; HL-60 Cells; Hot Temperature; Humans; Kinetics; Microsomes; Mycotoxins; Nuclear Proteins; Palmitic Acid; Phosphoproteins; Precursor Cell Lymphoblastic Leukemia-Lymphoma; RNA-Binding Proteins; Saccharomyces cerevisiae; Serine-Arginine Splicing Factors; Sphingolipids; Tumor Cells, Cultured

Sphingosine has been shown to inhibit cell growth in many cell lines although the mechanism of this effect remains obscure. More recently, D-erythro-sphingosine has been shown to act as an early inducer of dephosphorylation of the retinoblastoma gene product (pRb) in the lymphoblastic leukemia cell line MOLT-4 [Chao, R., Khan, W., & Hannun, Y.A. (1992) J. Biol. Chem., 267, 23459-23462]. In the current study, the role of the natural D-erythro-sphingosine in regulation of cell growth and pRb dephosphorylation was evaluated using chemically synthesized pure isomers of sphingosine. Of the four possible stereoisomers of sphingosine, D-erythro-sphingosine was most active in inducing dephosphorylation of pRb protein with an EC50% of 0.6 microM whereas its enantiomer L-erythro-sphingosine was 8-fold less potent with an EC50% of 5 microM. The dose responses for inhibition of cell growth were nearly identical to the EC50% for pRb dephosphorylation with D-erythro-sphingosine causing 50% inhibition at 0.6 microM whereas L-erythro-sphingosine was 5-6-fold less potent. All of the stereoisomers were taken up by the cells, and the greater potency of D-erythro-sphingosine was not due to differences in cellular uptake. The metabolism of D-erythro-sphingosine was also studied to evaluate the possible role of sphingosine metabolites on regulation of retinoblastoma protein. Evidence is provided against a role for ceramide or sphingosine 1-phosphate as mediators of the effects of sphingosine on pRb dephosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Cell Death; Cell Division; Drug Synergism; Fumonisins; Kinetics; Lysophospholipids; Mycotoxins; Phosphorylation; Precursor Cell Lymphoblastic Leukemia-Lymphoma; Retinoblastoma Protein; Sphingosine; Stereoisomerism; Tumor Cells, Cultured