Omega-3 long chain fatty acid synthesis is regulated more by substrate levels than gene expression.

The conversion of linoleic acid (LA) and alpha-linolenic acid (ALA) to long chain polyunsaturated fatty acids (LCPUFA) is known to involve desaturation and elongation steps. Although there is evidence that genes for these steps can be regulated by extremes of dietary PUFA, the degree to which there is meaningful regulation of LCPUFA levels in tissues by diet as a result of changes in expression of desaturase and elongase genes is unclear. In this study, we tested the effect of increasing ALA levels in diets of rats from 0.2% to 2.9% energy (en) against a constant LA level (1%en) on plasma and liver phospholipid LCPUFA content together with the expression of hepatic genes involved in PUFA metabolism, the desaturases FADS1 and FADS2, the elongases ELOV2 and ELOV5, and the transcription factors sterol regulatory element-binding protein-1c (SREBP-1c) and peroxisome proliferator-activated receptor alpha (PPARalpha). The levels of plasma and liver eicosapentaenoic acid (EPA) and docosapentaenoic acid (DPA) increased in proportion to dietary ALA whereas docosahexaenoic acid (DHA) increased only up to 1%en ALA. A low PUFA (0.4%en) reference diet stimulated the expression of delta 6 desaturase (FADS2) and elongase 2 (ELOVL2) when compared to higher PUFA diets. There was, however, no difference in the expression of any of the genes in rats, which were fed diets containing between 0.2%en and 2.9%en ALA and mRNA expression was unrelated to tissue/plasma LCPUFA content. These data suggest that the endogenous synthesis of n-3 LCPUFA from the precursor ALA is regulated independently of changes in the expression of the synthetic enzymes or regulatory transcription factor, and provides evidence that n-3 LCPUFA synthesis is regulated more by substrate competition for existing enzymes than by an increase in their mRNA expression.

Selective COX-2 inhibitors, eicosanoid synthesis and clinical outcomes: a case study of system failure.

Elucidation of differences between the active sites of COX-1 and COX-2 allowed the targeted design of the selective COX-2 inhibitors known as coxibs. They were marketed as non-steroidal anti-inflammatory drugs (NSAIDs) that had improved upper gastrointestinal (GI) safety compared with older non-selective NSAIDs such as diclofenac and naproxen. Two GI safety studies conducted with arthritis patients demonstrated that in terms of upper GI safety, celecoxib was not superior to diclofenac (CLASS study) but rofecoxib was superior to naproxen (VIGOR study). However, the VIGOR study revealed also that rofecoxib had increased cardiovascular (CV) risk compared with naproxen. This clinical outcome was supported by the existence of plausible eicosanoid-based biological mechanisms whereby selective COX-2 inhibition could increase CV risk. Nevertheless, the existence of CV risk with rofecoxib was successfully discounted by its pharmaceutical company owner, Merck & Co, with the assistance of specialist opinion leaders and rofecoxib achieved widespread clinical use for 4-5 years. Rofecoxib was withdrawn from the market when several clinical trials in colorectal cancer and post-operative pain revealed increased CV risk with not only rofecoxib, but also coxibs. The commercial success of rofecoxib provides a case-study of failure of the medical journal literature to guide drug usage. Attention to ethical issues may have provided a more useful guide for prescribers.

The nuclear origin of the non-phosphorylating NADH dehydrogenases of plant mitochondria.

The oxidation of matrix and cytosolic NADH by isolated beetroot and wheat leaf mitochondria was investigated to determine whether the rotenone-insensitive NADH dehydrogenases of plant mitochondria were the products of nuclear or mitochondrial genes. After aging beetroot tissue (slicing and incubating in a CaSO4 solution), the induction of the level of matrix NADH oxidation in the presence of rotenone was greatly reduced in mitochondria isolated from tissue treated with cycloheximide, a nuclear protein synthesis inhibitor. This was also true for the oxidation of cytosolic NADH. Mitochondria isolated from chloramphenicol-treated tissue exhibited greatly increased levels of both matrix and external rotenone-insensitive NADH oxidation when compared to the increase due to the aging process alone. This increase was not accompanied by an increase in matrix NAD-linked substrate dehydrogenases such as malic enzyme nor intra-mitochondrial NAD levels. Possible explanations for this increase in rotenone-insensitive NADH oxidation are discussed. Based on these results we have concluded that the matrix facing rotenone-insensitive NADH dehydrogenase of plant mitochondria is encoded by a nuclear gene and synthesis of the protein occurs in the cytosol.