Faculty Bibliography

In hepatocytes, vitamin E is secreted via the efflux pathway and is believed to associate with apolipoprotein B (apoB)-lipoproteins extracellularly. The molecular mechanisms involved in the uptake, intracellular trafficking, and secretion of dietary vitamin E by the intestinal cells are unknown. We observed that low concentrations of Tween-40 were better for the solubilization and delivery of vitamin E to differentiated Caco-2 cells, whereas high concentrations of Tween-40 and sera inhibited this uptake. Vitamin E uptake was initially rapid and then reached saturation. Subcellular localization revealed that vitamin E primarily accumulated in microsomal membranes. Oleic acid (OA) treatment, which induces chylomicron assembly and secretion, decreased microsomal membrane-bound vitamin E in a time-dependent manner. To study secretion, differentiated Caco-2 cells were pulse-labeled with vitamin E and chased in the presence and absence of OA. In the absence of OA, vitamin E was associated with intestinal high density lipoprotein (I-HDL), whereas OA-treated cells secreted vitamin E with I-HDL and chylomicrons. No extracellular transfer of vitamin E between these lipoproteins was observed. Glyburide, an antagonist of ABCA1, partially inhibited its secretion with I-HDL, whereas plasma HDL increased vitamin E efflux. An antagonist of microsomal triglyceride transfer protein, brefeldin A, and monensin specifically inhibited vitamin E secretion with chylomicrons. These studies indicate that vitamin E taken up by Caco-2 cells is stored in the microsomal membranes and secreted with chylomicrons and I-HDL. Transport via I-HDL might contribute to vitamin E absorption in patients with abetalipoproteinemia receiving large oral doses of the vitamin

PURPOSE OF REVIEW/OBJECTIVE:The assembly of intestinal lipoproteins is critical for the transport of fat and fat-soluble vitamins. In this review we propose a nomenclature for these lipoproteins and have summarized recent data about their intracellular assembly and factors that modulate their secretion. RECENT FINDINGS/RESULTS:The assembly and secretion of intestinal lipoproteins increases with the augmented synthesis of apoB, apoAIV and lipids. Chylomicron assembly begins with the formation of primordial, phospholipid-rich particles in the membrane, and their conversion to large chylomicrons occurs in the lumen of the smooth endoplasmic reticulum. Chylomicrons are transported from the endoplasmic reticulum via specialized vesicles to the Golgi for secretion. The identification of genetic mutations in chylomicron retention disease indicates that Sar1b may play a critical role in this process. In addition to chylomicron assembly, intestinal cells have been shown to transport dietary cholesterol via apoB-independent pathways, such as efflux. SUMMARY/CONCLUSIONS:Understanding the mechanisms involved in the intracellular transport of chylomicrons and chylomicron-independent secretion pathways are expected to be the next frontiers in the field of intestinal lipoprotein assembly and secretion.

Angiogenesis governs the progression of multiple myeloma (MM). Circulating endothelial cells (CECs) contribute to angiogenesis and comprise mature ECs and endothelial progenitor cells (EPCs). The present study sought to characterize CECs and their relation to disease activity and therapeutic response in 31 consecutive patients with MM. CECs, identified as CD34(+)/CD146(+)/CD105(+)/CD11b(-) cells, were 6-fold higher in patients compared to controls and correlated positively with serum M protein and beta(2)-microglobulin. Circulating EPCs displayed late colony formation/outgrowth and capillary-like network formation on matrigel; these processes were inhibited after effective thalidomide treatment. Co-expression of vascular endothelial growth factor receptor-2 (KDR) and CD133 characterized EPCs in MM, and KDR mRNA elevations correlated with M protein levels. In vitro exposure of ECs to thalidomide or its derivative CC-5013 inhibited gene expression of the receptors for transforming growth factor-beta and thrombin. Thus, elevated levels of CECs and EPCs covary with disease activity and response to thalidomide, underscoring the angiogenic aspect of MM and suggesting that angioblastlike EPCs are a pathogenic biomarker and a rational treatment target in MM. The results also highlight the anti-angiogenic properties of thalidomide and CC-5013 and further elucidate possible mechanisms of their effectiveness against MM. (Blood. 2005;105:3286-3294).

Fatty acids of varying lengths and saturation differentially affect plasma apolipoprotein B-100 (apoB-100) levels. To identify mechanisms at the level of production, rat hepatoma cells, McA-RH7777, were incubated with [(35)S]methionine and either fatty acid-BSA complexes or BSA alone. There were increases in labeled apoB-100 secretion with saturated fatty acids palmitic and myristic (MA) (153 +/- 20% and 165 +/- 11%, respectively, relative to BSA). Incubation with polyunsaturated docosahexaenoic acid (DHA) decreased secretion to 26 +/- 2.0%, while monounsaturated oleic acid (OA) did not change it. In pulse-chase studies, MA treatment resulted in reduced apoB-100 degradation, in agreement with its promotion of secretion. In triglyceride (TG) studies, synthesis was stimulated equally by OA, MA, and DHA, but TG secretion was relatively decreased with MA and DHA. With OA, the majority of newly secreted apoB100-lipoproteins was d < or = 1.006, but with MA, they were much denser (1.063 < d). Furthermore, the relative recruitment of newly synthesized TG to lipoproteins was impaired with MA. We conclude that mechanisms for effects of specific dietary fatty acids on plasma lipoprotein levels may include changes in hepatic production. In turn, hepatic production may be regulated by specific fatty acids at the steps of apoB-100 degradation and the recruitment of nascent TG to lipoprotein particles