Repository corticotropin injection versus corticosteroids for protection against renal damage in a focal segmental glomerulosclerosis rodent model

Negative Modulation of B Cell Activation by Melanocortin 1 Receptor Signaling Protects against Membranous Nephropathy

SIGNIFICANCE STATEMENT

Emerging evidence suggests that melanocortin neuropeptides-specifically adrenocorticotropic hormone-offer a novel, steroidogenic-independent therapeutic modality for membranous nephropathy (MN). The molecular mechanism underlying this beneficial effect, however, remains largely elusive. To investigate whether melanocortins modulate humoral immunity, the authors induced passive Heymann nephritis, a model of human MN, in wild-type and melanocortin 1 receptor (MC1R) knockout rats and treated them with melanocortin agents. Additional rats received adoptive transfer of bone marrow-derived cells beforehand from wild-type or MC1R knockout rats. The findings indicate that MC1R signaling plays a key role in negative modulation of B-cell activation and thereby suppresses humoral immune responses in passive Heymann nephritis, and suggest that MC1R signaling might offer a novel B cell-targeted therapeutic strategy for MN.

BACKGROUND

Emerging evidence suggests that the pituitary neuropeptide melanocortins-specifically, adrenocorticotropic hormone-offer a novel nonsteroidogenic therapeutic modality for membranous nephropathy (MN). However, the mechanism(s) of action remains elusive.

METHODS

To investigate whether melanocortins modulate humoral immunity, we induced passive Heymann nephritis (PHN), a model of MN, in wild-type (WT) and melanocortin 1 receptor (MC1R) knockout (KO) rats. We treated the animals with melanocortin agents-repository corticotropin injection, the nonsteroidogenic pan-melanocortin receptor agonist [Nle 4 , DPhe 7 ]-α-melanocyte stimulating hormone, the selective MC1R agonist MS05, vehicle gel, or phosphate-buffered saline-and evaluated kidney function, histology, and molecular changes. Additional rats received adoptive transfer of syngeneic bone marrow-derived cells beforehand from WT or MC1R KO rats.

RESULTS

KO of MC1R worsened PHN and this was associated with increased deposition of autologous immunoglobulin G (IgG) and complement C5b-9 in glomeruli and higher circulating levels of autologous IgG-evidence of a sensitized humoral immune response. Melanocortin therapy ameliorated PHN in WT rats, coinciding with reduced glomerular deposition of autologous IgG and C5b -9. The beneficial efficacy of melanocortins was blunted in KO rats but restored by adoptive transfer of syngeneic bone marrow-derived cells derived from WT rats. Mechanistically, MC1R was expressed in B lymphocytes and was negatively associated with B cell activation. MC1R agonism triggered the expression of microphthalmia-associated transcription factor in activated B cells in a cAMP-dependent mode and also repressed the expression of interferon regulatory factor 4 (a lymphoid transcription factor essential for B-cell development and maturation), resulting in suppressed plasma cell differentiation and IgG production.

CONCLUSIONS

MC1R signaling negatively modulates B cell activation and suppresses humoral immune responses in PHN, suggesting that MC1R signaling might offer a novel therapeutic target for MN.

Distinct binding and signaling activity of Acthar Gel compared to other melanocortin receptor agonists

PURPOSE

To compare the binding and agonistic activity of Acthar^® Gel and synthetic melanocortin receptor (MCR) agonists and examine how the activity of select agonists affects the in vivo production of corticosterone.

MATERIALS AND METHODS

In vitro binding was determined using concentration-dependent displacement of the ligand [¹²⁵I]Nle⁴, D-Phe⁷-α-melanocyte-stimulating hormone (α-MSH) on cells expressing MC1R, MC3R, MC4R, or MC5R. Functional activity was determined using a time-resolved fluorescence cyclic adenosine monophosphate (cAMP) assay in cells expressing MC1R, MC2R, MC3R, MC4R, or MC5R. In vivo corticosterone analyses were performed by measuring plasma corticosterone levels in Sprague Dawley rats.

RESULTS

Acthar Gel and synthetic MCR agonists exhibited the highest binding at MC1R, lowest binding at MC5R, and moderate binding at MC3R and MC4R. Acthar Gel stimulated the production of cAMP in all 5 MCR-expressing cell lines, with MC2R displaying the lowest level of full agonist activity, 3-, 6.6-, and 10-fold lower than MC1R, MC3R, and MC4R, respectively. Acthar Gel was a partial agonist at MC5R. The synthetic MCR agonists induced full activity at all 5 MCRs, with the exception of α-MSH having no activity at MC2R. Acthar Gel treatment had less of an impact on in vivo production of corticosterone compared with synthetic ACTH_1-24 depot.

CONCLUSIONS

Acthar Gel bound to and activated each MCR tested in this study, with partial agonist activity at MC5R and the lowest level of full agonist activity at MC2R, which distinguished it from synthetic MCR agonists. The minimal activity of Acthar Gel at MC2R corresponded to lower endogenous corticosteroid production.