Capacity of insect (Manduca sexta) prothoracic glands to secrete ecdysteroids: relation to glandular growth

Expressions of sugar transporters/trehalases in relation to PTTH-stimulated ecdysteroidogenesis in the silkworm, Bombyx mori

The prothoracic gland (PG) is the source of ecdysteoids in larval insects. Although numerous studies have been conducted on signaling networks involved in prothoracicotropic hormone (PTTH)-stimulated ecdysteroidogenesis in PGs, less is known about regulation of metabolism in PGs. In the present study, we investigated correlations between expressions of sugar transporter (St)/trehalase (Treh) genes and PTTH-stimulated ecdysteroidogenesis in Bombyx mori PGs. Our results showed that in vitro PTTH treatment stimulated expression of the St1 gene, but not other transporter genes. Expression of the Treh1 gene was also stimulated by PTTH treatment. An immunoblotting analysis showed that St1 protein levels in Bombyx PGs increased during the later stage of the last larval instar and were not affect by PTTH treatment. PTTH treatment enhanced Treh enzyme activity in a time-dependent manner. Blocking either extracellular signal-regulated kinase (ERK) signaling with U0126 or phosphatidylinositol 3-kinase (PI3K) signaling with LY294002 decreased PTTH-stimulated Treh enzyme activity, indicating a link from the ERK and PI3K signaling pathways to Treh activity. Treatment with the Treh inhibitor, validamycin A, blocked PTTH-stimulated Treh enzyme activity and partially inhibited PTTH-stimulated ecdysteroidogenesis. Treatment with either a sugar transport inhibitor (cytochalasin B) or a specific glycolysis inhibitor (2-deoxy-D-glucose, 2-DG) partially inhibited PTTH-stimulated ecdysteroidogenesis. Taken together, these results indicate that increased expressions of St1/Treh1 and Treh activity, which lie downstream of PTTH signaling, are involved in PTTH stimulation in B. mori PGs.

Nutritional sensitivity of fifth instar prothoracic glands in the tobacco hornworm, Manduca sexta

Insulin-regulated growth of the prothoracic glands appears to play a critical role in timing the last larval molt, and hence metamorphosis. The present study examined insulin signaling in relation to the growth and secretory activity of prothoracic glands in the tobacco hornworm, Manduca sexta. As larvae feed during the first half of the final larval stage, the prothoracic glands grow and ecdysone secretory capacity increases. During this period of growth, we verified the presence of insulin receptor transcript in the prothoracic glands and demonstrated that the glands were responsive to insulin, as evidenced by the in vitro phosphorylation of signaling proteins in the insulin pathway such as Akt/protein kinase B and FOXO. It was predicted that starvation would reduce ecdysone secretion with concomitant changes in insulin signaling. To test this prediction, larvae were starved and changes were quantified in two nutritionally sensitive transcripts, insulin receptor and the translation inhibitor 4EBP. In glands from starved larvae, growth and ecdysone secretory capacity were reduced, and insulin receptor and 4EBP transcripts were increased. The latter changes would be expected to accompany starvation in conjunction with enhanced insulin sensitivity and reduced protein synthesis. Increased transcription of insulin receptor and 4EBP strongly suggest that nutritional deprivation reduces the secretion of endogenous insulin-like hormones. When injected with insulin, 4EBP levels in the prothoracic glands of starved larvae decreased. Thus, insulin appeared to correct starvation-induced deficits in glandular protein synthesis. However, insulin injection did not enhance ecdysone secretion. Thus, although the prothoracic glands are insulin-responsive and insulin-like hormones may promote glandular growth as larvae feed, the effects of nutritional depletion on steroidogenesis in Manduca cannot be explained solely by reduced insulin.

In vitro and in vivo stimulation of extracellular signal-regulated kinase (ERK) by the prothoracicotropic hormone in prothoracic gland cells and its developmental regulation in the silkworm, Bombyx mori

In this study, we investigated activation of the extracellular signal-regulated kinase (ERK) by the prothoracicotropic hormone (PTTH) in prothoracic gland cells of the silkworm, Bombyx mori. The results showed that the PTTH stimulated ERK phosphorylation as this depends on time and dose and ecdysteroidogenic activity. The ERK phosphorylation inhibitors, PD 98059 and U0126, blocked both basal and PTTH-stimulated ERK phosphorylation and ecdysteroidogenesis. In addition, activation of glandular ERK phosphorylation by the PTTH appeared to be developmentally regulated with the refractoriness of gland cells to the PTTH occurring during the latter stages of both the fourth and last larval instars. Moreover, in vitro activation of ERK phosphorylation of prothoracic glands by the PTTH was also verified by in vivo experiments: injection of the PTTH into day 6 last instar larvae greatly increased the activity of glandular ERK phosphorylation and ecdysteroidogenesis. These results suggest that development-specific changes in ERK phosphorylation may play a role in PTTH stimulation of ecdysteroidogenesis.

