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PubMedDate not supplied

MDA468 growth inhibition by EGF is associated with the induction of the cyclin-dependent kinase inhibitor p21WAF1.

Epidermal growth factor (EGF) usually stimulates the proliferation of a variety of normal and malignant cells. In contrast, MDA468, a human breast cancer cell line with a very high number of EGF receptors, is growth inhibited in response to concentrations of EGF that stimulate most other cells. The purpose of this study was to elucidate the cellular mechanisms involved in EGF-induced growth inhibition. EGF treatment stimulated the sustained expression of the cyclin-dependent kinase (CDK) inhibitor p21WAF1. The p21WAF1 induction in EGF-treated MDA468 cells is probably p53-independent since these cells contain no active p53. The promoter for p21WAF1 gene contains binding sites for signal transducer and activator of transcription (STAT) and EGF is known to activate members of this family of transcription factors. Using electrophoretic mobility shift assays (EMSA), we found that EGF activates STAT1 and STAT3 in the MDA468 cells. These activated STATs specifically recognized the three conserved STAT-responsive elements in the p21WAF1 gene promoter, suggesting that STATs may be responsible for the p21WAF1 induction by EGF in MDA468 cells. The sustained rise in p21WAF1 in response to EGF is proposed to be a means of growth inhibition in these cells.

Breast Neoplasms↗

PubMedDate not supplied

Streptozotocin, an analog of N-acetylglucosamine, blocks the removal of O-GlcNAc from intracellular proteins.

Streptozotocin (STZ), an analog of N-acetylglucosamine (GlcNAc), is a specific toxin for the pancreatic beta cell. We found that treatment of rats with STZ results in an early beta-cell-specific increase in the level of intracellular protein modification by O-linked GlcNAc (O-GlcNAc). Using a model O-GlcNAc peptide based on the transcription factor Sp1, we show that treatment of cultured cells with STZ during peptide biosynthesis results in hyperglycosylation of the peptide as a result of the ability of STZ to specifically inhibit the activity of O-GlcNAc-selective N-acetyl-beta-D-glucosaminidase. Although this inhibitory activity of STZ probably can occur in all cells, we found, using in situ hybridization, that beta cells express very high levels of the mRNA encoding the enzyme responsible for cytoplasmic protein O-glycosylation, O-GlcNAc transferase (OGT). These findings suggest that the pancreatic beta cell is particularly sensitive to the toxicity of STZ because it expresses such high levels of OGT. When STZ blocks O-GlcNAc removal from intracellular proteins, the cell with the most rapid on-rate for O-GlcNAc, the beta cell, will experience the most rapid accumulation of this protein modification. Because we also show that the on-rate of O-GlcNAc is substrate driven in several cell types, we speculate that the beta cell, with its high level of OGT, may also respond to elevations of blood sugar with increased protein modification by O-GlcNAc. Thus, this proposed mechanism of STZ toxicity on the beta cell may result from an exaggeration of a heretofore unrecognized physiological response to glucose mediated through the high level of OGT in these cells.

Acetylglucosamine↗