Demonstration of a direct effect of inhibitors of the degradation of receptor-bound human choriogonadotropin on the steroidogenic pathway

Is calcium or cyclic AMP involved in the inhibitory effect on pituitary hormone secretion of the tripeptide aldehyde proteinase inhibitors?

The mechanism by which tripeptide aldehyde proteinase inhibitors decrease prolactin (PRL) and growth hormone (GH) secretion was studied. Agents known to modify the intracellular levels of cyclic adenosine monophosphate (cAMP) or cytosolic free calcium were used in monolayer cultures of the rat anterior pituitary gland. The phosphodiesterase inhibitor isobutyl-methylxanthine (IBMX), 8-bromo-cAMP and forskolin all stimulated PRL release. Boc-D-Phe-Pro-arginal (Boc-DPPA) at 1 mmol/l concentration was a potent inhibitor of basal PRL release and significantly decreased the effect of 8-Br-cAMP, forskolin or IBMX (0.5 mmol/l). Forskolin (1 mumol/l) stimulated ACTH, PRL and GH release and all these effects were decreased by 100 mumol/l of Boc-D-Phe-Phe-lysinal (Boc-DPPL). Neither tripeptide aldehyde affected the forskolin-induced rise in intracellular cAMP. Growth hormone releasing factor (hpGRF, 1 nmol/l) stimulated both GH release and intracellular cAMP generation; Boc-DPPL (100 mumol/l) significantly decreased stimulated GH release without affecting cAMP accumulation. Increasing medium K+ to 10 times normal level stimulated PRL release presumably by enhancing Ca2+ entry into the cells and 1 mmol/l Boc-DPPA decreased high potassium-stimulated PRL release. The ionophore A-23187 stimulated PRL release at 10 mumol/l but not at 1 mumol/l. At 1 mumol/l A-23187 prevented the Boc-DPPA-induced inhibition of PRL release. These findings suggest that the tripeptide aldehyde proteinase inhibitors inhibit PRL and GH release at a site beyond cAMP formation.

Inhibition of growth hormone and prolactin secretion by a serine proteinase inhibitor

The action of the tripeptide aldehyde t-butyloxycarbonyl-DPhe-Pro-Arg-H (boc-fPR-H), belonging to a family of serine proteinase inhibitors, on the release of immunoreactive prolactin (iPRL) and growth hormone (iGH) has been studied. In rat anterior pituitary cell cultures and pituitary quarters 1 mM boc-fPR-H inhibited basal iPRL and iGH release. Thyroliberin-induced iPRL release by cultured cells was also markedly inhibited with a concomitant accumulation of intra-cellular iPRL. During the short- and long-term exposure of cells to boc-fPR-H there no changes in total cell protein contents and in activities of some lysosomal marker enzymes. A wide scale of unchanged parameters characteristic for cellular metabolism indicated that the tripeptide aldehyde has no cytotoxic effect. The marked inhibition of basal as well as stimulated hormone release in the presence of the enzyme inhibitor might suggest that at least a portion of the hormones is released via a proteolytic enzyme-dependent process.

Liposome-mediated delivery of deoxyribonucleic acid to cells: enhanced efficiency of delivery related to lipid composition and incubation conditions

Delivery of liposome-encapsulated simian virus 40 (SV40) DNA to African green monkey Related to been used as a probe to study liposome--cell interactions and to determine conditions which favor the intracellular delivery of liposome contents to cells. The efficiency of DNA delivery by various liposome preparations (monitored by infectivity assays) was found to be dependent both on the magnitude of vesicle binding to cells and on the resistance of liposomes to cell-induced leakage of contents. Acidic phospholipids were much more effective in both binding and delivery, and phosphatidylserine (PS) was the best in both aspects. The inclusion of 50 mol % cholesterol in liposomes reduces the cell-induced leakage of vesicle contents (2--5-fold) and substantially enhances the delivery of DNA to cells (2--10-fold). Following incubation of cells with negatively charged liposomes containing SV40 DNA, infectivity can be enhanced greatly by brief exposure of the cells to glycerol solutions. In contrast, only slight enhancement by glycerol was observed for SV40 DNA encapsulated in neutral or positively charged liposomes. The results of competition experiments between empty phosphatidylcholine liposomes and DNA-containing PS liposomes also suggest possible differences in the interaction of neutral and negatively charged liposome preparations with cells. Morphological studies indicate that the glycerol treatment stimulates membrane ruffling and vacuolization and suggest that the enhanced uptake of liposomes occurs by an endocytosis-like process. Results obtained with metabolic inhibitors are also consistent with the interpretation that the enhancement of liposome delivery in glycerol-treated cells occurs via an energy-dependent endocytotic pathway. Pretreatment of cells with chloroquine, a drug which alters lysosomal activity, further enhanced infectivity in glycerol-treated cells (4-fold). This observation suggests the involvement of a lysosomal processing step at some point in the expression of liposome-encapsulated DNA and, more importantly, illustrates the possibility of altering cellular mechanism to engineer more efficient delivery by liposomes. Under optimal conditions determined in this study, the efficiency of liposome-mediated SV40 DNA delivery was increased more than 1000-fold over that obtained by simply incubating cells with liposomes. It is also demonstrated that these conditions enhance delivery of other molecules, besides DNA, which are encapsulated in liposomes.

