KPV and alpha-MSH Elevate Intracellular Calcium in Keratinocytes via MC-1R, Not Cyclic AMP
Background
Alpha-melanocyte-stimulating hormone (alpha-MSH) exerts well-documented anti-inflammatory effects believed to depend on cyclic AMP elevation downstream of MC-1R binding. The carboxyl-terminal tripeptide KPV — the smallest reported fragment retaining anti-inflammatory activity — was assumed to share this cAMP-dependent mechanism, yet direct evidence in human skin cells was lacking. Keratinocytes are a frontline immune-relevant cell type in skin inflammation, making them a critical model for testing MSH peptide signalling. Whether KPV operates through MC-1R or through an entirely different second-messenger cascade remained unresolved, leaving fundamental gaps in understanding how these peptides could be exploited therapeutically.
Study Design
Intracellular signalling of alpha-MSH, KPV (10⁻¹⁵–10⁻⁷ M), KP-D-V (10⁻¹⁵–10⁻⁷ M), and ACTH (10⁻¹⁵–10⁻⁷ M) was examined in two human keratinocyte models: immortalised HaCaT cells and primary normal human keratinocytes. Cyclic AMP levels were assayed alongside rapid intracellular calcium measurements. PIA, an adenosine agonist that inhibits the cyclic AMP pathway, was used as a pharmacological tool to unmask calcium responses in HaCaT cells. Chinese hamster ovary (CHO) cells stably transfected with MC-1R were used to confirm receptor-specific calcium signalling for alpha-MSH and KPV peptides.
Results
No elevation in cyclic AMP was detected in either HaCaT or normal human keratinocytes following treatment with alpha-MSH, KPV, or ACTH peptides at any tested concentration, directly contradicting the prevailing cAMP-dependence model for these cells. Rapid, acute intracellular calcium responses were observed in HaCaT keratinocytes to alpha-MSH, KPV, KP-D-V, and ACTH, each across the range 10⁻¹⁵–10⁻⁷ M — but only when PIA was co-present to suppress the cAMP pathway. Normal human keratinocytes responded to all the same peptides and additionally responded to ACTH 1-17 (10⁻¹³–10⁻⁷ M), a differential response not seen in HaCaT cells, suggesting cell-type-specific receptor expression or signalling competence.
Stable transfection of CHO cells with
MC-1Rdemonstrated that both alpha-MSH and KPV peptides directly elevated intracellular calcium via this receptor, confirmingMC-1Ras sufficient to mediate a calcium — not cAMP — second-messenger response.
Taken together, these findings establish that KPV's anti-inflammatory signalling in keratinocytes operates through a MC-1R-coupled, cAMP-independent calcium pathway, fundamentally reframing the mechanistic basis for this peptide class.
Key Findings
- No cyclic AMP elevation detected in HaCaT or normal keratinocytes with alpha-MSH, KPV, or ACTH at any dose tested.
- Intracellular calcium responses to KPV and KP-D-V observed across 10⁻¹⁵–10⁻⁷ M range in HaCaT cells, but only with PIA co-treatment.
- Normal keratinocytes additionally responded to ACTH 1-17 (10⁻¹³–10⁻⁷ M), unlike HaCaT cells.
- MC-1R-transfected CHO cells confirmed alpha-MSH and KPV peptides elevate intracellular calcium via MC-1R.
- KP-D-V (D-valine stereoisomer of KPV) produced equivalent calcium responses, supporting protease-stable analogue development.
Why It Matters
The assumed requirement for cyclic AMP in KPV and alpha-MSH anti-inflammatory action must be reconsidered — at least in keratinocytes, the operative second messenger appears to be intracellular calcium, not cAMP. For clinicians and researchers developing topical or systemic melanocortin-based therapies for skin inflammation, this means that potency screening based solely on cAMP readouts will miss or misrank active compounds. The finding that KP-D-V (the D-valine stereoisomer) produces the same calcium response as KPV is particularly relevant for formulation: D-amino acid substitution is commonly used to extend peptide half-life, and its retained receptor activity here strengthens the case for protease-resistant KPV analogues. Protocol-level implication: adenosine tone in the tissue microenvironment may gate whether calcium signalling is detectable, suggesting that local cAMP suppression could modulate KPV efficacy in vivo.
