Action of insect hormones at the cellular level. Morphological changes of a diploid cell line of Drosophila melanogaster, treated with ecdysone and several analogues in vitro

A critical period of ecdysterone action on sensitive clones of Drosophila cultured in vitro: the maturation of the cells

Ecdysterone-sensitive clones cultured in vitro were isolated from established cell lines of Drosophila melanogaster. The clones FC and 89K are ecdysterone-inducible for two enzymatic activities: acetylcholinesterase and beta-galactosidase. No activity could be detected in untreated cells, whereas after treatment with 50-250 nM ecdysterone, the activity appeared after one day and increased during 3-4 days. We wanted to modulate the response of the cells by varying the conditions of the hormonal stimulus. Mimicking the physiological situation of Drosophila (the ecdysterone peak corresponding to the molts is preceded by low levels) we pretreated the cells with a subthreshold concentration (1-5 nM) for 2 days and then we added the stimulating concentration of 50-250 nM ecdysterone. The enzymatic activities were then detectable within the following hours and the final level of induction was about twice the one of cells without pretreatment. Thus, the continuous presence of a subthreshold concentration of ecdysterone provokes the maturation of the cells which become able to respond to the hormonal stimulus by a quicker and higher enzymatic induction. The cellular maturation seems to be a critical period. It is altosid-sensitive. Altosid (a juvenile hormone analog) abolishes the effects of the ecdysterone-induced maturation.

Acetylcholinesterase induction by beta-ecdysone in Drosophila cell lines and their hybrids

Drosophila S3 line cells maintain a high basal level of acetylcholinesterase (AChE) activity, which is lost and then reinduced following exposure to the steroid, beta-ecdysone. Electrophoretic studies indicate that the basal and reinduced AChE activities may be different isozymes. In contrast, MDR line cells have a low basal AChE activity which is induced to a modest level by hormone. In S3/MDR hybrids the high basal level phenotype is extinguished, and the hormone-induced AChE level is modest. MDER line cells, which continue to grow in the presence of beta-ecdysone and are therefore termed resistant, show a low basal AChE level that is not further elevated by hormone. This phenotype and response also occurred in beta-ecdysone-resistant S3/MDER hybrids.

Tubulin content and synthesis in differentiating Drosophila cells in culture

Drosophila Kc cells exposed to physiological doses of the moulting hormone, beta-ecdysone, elongate, become motile, and subsequently aggregate. This pattern of morphogenesis was found to require the assembly of a microtubular cytoskeleton. Tubulin content was significantly increased in hormone-treated cells when compared to controls, as measured by a 3H-colchicine-binding assay. However, determinations of rates of tubulin synthesis and breakdown revealed no difference between control and hormone-treated cells for either parameter. When tubulin content was assayed by methods that do not depend on colchicine-binding activity, no difference between hormone-treated and control cells was observed. These results are discussed in terms of a model in which beta-ecdysone affects the distribution of tubulin in "assembly-active" and "assembly-inactive" pools.

The uptake and release of ponasterone A by the Kc cell line of Drosophila melanogaster

The association of ponasterone A (PNA) and 20-hydroxyecdysone with Kc cells is commensurate with their biological activity on this Kc cell line, the physiological activity ratio for PNA, 20-hydroxyecdysone and ecdysone is 1 : 50 : 2000, resp. Both association and release of [3H]-PNA are temperature-dependent, the activation energy was calculated as 16.7 cal (Arrhenius analysis). This association is compatible with unlabelled PNA and various ecdysteroids. The KD for PNA (Scatchard analysis) was estimated as 3.6 x 10(-9) M, giving the number of binding sites as approx. 1800 per cell.

Induction of acetylcholinesterase activity by beta-ecdysone in a Drosophila cell line

When cells of the Drosophila Kc-H line are treated with larger than or equal to 10(-8) molar beta-ecdysone, they extend long processes and acquire acetylcholinesterase activity. Thus, this permanent line, derived originally from embryol cultures, may be composed of cells having some neural or glial characteristics.

Juvenile hormone analogue counteracts growth stimulation and inhibition by ecdysones in clonal Drosophila cell line

Depending on concentration, ecdysones either stimulate or inhibit proliferation of a clonal Drosophila cell line. Both effects are counteracted by ethyl dichlorofarnesoate, a juvenile hormone analogue, which by itself is growth inhibitory. Qualitatively no difference was seen between alpha- and beta-ecdysone.

Small circular DNA in Drosophila melanogaster

Covalently closed small circular DNA isolated from Drosophila melanogaster is described. The small circular DNA is found in blastema stage eggs and in Schneider's cell culture line 2 and a cloned subline of line 2. It is heterogeneous in size, although the size distributions and mean sizes differ for each source. The small circular DNA from Schneider's line 2 cells ranges from 0.09-7.3 mum, with a mean contour length of 1.1 mum. This DNA has a buoyant density of 1.703 g/cc and appears to be present predominantly in the nuclear fraction of detergent-disrupted cells. the restriction enzyme EcoRl cleaves approximately 40% of the small circular DNA with a bias toward the larger size classes. Both logarithmic and stationary phase cells contain approximately 3-40 average sized small circular DNA molecules per cell, representing a maximum of 0.03% of the total cellular DNA. Exposure to cycloheximide or puromycin for 14 hr results in a 30 fold increase in the number of small circles per cell, but reduces the mean length of the circular DNA to 0.3 mum. The drug-amplified DNA has a buoyant density in the range of 1.698-1.703 g/cc. No amplification was seen in cells treated with either inhibitor for 3.5 hr. Ethidium bromide, cytosine arabinoside, beta-ecdysone, and insulin all had no significant effect on the amount per cell of either small circular DNA or mitochondrial DNA.

Influence of physical factors on the growth of insect cells in vitro. II. Sodium and potassium as osmotic pressure regulators of moth cell growth

The tolerance of a cell line (IMC-HZ-1) from a moth, Heliothis zea, for the monovalent cations Na+ and K+ were defined. Cells shifted to media containing more than 70 mM of K+ showed decreased growth rates. No evidence was obtained for Na+ toxicity. The osmotic pressure tolerances were influenced by the K+ concentration of the medium. The richer the medium was in K+, the narrower was the spectrum of osmotic pressure tolerance. Once the limit of K+ tolerance was exceeded, the rate of decline of growth was linear with respect to further increases in K+. This rate of decline was independent of osmotic pressure. The initial responses of cells during one subculture (2 to 4 population doublings) in media differing from the standard medium (used to maintain the cell line) were not reliable indicators of the growth potential of the cells. Continued subculture in such media resulted in an upward trend in population growth rates in most cases.