Receptor-mediated gonadotropin action in the ovary. Demonstration of acute dependence of rat luteal cells on exogenously supplied steroid precursor (sterols) for gonadotropin-induced steroidogenesis

Incubation of luteal cells with human, horse and rat sera, but not bovine sera resulted in enhanced basal and hCG-stimulated progesterone accumulation. The stimulatory effect of human or rat sera on basal, hCG- or 8 Br-cyclic AMP-induced progesterone synthesis in luteal cells was evident within 15-30 min after incubation, reaching a maximum after 3-4 h. The stimulatory effects of hCG and/or sera were blocked by inhibitors of RNA and protein synthesis. Similarly, lysosomotropic agents, chloroquine (100 microM) and ammonium chloride (10 mM), partly blocked the steroidogenic response of luteal cells to hCG and/or human or rat sera. Incubation of cells in the presence of 2-deoxyglucose, sodium azide and phenylmethylsulfonyl fluoride resulted in partial inhibition of progesterone secretion in response to hCG or sera. Fractionation of human or rat sera into various lipoprotein fractions demonstrated that LDL and HDL most effectively supported and potentiated the steroidogenic response to hCG. Lipoprotein-deficient serum, however, did not alter gonadotropin-induced steroid production. Incubation of luteal cells with increasing concentrations of h-LDL and h-HDL enhanced both basal and hCG-mediated steroidogenesis in a dose-related manner, although very high concentrations of these lipoproteins were inhibitory. Further, [3H]cholesterol from [3H]cholesteryl linoleate-LDL was incorporated into luteal cell progesterone and the extent of this incorporation was enhanced by hCG. Addition of excess unlabeled h-LDL, h-HDL, as well as r-HDL, drastically reduced the incorporation of radioactive label into progesterone. These studies suggest that (a) serum potentiation of steroidogenesis was due to presence of lipoproteins, mainly LDL and HDL, and (b) the lipoprotein-bound cholesterol is delivered into the luteal cells and utilized for steroidogenesis.

Characterization of the desensitized state of Leydig tumor cells

A perifusion system has been used to study the in vitro desensitization of isolated Leydig tumor cells. It was observed that the cells become refractory, as measured by decreased rates of steroidogenesis, during continuous perifusions with saturating concentrations of either human choriogonadotropin (CG), cholera toxin, or 8-bromo-cyclic AMP. Furthermore, an initial perifusion of the cells with either human CG, cholera toxin, or 8-bromo-cyclic AMP causes subsequent desensitization towards all three stimuli. Thus, each of these stimuli is equally effective in inducing a state of desensitization in these cells that is manifested by a steroidogenic lesion(s) distal to cyclic AMP formation. It was found that the post-cyclic AMP lesion(s) in the desensitized state occurs prior to the formation of pregnenolone. However, the decreased rates of steroidogenesis do not seem to arise from a depletion of intracellular cholesterol.

Modulation of steroidogenic activities in testis Leydig cells

Recent developments in the regulation of testicular Leydig-cell function support the following conclusions. (1) Enzymic activities involved in steroid production in the testis are mainly localized in Leydig cells. The aromatase enzyme complex for oestrogen production appears to be localized in Leydig cells as well as in Sertoli cells. (2) LH- (or hCG-) induced alterations of Leydig cells depend on dose and duration of exposure of the cell to the hormone. Locally produced oestradiol is probably involved in the inhibition of steroidogenesis. (3) The stimulatory action of LH on Leydig cells involves different proteins in concert with the activation of the cleavage activity of mitochondrial cholesterol side-chain. However, most of the functional properties of these proteins are yet unknown. (4) Different populations of Leydig cells are present in the testis. These different cell populations can be characterized by quantitatively and qualitatively different responses to hormones.

Effects of lysomotropic agents, and of microfilament- and microtubule-disrupting drugs on the activation of casein-gene expression by prolactin in the mammary gland

The organ-culture technique was used to investigate the effects of lysomotropic agents (NH4Cl and chloroquine) and of modifiers of microfilaments (cytochalasin B) and microtubules (colchicine) on the induction of casein synthesis and the accumulation of casein mRNA by prolactin in the rabbit mammary gland. Neither chloroquine nor NH4Cl altered the lactogenic action of prolactin. Cytochalasin B attenuated the response to prolactin in terms of casein synthesis. However, this drug did not hamper the accumulation of casein mRNA. Colchicine exhibited a marked specific inhibitory effect on the induction of casein synthesis. It also prevented the accumulation of casein mRNA. These results suggest that a putative degradation of the internalized prolactin--receptor complex by lysosomes is not strictly involved in prolactin action. In addition, the integrity of the microfilaments seems unnecessary in the process of casein-gene activation by prolactin. By contrast, the integrity of the microtubule network seems absolutely necessary to ensure the transmission of prolactin information to the nucleus.