Cycloheximide protection against actinomycin D cytotoxicity.

Area of Science:

• Cellular and Molecular Biology
• Pharmacology
• Toxicology

Background:

• Actinomycin D is a cytotoxic agent that inhibits transcription.
• The precise mechanisms underlying actinomycin D-induced cytotoxicity are not fully understood.
• Cycloheximide is known to inhibit protein synthesis.

Purpose of the Study:

• To investigate the protective effects of cycloheximide against actinomycin D cytotoxicity in mammalian cells.
• To elucidate the molecular mechanisms by which cycloheximide confers protection.
• To determine the role of c-myc mRNA in actinomycin D-induced cell death.

Main Methods:

• Chinese hamster ovary (CHO) cells and Swiss 3T3 cells were treated with cycloheximide and/or actinomycin D.
• Intracellular concentrations of actinomycin D were measured.
• Levels of rRNA and various mRNA species, including c-myc mRNA, were quantified.
• Total transcription rates were assessed.

Main Results:

• Cycloheximide pretreatment and concomitant treatment protected cells from actinomycin D cytotoxicity.
• Cycloheximide reduced intracellular actinomycin D levels by decreasing its binding to the acid-precipitable fraction.
• Actinomycin D depleted cytoplasmic c-myc mRNA, while cycloheximide elevated and stabilized c-myc mRNA levels.
• Cycloheximide further inhibited transcription but preserved cytoplasmic rRNA and most mRNA levels.

Conclusions:

• Reduced cytoplasmic levels of short-lived essential mRNA, such as c-myc mRNA, contribute to actinomycin D lethality.
• Cycloheximide's protective mechanism involves stabilizing and elevating cytoplasmic c-myc mRNA, counteracting its depletion by actinomycin D.
• Cycloheximide also protects by reducing the amount of actinomycin D bound within cells.