Corticotropin-induced desensitization: steroidogenic and cyclic AMP responses in superfused adrenocortical cells

Invited review: Adrenocortical function in avian and non-avian reptiles: Insights from dispersed adrenocortical cells

Herein we review our work involving dispersed adrenocortical cells from several lizard species: the Eastern Fence Lizard (Sceloporus undulatus), Yarrow's Spiny Lizard (Sceloporus jarrovii), Striped Plateau Lizard (Sceloporus virgatus) and the Yucatán Banded Gecko (Coleonyx elegans). Early work demonstrated changes in steroidogenic function of adrenocortical cells derived from adult S. undulatus associated with seasonal interactions with sex. However, new information suggests that both sexes operate within the same steroidogenic budget over season. The observed sex effect was further explored in orchiectomized and ovariectomized lizards, some supported with exogenous testosterone. Overall, a suppressive effect of testosterone was evident, especially in cells from C. elegans. Life stage added to this complex picture of adrenal steroidogenic function. This was evident when sexually mature and immature Sceloporus lizards were subjected to a nutritional stressor, cricket restriction/deprivation. There were divergent patterns of corticosterone, aldosterone, and progesterone responses and associated sensitivities of each to corticotropin (ACTH). Finally, we provide strong evidence that there are multiple, labile subpopulations of adrenocortical cells. We conclude that the rapid (days) remodeling of adrenocortical steroidogenic function through fluctuating cell subpopulations drives the circulating corticosteroid profile of Sceloporus lizard species. Interestingly, progesterone and aldosterone may be more important with corticosterone serving as essential supportive background. In the wild, the flux in adrenocortical cell subpopulations may be adversely susceptible to climate-change related disruptions in food sources and to xenobiotic/endocrine-disrupting chemicals. We urge further studies using native lizard species as bioindicators of local pollutants and as models to examine the broader eco-exposome.

Tripartite control of dynamic ACTH-cortisol dose responsiveness by age, body mass index, and gender in 111 healthy adults

BACKGROUND

Recent analyses in small cohorts suggest that pituitary hormones exert time-varying (viz., initial and delayed) dynamic dose-responsive effects on target glands, wherein down-regulating dynamics are inferable on a time scale of single pulses.

HYPOTHESIS

Age, body mass index (BMI), and sex modulate the rapid potency-down-regulating dynamics of pulsatile pituitary ACTH-adrenal cortisol coupling overnight.

LOCATION

The study was conducted at a clinical translational research unit.

SUBJECTS

Subjects included healthy adults (48 women, 63 men; aged 18-77 yr; BMI 18-42 kg/m(2)).

OUTCOMES

Outcomes included analytical dose-response estimates of endogenous ACTH efficacy, dynamic ACTH potency, and adrenal sensitivity from overnight 10-min ACTH-cortisol profiles.

RESULTS

Stepwise backward-elimination, multivariate-regression analysis revealed that in the combined cohorts (n = 111), age was associated with enhanced initial ACTH potency (R = 0.265, P = 0.005). Moreover, age and BMI jointly attenuated adrenal sensitivity (R = 0.334, P = 0.0017) and augmented down-regulated ACTH potency (R = 0.321 and P = 0.0028). Exploratory gender-segmented analyses showed that these outcomes might be explained by: (1) a negative effect of age in men on adrenal sensitivity (R = 0.270, P = 0.034) and (2) positive effects of age in men (R = 0.332, P = 0.0019) and BMI in women (R = 0.331, P = 0.024) on initial ACTH potency.

CONCLUSIONS

In healthy adults, adrenal sensitivity to endogenous ACTH pulses, ACTH efficacy, and ACTH potency is associated with age, BMI, and gender. These findings may explain conflicting data in earlier literature and introduce the need to control all three of age, BMI, and sex in future studies of the stress-adaptive axis.

