The role of lipotropins as hematopoietic factors and their potential therapeutic use

ACTH promotes chondrogenic nodule formation and induces transient elevations in intracellular calcium in rat bone marrow cell cultures via MC2-R signaling

Adrenocorticotropic hormone (ACTH) is among several melanocortin peptide hormones that are derived from proopiomelanocortin (POMC). ACTH has been found to enhance osteogenesis and chondrogenesis. We show that, in the presence of dexamethasone, ACTH dose-dependently increases chondrogenic nodule formation in bone marrow stromal cells (BMSC) from the Wistar Kyoto (WKY) rat. The nodules consist in condensed cells highly expressing alkaline phosphatase, Sox9 and type II collagen transcripts and a proteoglycan-rich matrix. Immunoblot analysis of crude membrane fractions has shown that these cells express three melanocortin receptors (MC-R), namely MC2-R, MC3-R and MC5-R and the melanocortin 2-receptor accessory protein (MRAP). To determine which of these receptors mediate ACTH-induced effects, we have used MC-R-specific peptides and the known agonist profiles of the receptors. Neither α-MSH, a strong agonist of MC5-R, nor γ2-MSH, a strong agonist of MC3-R, duplicates ACTH effects in rat BMSC. In addition, calcium flux has been examined as a mechanism for ACTH action at the MC2-R. Consistent with MC2-R and MRAP expression patterns in the BMSC cultures, ACTH-induced transient increases in intracellular calcium are increased with dexamethasone treatment. Neither α-MSH nor γ2-MSH affects calcium flux. Dexamethasone increases MC2-R and MRAP expression and POMC peptide expression and cleavage increasing the production of the lipolytic β-lipotropic hormone product. Therefore, the effects of ACTH in rat BMSC enriched for mesenchymal progenitors are consistent with an MC2-R signaling mechanism, with dexamethasone being capable of regulating components of the melanocortin system in these cells.

Involvement of human peroxisomes in biosynthesis and signaling of steroid and peptide hormones

Although peroxisomes exert essential biological functions, cell type-specific features of this important organelle are still only superficially characterized. An intriguing new aspect of peroxisomal function was recently uncovered by the observation that the peptide hormones β-lipotropin (β-LPH) and β-endorphin are localized to peroxisomes in various human tissues. This suggests a functional link between peptide hormone metabolism and peroxisomes. In addition, because endocrine manifestations that affect steroid hormones are often found in patients suffering from inherited peroxisomal disorders, the question has been raised whether peroxisomes are also involved in steroidogenesis. With this chapter, we will review several crucial aspects concerning peroxisomes and hormone metabolism.

Peroxisomal localization of the proopiomelanocortin-derived peptides beta-lipotropin and beta-endorphin

The peptide hormones ACTH, MSHs, β-lipotropin (β-LPH), and β-endorphin are all derived from the precursor molecule proopiomelanocortin (POMC). Using confocal laser microscopy and immunoelectron microscopy in human pituitary gland, we demonstrate a peroxisomal localization of β-endorphin and β-LPH in cells expressing the peroxisomal ATP-binding cassette-transporter adrenoleukodystrophy protein (ALDP). The peroxisomal localization of β-LPH and β-endorphin was not restricted to the pituitary gland but was additionally found in other human tissues that express high levels of ALDP, such as dorsal root ganglia, adrenal cortex, distal tubules of kidney, and skin. In contrast to the peptide hormones β-LPH and β-endorphin, which are derived from the C terminus of POMC, the N-terminal peptides ACTH, α-MSH, and γ-MSH were never detected in peroxisomes. This novel peroxisomal localization of β-endorphin and β-LPH in ALDP-positive cells was confirmed by costaining with ALDP and the peroxisomal marker catalase. Moreover, peroxisomal sorting of β-LPH could be modeled in HeLa cells by ectopic expression of a POMC variant, modified to allow cleavage and release of β-LPH within the secretory pathway. Although β-LPH and β-endorphin were only associated with peroxisomes in cells that normally express ALDP, the transporter activity of ALDP is not necessary for the peroxisomal localization, as demonstrated in tissues of X-linked adrenoleukodystrophy patients lacking functional ALDP. It remains to be elucidated whether and how the peroxisomal localization of POMC-derived hormones has a role in the endocrine dysfunction of peroxisomal disease.