Stage-dependent effects of starvation on the growth, metamorphosis, and ecdysteroidogenesis by the prothoracic glands during the last larval instar of the silkworm, Bombyx mori

The stage-dependent effects of starvation on the growth, metamorphosis, and ecdysteroidogenesis of the prothoracic glands during the last larval instar of the silkworm, Bombyx mori, were studied in the present study. When last instar larvae were starved beginning on day 1 of that instar, all larvae died between days 5 and 7 of the instar. Although the prothoracicotropic hormone (PTTH) release from the brain-corpus cardiacum-corpus allatum (BR-CC-CA) did not significantly change during starvation, a deficiency in PTTH signal transduction was maintained, which led to very low levels of hemolymph ecdysteroids after the beginning of starvation. However, when starvation began on day 3 of the last larval instar, the major hemolymph ecdysteroid peak, preceding larval-pupal transformation, occurred 1 day earlier than that in control larvae. Protein content of the prothoracic glands in day 3-starved larvae was maintained at a low level as compared to that of control larvae. The secretory activity of the prothoracic glands in day 3-starved larvae was maintained at a level similar to that of control larvae. However, the rate of ecdysteroidogenesis, expressed per microgram of glandular protein, was greatly enhanced in these starved larvae, indicating that upon starvation, larvae increased the ecdysteroid production rate to enhance the rate of survival.

Autocrine activation of DNA synthesis in prothoracic gland cells of the silkworm, Bombyx mori

Autocrine activation of DNA synthesis in prothoracic gland cells in last instar larvae of the silkworm, Bombyx mori, was studied using both a long-term in vitro organ culture system and immunocytochemical labeling with 5-bromo-2'-deoxyuridine (BrdU). When prothoracic glands were incubated in a small volume of culture medium (10 microl/gland), the numbers of DNA-synthesizing cells per gland increased significantly, and DNA synthesis was stimulated less by hemolymph, as compared with glands incubated in a large volume (50 microl/gland). Moreover, glands cultured in groups (6 glands per group in a 50-microl drop) also resulted in much higher levels of DNA synthesis than those cultured individually in a 50-microl drop. The mechanism by which alternation of the volume of the incubation medium results in changes in the levels of DNA synthesis was further examined. When prothoracic glands were incubated in medium (50-microl drop per gland) that was preconditioned with glands (in a 10-microl drop individually), a dramatic increase in DNA synthesis activity was also observed, indicating that prothoracic glands may release a factor that stimulates their own DNA synthesis. The growth-promoting factor was further characterized and it was found that the factor is heat stable, and its molecular weight was estimated to be between 1,000 and 3,000 Da. Moreover, the factor also stimulated corpus allatum cell DNA synthesis in vitro. Injection of concentrated putative growth-promoting factor into day 4 last instar-ligated larvae greatly increased cell DNA synthesis of the prothoracic glands, indicating the in vivo function of the present autocrine factor.

Temporal changes in DNA synthesis of prothoracic gland cells during larval development and their correlation with ecdysteroidogenic activity in the silkworm,Bombyx mori

DNA synthesis in prothoracic gland cells of the silkworm, Bombyx mori, was studied immunocytochemically after in vivo labeling with 5-bromo-2'-deoxyuridine (BrdU), and its developmental changes during the 3rd, 4th, and last larval instars were examined. During the early stages of both the 3rd and 4th larval instars, a dramatic increase in the number of DNA-synthesizing cells of the prothoracic glands was detected. However, during the latter stages of each instar, the number of DNA-synthesizing cells greatly decreased. The determination of glandular protein content showed that dramatic increases occurred during the latter stages of each larval instar. Comparison of changes in prothoracic gland cell DNA synthesis with ecdysteroidogenic activity showed that the increase in DNA synthesis precedes ecdysteroidogenesis. The cellular mechanism underlying changes in prothoracic gland cell DNA synthesis during the last two larval instars was further analyzed by determining the in vitro DNA synthesis of the glands, their responsiveness to hemolymph growth factors, and changes in the growth-promoting activity of hemolymph during development. It was found that both growth factors and the responsiveness of the prothoracic gland cells to growth factors from hemolymph may play roles in regulating DNA synthesis of gland cells.

Dietary influences over proliferating cell nuclear antigen expression in the locust midgut