Dose-response downregulation within the span of single interpulse intervals

Pituitary ACTH drives adrenal glucocorticoid (cortisol) pulses via a time-delayed asymptotic dose-response process. To test the postulate that ACTH stimulates cortisol secretion dynamically (unequally during the initiation and termination of a cortisol secretory burst), a mathematical formalism was developed in which dose-response hysteretic shifts were allowed, but not required, within the time evolution of ACTH-cortisol pulse pairs. A dual-waveform deconvolution model was used to quantify cortisol secretion rates and reconstruct ACTH concentration profiles in 28 healthy adults previously sampled every 10 min for 24 h in the unstressed state (8,120 measurements). ACTH concentration-cortisol secretion dose-response functions were then estimated in each subject 1) without hysteresis (base model) and with allowances for possible hysteresis in 2) ACTH potency, 3) adrenal sensitivity, and 4) ACTH efficacy. Model residual error was 40% lower in the potency and sensitivity models and 20% lower in the efficacy model than in the base model (P < 0.001). Mean time shifts for inferable hysteretic inflection were model-independent, i.e., grand mean (95% confidence interval) 22 (12-39) min after the onset of a cortisol secretory burst. Half-maximally effective ACTH concentrations (EC(50)) differed before and after hysteretic inflection within individual pulses: 1) 9.4 and 54 ng/l in the potency model (P < 0.001) and 2) 8.9 and 123 ng/l in the sensitivity model (P < 0.001) compared with 16 ng/l in the no-hysteresis model (P < 0.001). In the efficacy-shift model, estimated maximal ACTH drive varied by 17-fold within cortisol secretory bursts (from 22 to 1.3 nmol.l(-1).min cortisol secretion(-1), P < 0.001). The collective results introduce the basis for modeling the dynamics of rapid, reversible physiological downregulation within the span of single interpulse intervals in vivo. This construct should have utility in parsing mechanisms of physiological regulation in other integrative systems.

Endogenous ACTH concentration-dependent drive of pulsatile cortisol secretion in the human

According to current regulatory concepts, pulsatile ACTH concentrations (CON) stimulate time-lagged cortisol secretion rates (SEC) via an implicit CON-SEC dose-response relationship. The present analyses reconstruct nonlinear properties of this in vivo agonist-response interface noninvasively in order to investigate pulse-by-pulse coupling consistency and to obviate the need to infuse isotopes or exogenous effectors, which may disrupt pathway interactions. This approach required an ensemble strategy of 1) measuring ACTH and cortisol CON in plasma sampled every 10 min for 24 h in 32 healthy adults, and 2) estimating simultaneously a) variable-waveform ACTH and cortisol SEC bursts superimposed upon fixed basal SEC; b) biexponential kinetics of ACTH and cortisol disappearance; c) nonequilibrium exchange of cortisol among free and cortisol-binding globulin (CBG)- and albumin-bound moieties; d) two SEC-burst shapes demarcated by a statistically defined day/night boundary; e) feedforward efficacy, potency, and sensitivity; and f) stochastic variability in feedforward measures over time. Thereby, we estimate 1) ACTH SEC (microg.l(-1).day(-1)) of 0.27 +/- 0.04 basal and 0.87 +/- 0.07 pulsatile (means +/- SE); 2) cortisol SEC (micromol.l(-1).day(-1)) of 0.10 +/- 0.01 basal and 3.5 +/- 0.20 pulsatile; 3) free cortisol half-lives (min) of 1.8 +/- 0.20 (diffusion/advection) and 4.1 +/- 0.30 (elimination) and a half-life of total cortisol of 49 +/- 2.4 and of ACTH of 20 +/- 1.3; 4) ACTH potency (EC(50), ng/l) of 26 +/- 2.4, efficacy (nmol.l(-1).min(-1)) 10 +/- 1.8, and sensitivity (slope units) 0.65 +/- 0.09; 5) night/day augmentation of ACTH and cortisol SEC-burst mass by 2.1- and 1.7-fold (median); 6) abbreviation of the modal time to maximal ACTH and cortisol SEC rates by 4.4- and 4.3-fold, respectively, after a change point clock time of 0205 (median); 7) in vivo percentage distribution of cortisol as 6% free, 14% albumin bound, and 80% CBG bound with an absolute free cortisol CON (nmol/l) 11.5 +/- 0.54; and 8) significant (mean CV) stochastic variability in feedforward efficacy (140%), potency (38%), and sensitivity (56%) within the succession of paired ACTH/cortisol pulses of any given subject. In conclusion, the present composite formulation illustrates a platform for dissecting mechanisms of in vivo regulation of effector-response properties noninvasively in the corticotropic axis of the uninfused individual.

