Corticosteroid-binding globulin contributes to the neuroendocrine phenotype of mice selected for extremes in stress reactivity.

Increasing evidence indicates an important role of steroid-binding proteins in endocrine functions, including hypothalamic-pituitary-adrenal (HPA) axis activity and regulation, as they influence bioavailability, local delivery, and cellular signal transduction of steroid hormones. In the plasma, glucocorticoids (GCs) are mainly bound to the corticosteroid-binding globulin (CBG) and to a lesser extend to albumin. Plasma CBG levels are therefore involved in the adaptive stress response, as they determine the concentration of free, biologically active GCs. In this study, we investigated whether male mice with a genetic predisposition for high-reactivity (HR), intermediate-reactivity (IR), or low-reactivity (LR) stress-induced corticosterone (CORT) secretion present different levels of free CORT and CORT-binding proteins, basally and in response to stressors of different intensity. Our results suggest a fine control interaction between plasma CBG expression and stress-induced CORT release. Although plasma CBG levels, and therefore CBG binding capacity, were higher in HR animals, CORT secretion overloaded the CBG buffering function in response to stressors, resulting in clearly higher free CORT levels in HR compared with IR and LR mice (HR>IR>LR), resembling the pattern of total CORT increase in all three lines. Both stressors, restraint or forced swimming, did not evoke fast CBG release from the liver into the bloodstream and therefore CBG binding capacity was not altered in our three mouse lines. Thus, we confirm CBG functions in maintaining a dynamic equilibrium between CBG-bound and unbound CORT, but could not verify its role in delaying the rise of plasma free CORT immediately after stress exposure.

Comparative effect of FGF2, synthetic peptides 1-28 N-POMC and ACTH on proliferation in rat adrenal cell primary cultures.

There is evidence that pro-opiomelanocortin (POMC)-derived peptides other than adrenocorticotropic hormone (ACTH) have a role in adrenal cell proliferation. We compared the activity of synthetic rat N-terminal POMC fragment 1-28 with disulfide bridges (N-POMC(w)) and without disulfide bridges (N-POMC(w/o)), with the activity of fibroblast growth factor (FGF2), a widely studied adrenal growth factor, and ACTH, in well-characterized pure cultures of both isolated adrenal Glomerulosa (G) and Fasciculata/Reticularis (F/R) cells. Three days of FGF2-treatment had a proliferative effect similar to serum, and synthetic peptide N-POMC(w) induced proliferation more efficiently than N-POMC(w/o). Moreover, both induced proliferation via the ERK1/2 pathway. In contrast, sustained ACTH treatment decreased proliferation and viability through apoptosis induction, but not necrosis, and independently of PKA and PKC pathways. Further elucidation of 1-28 POMC signal transduction is of interest, and primary cultures of adrenal cells were found to be useful for examining the trophic activity of this peptide.

Differences between the growth regulatory pathways in primary rat adrenal cells and mouse tumor cell line.

In this study, DNA synthesis, phosphorylation of ERK1/2 and CREB proteins, as well as induction of c-Fos protein, were examined in rat adrenocortical, glomerulosa and fasciculata/reticularis cells, as well as in the Y1 cell line. We found that FGF2 was mitogenic only in glomerulosa cells and although ACTH did not activate ERK1/2, it did activate CREB protein, indicating efficient transduction of signals initiated in the ACTH receptors of rat adrenocortical cells. The FGF2 activated ERK1/2 in rat adrenal cells by a mechanism that might be modulated by upstream PKA pathway phosphorylation of MEK and despite the nonmitogenic effect of ACTH on rat adrenal cells it effectively induces c-Fos protein. The results presented herein describe distinct differences between the ACTH and FGF2 signal transduction mechanisms seen in adrenocortical cells and those observed in the Y1 cell line, indicating that, in vitro, ACTH blockage of the mitogenic effect occurs in normal adrenal cells after induction of c-Fos protein.