Abstract B011: The anti-cancer effects of vitamin D are blocked postpartum, due to suppression of vitamin D metabolism in the involuting liver
Authors: Bernhardt Sarah M , Schedin Pepper 
Published: December 2022

Abstract
Postpartum mammary gland involution is a physiologic window of increased breast cancer risk. It has been proposed that the poor prognosis associated with postpartum breast cancer is due to the involuting mammary microenvironment promoting progression of indolent lesions to invasive disease. As such, the involuting gland has been implicated as a target for preventive strategies. Vitamin D has anti-cancer properties, and there are data demonstrating that vitamin D supplementation protects against breast cancer progression in mouse models. Moreover, vitamin D deficiency is prevalent in postpartum women, suggesting that vitamin D supplementation during the vitamin D-deficient, at-risk window of involution may be an approach for preventing the progression of indolent lesions. Here, we characterized how vitamin D deficiency and supplementation affect tumor growth in a mouse model of postpartum breast cancer. Vitamin D deficiency and sufficiency were established in BALB/c mice through feeding diets containing low or high levels of vitamin D. Quantification of serum 25(OH)D verified that diets resulted in vitamin D deficiency (25.8±4.3nmol/L;mean±stdev) or sufficiency (66.4±11.7nmol/L) at levels comparable to humans. Murine mammary cancer cells (D2A1, 2×104 cells, 20µL) were injected into mammary fatpads of involuting (2 days post wean), or age-matched nulliparous mice (n=5-12 mice/group), and tumor growth tracked for 4 weeks. Independent of vitamin D status, tumors exposed to the involuting mammary gland grew 1.8-fold larger than tumors exposed to the nulliparous gland (p&lt;0.001); consistent with prior data showing that the microenvironment of the involuting gland is tumor promotional. Interestingly, while vitamin D supplementation of nulliparous mice associated with a 3.4-fold reduction in tumor growth (p=0.03), vitamin D supplementation of involution mice did not reduce tumor growth. Dietary vitamin D is metabolized to its active form in the liver. Our group has previously shown that the liver also undergoes weaning-induced involution. Thus, we tested if involution of the liver impairs metabolism of vitamin D to its active form, potentially contributing to the null effects of vitamin D supplementation in involution mice. We collected serum from mice on vitamin D deficient and supplemented diets across reproductive time points (nulliparous, lactation, involution days 2, 4, 6), and found that vitamin D supplementation during involution was insufficient to restore serum vitamin D to levels of sufficiency. Further, gene expression analysis of livers show that expression of Cyp2r1 and Cyp27a1—genes involved in vitamin D activation—were reduced during involution. Together, these findings suggest that impaired metabolism of vitamin D during involution may reduce the availability of active vitamin D, and contribute to the null effect observed in involution mice. Understanding the mechanisms by which mammary gland involution influences the anti-cancer effects of vitamin D is required to optimize cancer prevention strategies that target this window.
Citation Format: Sarah M Bernhardt, Pepper Schedin. The anti-cancer effects of vitamin D are blocked postpartum, due to suppression of vitamin D metabolism in the involuting liver [abstract]. In: Proceedings of the AACR Special Conference on Rethinking DCIS: An Opportunity for Prevention?; 2022 Sep 8-11; Philadelphia, PA. Philadelphia (PA): AACR; Can Prev Res 2022;15(12 Suppl_1): Abstract nr B011.