It doesn’t pay to be overconfident. There is so much we don’t understand. Our latest humility lesson comes from Finland. Researchers there have discovered that in some cells at least, 1a,25-dihydroxy-vitamin D [1a,25(OH)2D, which is sometimes written as 1,25(OH)2D] is not the only form of vitamin D that can activate the nuclear vitamin D receptor.
Nuclear receptors are found only in animal cells. They up- and down-regulate gene activity. In human cells there are 48 of them, one of which is activated, as we shall see, by various forms of vitamin D. Activating its nuclear receptor isn’t the only metabolic effect of vitamin D, but it’s clearly an important part of the mix.
The researchers set up this experiment using two types of cells. One type was from the human prostate gland and the other type was a modified mouse cell that couldn’t produce the enzyme that converts the storage form of vitamin D, 25-hydroxyvitamin D [25(OH)D] into the classically active form, 1a,25(OH)2D.
Their idea was to split the cells into separate groups, give each group one of three different types of vitamin D, and determine what genes were turned on or off in response to the vitamin D. The mouse cells were included in the experiment to make sure that in the 25(OH)D group the results weren’t actually 1a,25(OH)2D results because the cell had converted the one into the other. Since the mouse cells were unable to do the conversion, the researchers could be sure that any results they saw in that group were in fact due to 25(OH)D.
The third type of vitamin D the researchers included in the experiment was 24R,25 dihydroxy-vitamin D [24R,25(OH)2D]. This version is the first breakdown product of both 25(OH)D and 1a,25(OH)2D. The breakdown depends on an enzyme called 24-hydroxylase, which is made by a gene called CYP24A1. We’ll get back to this gene in a moment.
In both the human and the mouse cells, the researchers found that all three versions of vitamin D activated the vitamin D receptor and, with some overlap, regulated different genes. In the human prostate cell, a total of 441 different genes were regulated up or down. Figure 1 shows the percentage of those 441 genes regulated by each version of vitamin D, as well as their overlap.
One of the big surprises here is that the version of vitamin D that regulates the most genes (175) is 24R,25(OH)2D. This version of vitamin D had previously been shown to have some activity in vitamin D receptor knockout mice, but the authors say this is the first time it’s been shown to activate the vitamin D receptor. The next big surprise is that the version of vitamin D that regulates the second-most number of genes (171) is 25(OH)D. What we heretofore considered the “activated” form of vitamin D, 1a,25(OH)2D, regulates a total of 164 genes in this particular kind of prostate cell.
One of the interesting findings is that the gene that both 25(OH)D and 1a,25(OH)2D up-regulated most strongly was CYP24A1 – the gene that creates the enzyme that converts these forms of vitamin D into 24R,25(OH)2D, beginning their own breakdown process. And 24R,25(OH)2D had no effect on this gene at all.
The researchers identified the types of genes that were being regulated in several different ways. One of these analyses looked into the network functions of genes exclusively regulated by a single version of vitamin D. 1a,25(OH)2D appears to be the version involved in cardiovascular, antimicrobial, and inflammatory health, while 25(OH)D and 24R,25(OH)2D seem more involved in cell-level processes which likely relate to cancer and neurological disease.
We have so much to learn.