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

Abstract

Opioids and nitric oxide (NO) interact functionally in different systems. NO-generating agents decrease the activity of opioid agonists, prevent opioid tolerance, and are used in opioid withdrawal syndromes. There exist, however, few reports indicating a direct interaction of the two systems. T47D human breast cancer cells in culture express opioid receptors, and opioid agonists inhibit their growth, while they release high amounts of the NO-related molecules NO2-/NO3- to the culture medium. We have used this system to assay a possible direct interaction of opiergic and nitric oxide systems. Our results show that δ-or μ-acting opioid agonists do not modify the release of NO2-/NO3-. In contrast, κ-acting opioid agonists (ethylketocyclazocine, and αs1-casomorphine) decrease the release of NO2-/NO3-, in a time- and dose-dependent manner. The general opioid antagonist diprenorphine (10-6 M) produce a similar NO2-/NO3- release inhibition, indicating a possible non-opioid-receptor mediated phenomenon. In addition, ethylketocyclazocine, αs1-casomorphin and diprenorphine directly inhibit NOS activity: agonists, interact with both calcium-dependent and independent NOS-isoforms, while the antagonist diprenorphine modifies only the activity of the calcium-dependent fraction of the enzyme. Analysis of this interaction revealed that opioids modify the dimeric active form of NOS, through binding to the reductase part of the molecule, acting as non-competitive inhibitors of the enzyme. This interaction opens interesting new possibilities for tumor biology and breast cancer therapy.