Opioids are non-competitive inhibitors of nitric oxide synthase in T47D human breast cancer cells

Opioids and nitric oxide (NO) interact functionally in different systems. N O-generating agents decrease the activity of opioid agonists, prevent opioi d tolerance, and are used in opioid withdrawal syndromes. There exist, howe ver, few reports indicating a direct interaction of the two systems. T47D h uman breast cancer cells in culture express opioid receptors, and opioid ag onists inhibit their growth, while they release high amounts of the NO-rela ted molecules NO2-/NO3- to the culture medium. We have used this system to assay a possible direct interaction of opiergic and nitric oxide systems. O ur results show that delta- or p-acting opioid agonists do not modify the r elease of NO2-/NO3-. In contrast, K-acting opioid agonists (ethylketocyclaz ocine, and alpha (S1)-casomorphine) decrease the release of NO2-/NO3-, in a time- and dose-dependent manner. The general opioid antagonist diprenorphi ne (10(-6) M) produce a similar NO2-/NO3- release inhibition, indicating a possible non-opioid-receptor mediated phenomenon. In addition, ethylketocyc lazocine, alpha (S1)-casomorphin and diprenorphine directly inhibit NOS act ivity: agonists, interact with both calcium-dependent and independent NOS-i soforms, while the antagonist diprenorphine modifies only the activity of t he calcium-dependent fraction of the enzyme. Analysis of this interaction r evealed that opioids modify the dimeric active form of NOS, through binding to the reductase part of the molecule, acting as non-competitive inhibitor s of the enzyme. This interaction opens interesting new possibilities for t umor biology and breast cancer therapy.