delta-9-Tetrahydrocannabinol: effect on macromolecular synthesis in human and other mammalian cells

The principal psychoactive component of marihuana is delta-9-tetrahydrocannabinol. This compound at 10(-5) molar concentration in the medium of human cell cultures appeared to inhibit DNA, RNA, and protein synthesis by 50, 40, and 30% respectively, as measured by incorporation of radioactive precursors into acid-insoluble cell fractions in human diploid fibroblasts, human neuroblastoma cells, and mouse neuroblastoma cells. While delta-9-tetrahydrocannabinol inhibited semiconservative DNA synthesis, it had no effect on DNA repair synthesis in human cells as assayed by the photolysis of 5-bromodeoxyuridine incorporation into DNA during repair after ultraviolet radiation damage. Delta-9-tetrahydrocannabinol also had no effect on rejoining of DNA single-strand breaks induced by gamma-rays. The nonspecificity of the inhibition of macromolecular synthesis by delta-9-THC suggested a possible interference with uptake of radioactive precursors. However, experimentation has shown that this depression of macromolecular synthesis cannot be accounted for by reduced transport of radioactive precursors into the cell because the rate of transport of these precursors into the cell is essentially the same in the presence or absence of delta-9-THC. Pool sizes of macromolecular precursors as measured radioisotopically (3H-thymidine, 3H-uridine, 14C-leucine) appear to be reduced about 50%, and this reduced pool size could fully account for the reduced macromolecular synthesis seen in the presence of delta-9-THC. We do not know what causes this apparent reduction of pool sizes in the presence of delta-9-THC.