Characterization and regulation of angiotensin and corticotropin receptors on cultured bovine adrenal cells

Cultured bovine adrenal fasciculata cells were used to characterize angiotensin II (A-II) and corticotropin (ACTH) receptors and to study their homologous and heterologous regulation. These cells contain one type of high affinity binding sites for A-II (KD congruent to 2.4 +/- 0.3 10(-9) M) and about 100000 sites/cell. Photoaffinity labeling followed by SDS-PAGE under reducing conditions revealed a single macromolecule of apparent MR 65,000. Treatment of cells with increasing concentrations of A-II produced down-regulation of its own receptors and marked homologous and heterologous (ACTH) steroidogenic desensitization. However, the desensitization was not correlated with receptor loss and was mainly due to alterations of the steroidogenic pathway. Pretreatment of cells with ACTH also reduced A-II receptors, but this was not associated with steroidogenic desensitization. Bovine fasciculata cells contain two binding sites for ACTH: one of high affinity (KD congruent to 2.6 +/- 0.4 10(-10) M) and low capacity (2030 +/- 390 sites/cell) and the other of low affinity and high capacity. Affinity cross-linking of ACTH to plasma membranes prepared from adrenal cells revealed a labeled macromolecule of apparent MR 43000. However, cross-linking experiments to intact cells revealed, both under reducing and non-reducing conditions, two labeled macromolecules of apparent MR of 123000 and 43000. Pretreatment of cells with ACTH enhanced its receptor and the cAMP and cortisol responses to further ACTH stimulation. These effects were time- and dose-dependent. The maximal effects were observed at 10(-10) to 10(-9) M. A-II alone had no effect but it blocked partially the stimulatory action of ACTH.

Corticotropin positively regulates its own receptors and cAMP response in cultured bovine adrenal cells

Bovine fasciculata adrenal cells contain specific high-affinity (KD approximately 2.3 +/- 0.4 x 10(-10) M) and low-capacity (1910 +/- 300 sites per cell) corticotropin (ACTH) receptors. Pretreatment of cells with ACTH, caused in a time-(maximum effect at 48 hr) and dose-(ED50 approximately 10(-11) M, Vmax = 10(-10) to 10(-9) M) dependent manner an increase in ACTH binding. This was due to a 4-fold increase in the number of binding sites without modification of the binding affinity. The same pretreatment also enhanced the cAMP response to further ACTH stimulation in a dose-dependent manner (ED50 approximately 10(-11) M) and to a lesser extent the response to forskolin. However, pretreatment with higher concentrations of ACTH (10(-8) M) reduced the binding and the cAMP response when compared to the effect of 10(-9) M. These ACTH effects, which were mimicked by 8-bromoadenosine 3',5'-cyclic monophosphate, required de novo protein synthesis. Pretreatment with 10(-13) to 10(-11) M ACTH also enhanced the steroidogenic responsiveness to further hormonal stimulation. However, at higher concentrations the hormone induced an apparent steroidogenic desensitization that was probably related to a depletion of endogenous cholesterol, since cortisol production in the presence of 22-(R)-hydroxycholesterol was increased. Neither angiotensin-II nor atrial natriuretic factor alone modified ACTH receptors, but angiotensin-II partially blocked the stimulatory effect of ACTH. Thus, ACTH is one of the few polypeptide hormones having a positive trophic effect on its own receptors and target-cell responsiveness.

The inhibitory GTP-binding protein (Gi) occurs in rat Leydig cells and is differentially modified by lutropin and 12-O-tetradecanoylphorbol 13-acetate

Luteinizing-hormone (LH)-stimulated cyclic AMP production in rat testis Leydig cells was desensitized by both LH and 12-O-tetradecanoylphorbol 13-acetate (TPA). However, TPA, but not LH, enhanced the subsequent response to cholera toxin. Treatment of the cells with pertussis toxin potentiated cyclic AMP production in both control and LH-desensitized cells, but did not potentiate further the responses obtained by TPA pretreatment. The results implicate the presence of an inhibitory GTP-binding protein (Gi), which may be inhibited by TPA. The presence of a Gi-like protein within the plasma membrane of Leydig cells was demonstrated by pertussis-toxin-catalysed [32P]ADP-ribosylation of a Mr-40000-41000 protein.

Interrenal activity in the female lizard Lacerta vivipara J.: in vitro response to ACTH 1-39 and to [Sar1, Val5] angiotensin II (ANG II)

A perifusion system technique was developed in order to determine in vitro the respective roles of ACTH and ANG II in the regulation of adrenal steroidogenesis in the lizard Lacerta vivipara. Synthetic human ACTH 1-39, administered as 20-min pulses, stimulated corticosterone (B) and aldosterone (A) release in a dose-dependent manner. The increase in corticosterone output was higher than that in aldosterone output, leading to an enhancement of the B/A ratio. Iterative stimulations with 1 nM ACTH (20-min pulses every 120 min) led to reproducible increases in corticosterone and aldosterone release. Prolonged stimulation with 1 nM ACTH (up to 240 min) caused a sustained increase in corticosteroid release, suggesting that, in the lizard, ACTH does not induce any desensitization phenomenon. The angiotensin II analogue [Sar1, Val5] ANG II also stimulated corticosterone and aldosterone release in a dose-dependent manner; the stimulatory effects of ANG II on both steroids were very similar. These results indicate that, in lizards, ACTH plays a major role in the regulation of adrenal steroidogenesis. Since ANG II stimulates the production of gluco- and mineralocorticoids, our data raise the question of the existence of two cell types synthesizing corticosterone and aldosterone, respectively, in reptiles.

