Stimulation of cytoplasmic actin gene transcription and translation in cultured Drosophila cells by ecdysterone

Apolipoprotein E acts to increase nitric oxide production in macrophages by stimulating arginine transport

Previous studies have shown that apolipoprotein E (apoE) plays a role in immune function by modulating tissue redox balance. Using a mouse macrophage cell line (RAW 264.7), we have examined the mechanism by which apoE regulates nitric oxide (NO) production in macrophages. ApoE potentiates NO production in immune activated RAW cells in combination with lipopolysaccharide or polyinosinic:polycytidylic acid (PIC), agents known to induce expression of inducible nitric oxide synthase mRNA and protein. The effect is not observed with apolipoprotein B or heat-inactivated apoE. The combination of PIC plus apoE produced more NO than the level expected from an additive effect of PIC and apoE alone. Furthermore, this increase was observed at submaximal extracellular arginine concentrations, suggesting that apoE altered arginine (substrate) availability. Examination of [(3)H]arginine uptake across the cell membrane demonstrated that arginine uptake was increased by PIC but further increased by PIC plus apoE. Treatment of RAW cells with apoE was associated with an increased apparent V(max) and decreased affinity for arginine as well as a switch in the induction of mRNA for subtypes of cationic amino acid transporters (CAT). Treatment of RAW cells with PIC plus apoE resulted in the loss of detectable CAT1 mRNA and expression of CAT2 mRNA. Regulation of arginine availability is a novel action of apoE on the regulation of macrophage function and the immune response.

Transcript length heterogeneity at the small heat shock protein genes of Drosophila

Expression of the small heat shock protein (hsp) genes can be induced in cultured Drosophila cells by high temperature shock and by exposure to physiological doses of the insect molting hormone ecdysterone. Northern blot analysis was performed in order to compare the size of small hsp transcripts synthesized in response to these two stimuli. Transcripts from several other genes were also examined. Two types of length heterogeneity were observed for the small hsp gene transcripts. One involved the synthesis of what are designated as long form transcripts during heat shock; small hsp messenger RNAs extended at the 3' end by some 1.5 X 10(3) base-pairs. The second type of size heterogeneity observed is based on differences in the length of the poly(A) tail. The results of S1 nuclease protection analysis provided evidence that different initiation sites are not used for hsp 22 mRNA transcription in response to the two stimuli.

Evidence for the existence of an actin-derived protein in ataxia-telangiectasia lymphoblastoid cell lines

In a number of lymphoblastoid cell lines from individuals with the genetic disease ataxia-telangiectasia (A-T), a decrease in the levels of actin and a concomitant increase in the levels of a protein species of 37K has been observed to occur following high cell density. In this paper we describe ataxia-telangiectasia lymphoblastoid lines that show this response constitutively, and demonstrate that this 37K protein is most likely an actin-derived peptide. This is because of extensive similarity between the two proteins in the spectrum of peptides produced by proteolytic fingerprinting. It appears likely that this 37K protein is similar to the well documented protease-resistant actin fragments produced by protease digestion of actin in vitro. Examination of a number of possible initiators found no clear signal that promoted this response, although the response is clearly very dependent on cell culture conditions. The possible involvement of a cytoskeletal anomaly in ataxia-telangiectasia is discussed.

Ecdysteroids accelerate mitoses in accessory glands of beetle pupae

During the 9-day pupal period of Tenebrio molitor (the mealworm beetle), the cells of the male accessory glands undergo divisions for 7 days. There are two maxima in the mitotic activity in the glands in vivo, one at 1 day and the other at 4 days. The latter peak coincides with the large surge of ecdysterone occurring in the pupal stage. By the use of in vitro culture techniques, it has been demonstrated that the first bout of mitosis in both glands proceeds in basal medium, while the second bout of mitosis requires a physiological level of ecdysterone. Ecdysone was less effective than ecdysterone. Sensitivity to ecdysterone did not change significantly between Day 1 and Day 4 of the pupal stage. The results are discussed in relation to the effects of ecdysterone on cell division in mesodermal and ectodermal derivatives.

Drosophila cells and ecdysterone: a model system for gene regulation

When Drosophila cell lines are exposed to physiological doses of the steroid molting hormone, ecdysterone, they enter mitotic arrest and differentiate morphologically. These responses are accompanied by specific changes in gene expression. Several enzyme activities (acetylcholinesterase, beta-galactosidase, dopa decarboxylase, and catalase) are induced and the synthesis of a cytoplasmic actin and the four small heat-shock proteins is initiated. Several of these ecdysterone inducible genes have been physically isolated and characterized, in several cases by DNA sequencing. Current studies focus on introducing cloned ecdysterone inducible genes into responsive cells by DNA mediated transfection. Once it is clear that these introduced genes acquire the normal pattern of hormone-regulated gene expression in the cell, in vitro mutagenesis can be used before transfection to modify their structure. Transient expression, then, can become a functional assay to define regions of DNA flanking the coding region of inducible genes that are needed for proper gene expression and regulation in cultured cells.

Steroid and high-temperature induction of the small heat-shock protein genes in Drosophila

Transcription of the four small heat-shock protein genes of Drosophila melanogaster can be induced in cultured cells by high-temperature shock, or by physiological doses of the moulting hormone, ecdysterone. We have characterized and compared the two induction events, focusing on hsp22 and hsp23, in terms of rates of heat-shock protein synthesis, transcription rate, messenger RNA abundance and mRNA half-life. The results indicate that relative to hsp22, the rate of hsp23 synthesis is significantly greater during recovery from heat shock and during ecdysterone induction. This difference is not due to differences in transcription rate, but rather reflects differences in mRNA stability and translational efficiency. One intriguing finding is that hsp message stability is temperature-dependent; hsp transcripts are two to three times more stable at 35 degrees C than at 25 degrees C. The possible mechanism and significance of this phenomenon are discussed.