Pushing the limits of the scanning mechanism for initiation of translation

Structure, function, and genetics of lipoprotein (a)

Transcription factor NF-E2-related factor 2 (Nrf2) regulates the induction of antioxidative proteins, including heme oxygenase-1 (HO-1). Nrf2 is sequestered in the cytoplasm by Keap1 under unstimul...

Atypical protein kinase C mediates activation of NF-E2-related factor 2 in response to oxidative stress

Phospholipase D (PLD) has emerged as a regulator of several critical aspects of cell physiology. PLD, which catalyzes the hydrolysis of phosphatidylcholine (PC) to phosphatidic acid (PA) and choline, is activated in response to stimulators of vesicle transport, endocytosis, exocytosis, cell migration, and mitosis. Dysregulation of these cell biological processes occurs in the development of a variety of human tumors. It has now been observed that there are abnormalities in PLD expression and activity in many human cancers. In this review, evidence is summarized implicating PLD as a critical regulator of cell proliferation, survival signaling, cell transformation, and tumor progression.

https://mcr.aacrjournals.org/content/molcanres/1/11/789.full.pdf

Phospholipase D in cell proliferation and cancer.

It has been estimated that ∼37% of the US population judged to be at high risk for developing coronary artery disease (CAD), based on the National Cholesterol Education Program guidelines, have increased plasma lipoprotein(a) [Lp(a)], whereas Lp(a) is increased in only 14% of those judged to be at low risk. Therefore, the importance of establishing a better understanding of the relative contribution of Lp(a) to the risk burden for CAD and other forms of vascular disease, as well as the underlying mechanisms, is clearly evident. However, the structural complexity and size heterogeneity of Lp(a) have hindered the development of immunoassays to accurately measure Lp(a) concentrations in plasma. The large intermethod variation in Lp(a) values has made it difficult to compare data from different clinical studies and to achieve a uniform interpretation of clinical data. A workshop was recently convened by the National Heart, Lung, and Blood Institute (NHLBI) to evaluate our current understanding of Lp(a) as a risk factor for atherosclerotic disorders; to determine how future studies could be designed to more clearly define the extent to which, and mechanisms by which, Lp(a) participates in these processes; and to present the results of the NHLBI-supported program for the evaluation and standardization of Lp(a) immunoassays. This report includes the most recent data presented by the workshop participants and the resulting practical and research recommendations.

Report of the National Heart, Lung, and Blood Institute Workshop on Lipoprotein(a) and Cardiovascular Disease: Recent Advances and Future Directions

Lipoprotein(a) [Lp(a)] is a quantitative genetic trait in human plasma associated with atherothrombotic disease. The major determinant of Lp(a) concentration is the apolipoprotein(a) [apo(a)] gene locus. Variation in the number of kringle IV repeats (K-IV VNTR) in apo(a) has a direct effect on Lp(a) concentrations but explains only a fraction of the large intra- and inter-population variance in Lp(a) levels. Effects on Lp(a) of other intragenic polymorphisms including a pentanucleotide repeat (PNRP) in the promoter likely reflect allelic associations with as yet unidentified sequence variation in the apo(a) gene. We have studied a candidate C-->T transition in two European and two African populations. This polymorphism in the 5' region of the apo(a) gene creates an ATG start codon thereby reducing apo(a) translation in vitro by 60%. All samples were also analyzed for the K-IV VNTR and the PNRP to stratify for their effects and to consider allelic associations. Consistent with the in vitro effect the C-->T transition was associated with a significant reduction in Lp(a) levels in both African populations ( P < 0.0056). In Caucasians, however, the effect was not significant. This was explained by linkage disequilibrium of the +93 T with apo(a) alleles of intermediate length (K-24-K-34) and with nine PNRs. In Europeans these alleles are associated with low Lp(a) which makes any potential effect of the +93 T undetectable in the total sample. From our results we conclude (i) that the +93 C/T polymorphism is the second known intragenic apo(a) polymorphism which affects Lp(a) levels directly in vivo ; (ii) that allelic associations may mask the effect of a mutation; and (iii) that heterogeneity of an effect of a mutation across populations does not disprove causality.

Significant impact of the +93 C/T polymorphism in the apolipoprotein(a) gene on Lp(a) concentrations in Africans but not in Caucasians: confounding effect of linkage disequilibrium.

The activation of phospholipase D (PLD) by transforming Ras is well documented. Although two distinct PLD isoforms, PLD1 and PLD2, have been cloned from mammalian cells, it has remained unclear whether both isoenzymes are activated by Ras and, if this is the case, whether they are stimulated by a common mechanism. In the present study we show that expression of transforming Ras in HC11 mouse mammary epithelial cells enhanced the activity of endogenous PLD. Co-expression of Ras with either PLD1b or PLD2 resulted in elevated activities of both PLD isoenzymes in HC11 cells, indicating that transforming Ras was capable of activating both PLD isoforms in vivo. Ras-induced activation of PLD was resistant to the protein kinase C (PKC) inhibitor GF109203X, which preferentially affects conventional- and novel-type PKCs, but sensitive to Ro-31-8220, which inhibits atypical PKCs more effectively. Co-transfection of atypical PKC-iota with either PLD1b or PLD2 led to a selective activation of PLD2 by PKC-iota, whereas PLD1b was not affected. PLD1b, however, was found to be a potent activator of PKC-iota, whereas PLD2 was less effective in this respect. The data suggest that PKC-iota acts upstream of PLD2 and that PLD1b is implicated in the activation of PKC-iota. The data are discussed as indicating a putative signalling cascade comprising Ras-->PLD1b-->PKC-iota-->PLD2. Evidence for the implication of this pathway in the transcriptional regulation of cyclin D1 is also presented.

Michaela Windegger

Papers

Significant impact of the +93 C/T polymorphism in the apolipoprotein(a) gene on Lp(a) concentrations in Africans but not in Caucasians: confounding effect of linkage disequilibrium.

Regulation of phospholipase D isoenzymes by transforming Ras and atypical protein kinase C-iota.