Alpha-tocopherol succinate increases cyclooxygenase-2 activity: Tissue-specific action in pregnant rat uterus in vitro

Abstract

Aims

Lipid soluble vitamin E plays a role in several physiological mechanisms, however, the mechanism of this action is controversial. We investigated how tocopherol (α-tocopherol acid succinate) influences the effects of cyclooxygenase inhibitors (COXi) in the smooth muscles.

Main methods

The contractility of the samples from 22-day-pregnant myometrium and non-pregnant myometrium and trachea was determined in an isolated organ bath in vitro. The activity of cyclooxygenase enzymes (COX) was also measured in the tissues.

Key findings

Diclofenac (10^− 9–10^− 5 M) and rofecoxib (10^− 10–10^− 5 M) decreased the contractions in non-pregnant and 22-day-pregnant uteri. Tocopherol (10^− 7 M) increased the relaxant effect only in pregnant uteri. Both diclofenac (10^− 9–10^− 5 M) and rofecoxib (10^− 10–10^− 5 M) reduced the tracheal tones, while they were slightly intensified by pretreatment with tocopherol (10^− 7 M). Tocopherol enhanced the contractions of pregnant uteri. Tocopherol (10^− 7 M) itself can induce the cyclooxygenase activity and shift the COX-1 and COX-2 ratio to COX-2. The lowest COX activity was found in non-pregnant uteri, while the highest one was in the trachea.

Significance

The COX enzymes, especially COX-2, play an important role in the contraction of pregnant uteri in rat. Tocopherol has a tissue specific COX-2 activity increasing effect in pregnant rat uterus but has no such action in non-pregnant uteri or tracheal tissue. Hereby, tocopherol may intensify selectively the uterine relaxing effect of COX-2 inhibitors in preterm contractions. However, tocopherol can enhance the contractile response of pregnant uterus that may increase the risk of premature contractions.

Introduction

Vitamin E is a lipid-soluble compound with high peroxyl radical scavenger ability. Vitamin E includes eight natural analogs, such as α-, β-, γ-, and δ-tocopherols and α-, β-, γ-, and δ-tocotrienols, however, their activity is quite various [1]. α-tocopherol is the most frequently used analog. Over the past thirty years, many studies have been published on the biological effects and functions of vitamin E, e.g. it is needed for reproduction [2] and may help to prevent Alzheimer's disease, non-alcoholic fatty liver diseases (NAFLD and NASH) of non-diabetic patients, atherosclerosis and certain types of cancer [3], [4], [5], [6], [7]. The mechanism of action of vitamin E has not yet been fully clarified. It can get across the cell membrane and can bind to many intracellular receptors and enzymes and may lead to modifying their functions [8]. For example, tocopherols can increase the activity of estrogen receptor β (ERβ) [9], inhibit activities of protein kinase C alpha [10], phospholipase A2 [11] and protein tyrosine kinases [12]. Moreover, vitamin E can also influence the action of other drugs by its antioxidant effect that shows tissue specificity. The tracheal tone-reducing effect of β₂-agonist terbutaline was decreased in female estrous rat by pretreatment with α-tocopherol, while in pregnant uterus the pretreatment was inefficient. This difference was explained by the various oxidative states of smooth muscles [13].

The interaction between tocopherols and cyclooxygenase enzymes (COX) was investigated in a few studies. Wu et al. [14] reported vitamin E inhibited the activity of cyclooxygenase enzymes in human aortic endothelial cells. Moreover, vitamin E can affect the different steps of the arachidonic acid cascade, but this effect may be diverse in tissues. According to literature, prostaglandin E2 (PGE₂) production was reduced in mouse [15], [16] and rat [17] macrophages by vitamin E. Analogs of tocopherols have different effects on COX. Alpha-tocopherol succinate inhibited more efficiently the LPS-stimulated PGE₂ production in macrophages and the COX activity in human lung epithelial cells than the other analogs [18], [19].

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit COX-1 or COX-2 enzymes, hereby decrease the liberation of prostaglandins (PG). The low level of PGE₂ induces the relaxation of the uterine smooth muscle, although the clinical use of NSAIDs for the prevention of preterm birth is limited because of the side effects [20]. Non-COX selective NSAIDs, especially acetyl-salicylic acid, are responsible for aspirin-induced asthma (AIA), while COX-2 selective compounds seem to be helpful not to exacerbate asthmatic symptoms [21].

We hypothesize that tocopherols modifies the COX activity and the effect of NSAIDs that may have significance in smooth muscle contractions. Accordingly, the aim of this study is to investigate how α-tocopherol acid succinate (tocopherol) influences the effects of non-selective and selective COX inhibitors (COXi) in the non-pregnant and pregnant uterine, and tracheal smooth muscle contractions in rats in vitro.