Studies on the mechanism of corticotrophin-mediated desensitization of corticosterone secretion by rat adrenocortical cells

Long-term exposure of the adrenocortical cells in vivo or in vitro to high concentrations of ACTH results in a diminution of the responsiveness of these cells to a subsequent stimulation of corticosterone release by ACTH. Conflicting studies have been published on the mechanism of this 'desensitization' phenomenon. Dispersed adrenocortical cells prepared from the hypertrophic/hyperplastic adrenal glands of rats bearing the ACTH/PRL-secreting rat pituitary tumour 7315a showed an increased basal release of corticosterone, but had lost their ability to respond further to ACTH. However, corticosterone release in response to dibutyryl cyclic AMP (dbcAMP), cholera toxin and forskolin remained intact. Pretreatment of normal rats for 3, 9 and 21 days with 50 micrograms/rat/day of a long-acting ACTH depot preparation induced a dose-dependent increase in basal corticosterone release by the adrenocortical cells prepared from these animals and a dose-dependent decrease in the sensitivity to ACTH. However, the responsiveness of the adrenocortical cells prepared from the adrenal glands of control and ACTH pretreatment rats to dbcAMP, cholera toxin and forskolin was similar. In addition, pretreatment with ACTH in vivo did not affect the sensitivity of the adrenocortical cells in vitro to calmodulin inhibition by haloperidol and 11 beta-hydroxylase inhibition by etomidate. It is concluded that long-term exposure of the adrenal gland to high concentrations of ACTH in vitro results in an excessive activation of corticosterone release by the adrenocortical cells in vitro, which is accompanied by a loss of sensitivity to ACTH.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of a simplified perifusion system of rat zona glomerulosa. Effect of cytochalasin B on spontaneous and ACTH-stimulated corticosteroidogenesis

The aim of the present study was to develop a perifusion technique for rat adrenal glomerulosa slices as a model to study the kinetics of corticosteroid production in vitro. Perifusion of adrenal tissue with increasing concentration of ACTH (3.16 X 10(-12) to 10(-9) M) led to a dose-related stimulation of corticosterone and aldosterone secretion. Administration of cytochalasin B did not alter the basal secretion of corticosterone and aldosterone. In addition, cytochalasin B did not modify the response of glomerulosa tissue to ACTH. The results indicate that the perifusion model for glomerulosa fragments may provide valuable information, concerning the kinetics of steroid production and its regulation at the cellular level.

Functional characterization of luteinizing hormone responsiveness and desensitization in perifused interstitial cells

The characteristics of the steroidogenic response of isolated rat testis interstitial cells to repeated 10 min pulses of ovine LH at 2 h intervals were examined in a Bio-gel perifusion system. Maximal responsiveness of the perifused interstitial cells could be maintained for 6-8 h. Thereafter, both basal and LH-stimulated testosterone production declined gradually despite supplementation of the perifusion medium with 1% and 5% foetal calf serum or 0.5 microgram/ml insulin. In contrast to the time-related steroidogenic decline, a dose-dependent refractoriness of the interstitial cells could be induced by repeated exposure to LH pulses from 0.01 to 10 ng/ml during the first 6 h of perifusion. The higher the stimulating dose of LH, the greater was the rate and magnitude of the resultant desensitization. With lower doses (0.01 and 0.1 ng/ml) of LH, an initial sensitization or priming effect was also observed. These changes in steroidogenic response occurred in the absence of any significant alterations in LH/hCG receptor binding of the perifused interstitial cells, nor could the refractory state be overcome by stimulation with analogues of cAMP. The perifused interstitial cells, when desensitized with low doses of LH (0.1 ng/ml), were capable of increasing or maintaining testosterone production in response to further stimulation with higher doses of LH (1 and 10 ng/ml). The mechanism(s) underlying the in vitro desensitization of perifused interstitial cells by LH may best be explained on the basis of the interaction between the negative effects of substrate depletion and the positive influence of mobilization of substrate(s) into the metabolically active pool for cholesterol side-chain cleavage. It was concluded that the dose of LH used in the pulsatile stimulation of perifused interstitial cells is critically important not only in determining the total amount of testosterone produced, but also in the pattern of response in terms of the degree of sensitization and desensitization induced.