Calcitriol in melanoma cells produced two very different responses

Calcitriol in melanoma cells did not produce anything close to a uniform response. After 24 hours of exposure to the active form of vitamin D, one melanoma cell line lost viability, showed mitochondrial disruption and migrated less. Another exposed to the same concentrations was largely unaffected, and its viability even increased slightly at the lowest dose.

That contrast is the most revealing feature of a new study in Medical Oncology. Rather than simply adding another laboratory result to the long-running discussion of vitamin D and cancer, the work illustrates how differently two melanomas can respond to the same molecular intervention.

The researchers used two human melanoma cell lines, A375 and SK-MEL-28. Cells were exposed for 24 hours to calcitriol at concentrations of 1, 10 or 50 nM. The team then assessed viability, mitochondrial membrane potential, cell migration, formation of apoptotic bodies and several components of extracellular nucleotide signaling.

One cell line responded on several levels

A375 cells were sensitive to calcitriol across all three concentrations tested. Their viability decreased, and so did their mitochondrial membrane potential, an indicator of mitochondrial functional state.

Microscopy also revealed more nuclear fragmentation and apoptotic bodies after treatment, while cell counts declined as calcitriol concentration increased.

The effect extended to cell behavior. In a wound-healing assay, A375 cells exposed to calcitriol were less able to migrate into the artificial gap created in the cell layer. All three concentrations significantly inhibited wound closure after 24 hours.

The researchers then examined purinergic signaling, a communication system in which extracellular nucleotides such as ATP, ADP and AMP, together with adenosine, act as signaling molecules rather than simply as components of cellular energy metabolism.

Two enzymes in this system, CD39 and CD73, participate in the conversion of extracellular nucleotides toward adenosine. This pathway has attracted considerable attention in cancer because extracellular adenosine can help shape an immunosuppressive tumor environment.

In A375 cells, calcitriol changed ectonucleotidase activity. At 10 and 50 nM it also reduced expression of the genes encoding CD39 and CD73. The same concentrations lowered gene expression of IL-6 and NLRP3, two inflammatory signaling components implicated in cancer biology.

Those findings need a specific qualification. The study measured gene expression rather than the corresponding protein abundance, one of the limitations acknowledged by the authors. It also did not measure an immune response: these were isolated cancer cells in culture, not a complete tumor microenvironment containing immune and stromal cells.

The second melanoma behaved differently

SK-MEL-28 cells produced a strikingly different result.

Calcitriol did not reduce their viability in the way it did in A375 cells and did not produce the same change in mitochondrial membrane potential. At the lowest concentration tested, 1 nM, cell viability actually increased slightly but significantly.

This does not mean that calcitriol generally promotes melanoma growth. The observation applies to one cell line under a specific laboratory condition. Its significance lies elsewhere: melanoma cells carrying the same broad disease label can differ substantially in their molecular sensitivity.

The authors point to previous work showing relatively low functional vitamin D receptor activity in SK-MEL-28 compared with several other melanoma cell lines. That could help explain its resistance to low nanomolar calcitriol, but the present study did not directly test that mechanism.

The contrast between A375 and SK-MEL-28 is therefore both the study's most interesting finding and one of its major limitations. Only two melanoma cell lines were examined. The authors explicitly note that additional cell lines would be needed to establish how broadly these effects apply.

More importantly, the entire study was performed in vitro. No animals or patients were treated, and reducing the viability of cultured cancer cells does not demonstrate that vitamin D can treat melanoma in people.

The compound used was also calcitriol, the hormonally active form of vitamin D, at controlled nanomolar concentrations. These experiments are not equivalent to taking ordinary vitamin D supplements.

The most useful message from the study is therefore more specific than “vitamin D fights cancer.” In one melanoma cell line, calcitriol affected viability, mitochondria, migration and signaling pathways at the same time. In another melanoma line, most of that response disappeared.

A single cancer diagnosis does not necessarily imply a single cellular biology—or a single response to the same molecule.