Nuclear receptor agonists stimulate release of arachidonic acid from rat liver cells

Proteasome inhibitors: their effects on arachidonic acid release from cells in culture and arachidonic acid metabolism in rat liver cells

Background

I have postulated that arachidonic acid release from rat liver cells is associated with cancer chemoprevention. Since it has been reported that inhibition of proteasome activities may prevent cancer, the effects of proteasome inhibitors on arachidonic acid release from cells and on prostaglandin I₂ production in rat liver cells were studied.

Results

The proteasome inhibitors, epoxomicin, lactacystin and carbobenzoxy-leucyl-leucyl-leucinal, stimulate the release of arachidonic acid from rat glial, human colon carcinoma, human breast carcinoma and the rat liver cells. They also stimulate basal and induced prostacycin production in the rat liver cells. The stimulated arachidonic acid release and basal prostaglandin I₂ production in rat liver cells is inhibited by actinomycin D.

Conclusions

Stimulation of arachidonic acid release and arachidonic acid metabolism may be associated with some of the biologic effects observed after proteasome inhibition, e.g. prevention of tumor growth, induction of apoptosis, stimulation of bone formation.

Thiazolidinediones increase arachidonic acid release and subsequent prostanoid production in a peroxisome proliferator-activated receptor γ-independent manner

Thiazolidinedione, peroxisome proliferator-activated receptor gamma (PPARgamma) agonist, has been used as an anti-diabetic drug and as an useful tool to elucidate multiple PPARgamma functions by in vitro and in vivo studies. We investigated the effects of thiazolidinediones on prostanoid production in lipopolysaccharide-stimulated cells. The high concentrations (>10 microM) of rosiglitazone and pioglitazone significantly increased lipopolysaccharide-stimulated prostanoid production such as thromboxane A2 and prostaglandin E2. However, PPARgamma antagonist could not inhibit them. In PPARgamma-deficient cells, thiazolidinediones increased prostaglandin E2 production. Thiazolidinediones increased arachidonic acid (AA) release from the cell membrane by not stimulating AA releasing process involving several phospholipase A2s but inhibiting AA reuptaking process. The expression of cyclooxygenase-1 and cyclooxygenase-2 were not affected by thiazolidinediones. In this study, we demonstrated that high concentrations of TZDs increased AA release by the inhibition of AA reuptaking process, leading to subsequent increase in the prostanoid production in a PPARgamma-independent manner. This mechanism provides useful information for the elucidation of multiple PPARgamma functions and diabetic drug therapy.

Tamoxifen stimulates arachidonic acid release from rat liver cells by an estrogen receptor-independent, non-genomic mechanism

Background

Tamoxifen is widely prescribed for the treatment of breast cancer. Its success has been attributed to the modulation of the estrogen receptor. I have previously proposed that the release of arachidonic acid from cells may also mediate cancer prevention.

Methods

Rat liver cells were radiolabelled with arachidonic acid. The release of [³H] arachidonic acid after various times of incubation of the cells with tamoxifen was measured.

Results

Tamoxifen, at micromolar concentrations, stimulates arachidonic acid release. The stimulation is rapid and is not affected by pre-incubation of the cells with actinomycin or the estrogen antagonist ICI-182,780.

Conclusions

The stimulation of AA release by tamoxifen is not mediated by estrogen receptor occupancy and is non-genomic.

Does the release of arachidonic acid from cells play a role in cancer chemoprevention?

COX-2 is overexpressed in cancer cells and has become a major target for cancer preventive drugs. NSAIDs, retinoids, antioxidants, and PPAR agonists, reported to be chemopreventive, suppress COX-2 synthesis. NSAIDs also have been shown to be chemopreventive independent of COX-2 activity. Common to all of these compounds is their ability to release arachidonic acid (AA) from rat liver cells in culture. Most of these compounds inhibit induced PGI2 production. Vitamin D3 and tamoxifen, however, not only stimulate the release of AA from cells: they amplify rather than inhibits induced COX activity. In view of the many activities attributable to AA, I propose that its release and accumulation could initiate molecular reactions that lead to apoptosis and eventually to suppression of cancer. Some drugs shown to release AA from cells and affect PGI2 production-e.g., thiazolidinediones and statins are widely used for conditions unrelated to cancer. In vivo studies could reveal whether they can also function as cancer preventive agents.

Statins stimulate arachidonic acid release and prostaglandin I2 production in rat liver cells

Statins inhibit 3-hydroxy-3-methylglutaryl (HMG-CoA) reductase, the rate limiting step in cholesterol synthesis. They are, therefore, used clinically to lower cholesterol and prevent atherosclerosis. Statins have beneficial effects on multiple organ systems. Some of these effects are found in the absence of significant changes in cholesterol levels. Polyunsaturated fatty acids also inhibit HMG-CoA reductase and have many of the same beneficial effects of statins. Four statins (mevastatin, lovastatin, simvastatin and atorvastatin) have been tested in rat liver cells for their effect on arachidonic acid (AA) release and prostaglandin I2 production induced in the presence of lactacystin and 12-O-tetradecanoylphorbol-13-acetate. Each statin stimulated release of AA and induced prostaglandin I2 production. Mevalonate, the product of HMG-CoA reductase, did not reduce the stimulation observed in the presence of simvastatin indicating that HMG-CoA reductase activity is not involved. In view of the multiple biologic properties of AA, the AA released as a result of the action of the statins may play a role in some of the pharmacological effects attributed to these drugs.