# Dietary vitamin D is a novel modulator of tumor engraftment through regulation of GC protein abundance

## TL;DR

This study shows that dietary vitamin D can influence tumor growth by regulating the GC protein, which helps the immune system fight cancer cells.

## Contribution

The study reveals that dietary vitamin D modulates GC protein levels, offering a novel pathway for vitamin D to enhance tumor immunity.

## Key findings

- Absence of GC protein significantly increased tumor engraftment in both breast and prostate cancer models.
- Dietary vitamin D dosage showed a U-shaped relationship with serum GC protein abundance.
- High vitamin D doses may enhance immunity and reduce tumor burden through GC protein regulation.

## Abstract

The vitamin D binding protein, the GC protein, is a multifunctional protein that binds circulating vitamin D and also increases macrophage killing of tumor cells. Injecting exogenous GC protein concurrent with experimental tumor implant decreases tumor engraftment rate. Until now serum abundance of this protein was thought to be controlled by estrogen, glucocorticoids and inflammatory cytokines, but, not by vitamin D itself(1, 2). Nonetheless, increasing dietary vitamin D is thought to increase serum vitamin D, which is 98% bound by the GC protein.

Based on the protection that excess GC protein offers we sought to determine if decreased GC protein abundance might decrease tumor immunity. Relatedly, we theorized, by contrast to the current model, that dietary vitamin D might affect serum abundance of GC protein. If exogenous vitamin D alters available GC levels, then this effect might indicate a novel pathway by which vitamin D enhances immunity. To examine these possibilities, we examined the effect of GC protein absence on tumor persistence or engraftment on two different and common tumor types (prostate cancer and breast cancer). We further examined the relationship between dietary vitamin D and serum GC abundance. We found that absence of GC protein allowed significantly more engraftment of breast tumor cells in female mice and of prostate tumor cells in male mice. Further, we found a U-shaped response of serum GC protein to dietary vitamin D dosage as well as to serum vitamin D, indicating the potential benefit of high exogenous doses to enhance immunity and reduce tumor burden.