Cytokinesis mediated through the recruitment of cortexillins into the cleavage furrow.

The fact that substrate-anchored Dictyostelium cells undergo cytokinesis in the absence of myosin II underscores the importance of other proteins in enabling the cleavage furrow to constrict. Cortexillins, a pair of actin-bundling proteins, are required for normal cleavage. They are targeted to the incipient furrow in wild-type and, more prominently, in myosin II-null cells. No other F-actin bundling or cross-linking protein tested is co-localized. Green fluorescent protein fusions show that the N-terminal actin-binding domain of cortexillin I is dispensable and the C-terminal region is sufficient for translocation to the furrow and the rescue of cytokinesis. Cortexillins are suggested to have a targeting signal for coupling to a myosin II-independent system that directs transport of membrane proteins to the cleavage furrow.     

Regulation of sterol 27-hydroxylase and an alternative pathway of bile acid biosynthesis in primary cultures of rat hepatocytes

In man, hepatic mitochondrial sterol 27-hydroxylase and microsomal cholesterol 7-hydroxylase initiate distinct pathways of bile acid biosynthesis from cholesterol, the “acidic” and “neutral” pathways, respectively. A similar acidic pathway in the rat has been hypothesized, but its quantitative importance and ability to be regulated at the level of sterol 27-hydroxylase are uncertain. In this study, we explored the molecular regulation of sterol 27-hydroxylase and the acidic pathway of bile acid biosynthesis in primary cultures of adult rat hepatocytes. mRNA and protein turnover rates were approximately 10-fold slower for sterol 27-hydroxylase than for cholesterol 7-hydroxylase. Sterol 27-hydroxylase mRNA was not spontaneously expressed in culture. The sole requirement for preserving sterol 27-hydroxylase mRNA at the level of freshly isolated hepatocytes (0 h) after 72 h was the addition of dexamethasone (0.1 μM; > 7-fold induction). Sterol 27-hydroxylase mRNA, mass and specific activity were not affected by thyroxine (1.0 μM), dibutyryl-cAMP (50 μM), nor squalestatin 1 (150 nM-1.0 μM), an inhibitor of cholesterol biosynthesis. Taurocholate (50 μM), however, repressed sterol 27-hydroxylase mRNA levels by 55%. Sterol 27-hydroxylase specific activity in isolated mitochondria was increased > 10-fold by the addition of 2-hydroxypropyl-β-cyclodextrin. Under culture conditions designed to maximally repress cholesterol 7-hydroxylase and bile acid synthesis from the neutral pathway but maintain sterol 27-hydroxylase mRNA and activity near 0 h levels, bile acid synthesis from [¹⁴C]cholesterol remained relatively high and consisted of β-muricholate, the product of chenodeoxycholate in the rat. We conclude that rat liver harbors a quantitatively important alternative pathway of bile acid biosynthesis and that its initiating enzyme, sterol 27-hydroxylase, may be slowly regulated by glucocorticoids and bile acids.     

The steroids of 2000-year-old human coprolites.

Six samples of human coprolites, some more than 2,000 years old, were analyzed for fecal steroid composition. Despite this very lengthy period of storage, the fecal steroids of coprolites were remarkably similar to those of stool samples collected today. The sterol nucleus was clearly rather stable under the dry environmental conditions of the Nevada Caves. The steroid content (microgram/g dried weight) of coprolite was low in comparison to that of modern man. The bile acid/cholesterol and plant sterol/cholesterol ratios of the coprolite, however, were similar to these ratios of the stools of modern man. In the six coprolites, an average 73% of the neutral steroids was digitonin-precipitable. This precipitate was composed of cholesterol and three plant sterols (campesterol, stigmasterol, and beta-sitosterol) and their bacteria-modified products. A portion of the neutral steroids had been converted to products tentatively identified as epimers of these steroids. Individual bile acids were identified in the coprolite. The bile acid composition of the coprolite was similar to that of the stool of modern man.