We have studied the influence of variations in dietary protein (P) and digestible carbohydrate (C), the quantity of food eaten, and insect age during the fifth instar on the expression of the proliferating cell nuclear antigen(PCNA) in the epithelial cells of the midgut (with special reference to the midgut caeca) in the African migratory locust, Locusta migratoria. Densitometric analysis of PCNA-immunostained cells was used as an indirect measure of the levels of expression of PCNA, and a PCNA cellular index(PCNA-I) was obtained. Measurements of the DNA content of the cells have also been carried out by means of microdensitometry of Feulgen-stained, thick sections of midgut. A comparison between the PCNA nuclear level and the DNA content was performed. The PCNA levels were significantly different among the cells of the five regions studied: caeca, anterior ventricle, medial ventricle, posterior ventricle and ampullae of the Malpighian tubules. We have studied in more detail the region with highest PCNA-I, i.e. the caeca. The quality and the quantity of food eaten under ad libitum conditions were highly correlated with both the PCNA and DNA levels in the caeca cells. Locusts fed a diet with a close to optimal P:C content (P 21%, C 21%) showed the highest PCNA and DNA content. In locusts fed a food that also contained a 1:1 ratio of P to C but was diluted three-fold by addition of indigestible cellulose (P 7%, C 7%), a compensatory increase in consumption was critical to maintaining PCNA levels. Our measurements also showed that the nuclear DNA content of the mature and differentiated epithelial cells was several-fold higher than the levels in the undifferentiated stem cells of the regenerative nests. These results, combined with the low number of mitotic figures found in the regenerative nests of the caeca and the marked variation in PCNA levels among groups, suggest that some type of DNA endoreduplication process may be taking place. Our data also indicate that the DNA synthetic activity in the midgut is related to feeding in locusts. The possible dietary and nutritional regulatory mechanisms and the significance of the differences found are discussed.

PTTH-stimulated ecdysone secretion is dependent upon tyrosine phosphorylation in the prothoracic glands of Manduca sexta

PTTH stimulates ecdysteroid secretion by the insect prothoracic glands. The peptide activates cAMP synthesis in a calcium-dependent manner, ultimately enhancing ecdysteroid synthesis. We have found that PTTH stimulates a rapid increase in tyrosine phosphorylation of at least four proteins in the prothoracic glands of larval Manduca sexta, as seen on Western blots of glandular lysates probed with antibody directed against phosphotyrosine. PTTH-stimulated tyrosine phosphorylation is blocked by an inhibitor of Src family tyrosine kinases, 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]-pyrimidine (PP1). The inhibitor also blocks PTTH-stimulated ecdysone secretion, as well as PTTH-stimulated cAMP synthesis. Direct activation of the catalytic subunit of adenylyl cyclase by forskolin is not affected by PP1. In addition, ecdysteroid secretion stimulated by the cAMP analog dbcAMP is not blocked by PP1. These findings point to an important role for a Src-family tyrosine kinase at a very early step in the PTTH signaling pathway, prior to the activation of adenylyl cyclase.

Induction of DNA synthesis by 20-hydroxyecdysone in the prothoracic gland cells of the silkworm Bombyx mori during the last larval instar

DNA synthesis in the prothoracic gland cells of the silkworm Bombyx mori was studied immunocytochemically after in vivo labeling with 5-bromo-2'-deoxyuridine (BrdU), and its developmental changes during the last larval instar were examined. During the first 3 days of the last larval instar, no DNA-synthesizing cells were detected. On day 4, the number of DNA-synthesizing cells increased and peaked on day 7. When larvae were fed 20-hydroxyecdysone-supplemented mulberry leaves throughout the last larval instar, the number of DNA-synthesizing cells dramatically increased 2 days after 20-hydroxyecdysone treatment and reached a high level on day 5. The mechanism by which 20-hydroxyecdysone treatment induces DNA synthesis of prothoracic gland cells was further examined by using continuous in vitro BrdU labeling for a 2-day incubation. We conclude that 20-hydroxyecdysone may exert its growth-promoting action indirectly.

Activation of an extracellular signal-regulated kinase (ERK) by the insect prothoracicotropic hormone

Ecdysteroid hormones are crucial in controlling the growth, molting and metamorphosis of insects. The predominant source of ecdysteroids in pre-adult insects is the prothoracic gland, which is under the acute control of the neuropeptide hormone prothoracicotropic hormone (PTTH). Previous studies using the tobacco hornworm, Manduca sexta, have shown that PTTH stimulates ecdysteroid synthesis via a series of events, including the activation of protein kinase A and the 70 kDa S6 kinase (p70(S6k)). In this study, PTTH was shown to stimulate also mitogen-activated protein kinase (MAPK) phosphorylation and activity in the Manduca prothoracic gland. The MAPK involved appears to be an extracellular signal-regulated kinase (ERK) homologue. The ERK phosphorylation inhibitors PD 98059 and UO 126 blocked basal and PTTH-stimulated ERK phosphorylation and ecdysteroid synthesis. PTTH-stimulated ERK activity may be important for both rapid regulation of ecdysteroid synthesis and for longer-term changes in the size and function of prothoracic gland cells.

Insulin receptor-like tyrosine kinase in the tobacco hornworm, Manduca sexta

Phosphotyrosine-containing proteins are present in the prothoracic glands, muscle, and fat body of Manduca sexta, as determined by immunoprecipitation followed by kinase assay, and by Western blotting. One such protein (M(r) 178,000) can also be immunoprecipitated using antibodies directed against the human insulin receptor and insulin receptor substrate. The 178 kD protein appears to be expressed more strongly in prothoracic glands removed just prior to wandering (days 3-4) and prior to pupation (days 7-9), and phosphorylation of the protein is enhanced by an M. sexta brain factor. The results suggest that a tyrosine-kinase-linked molecule similar to the insulin receptor may play a regulatory role in M. sexta.