Vitamin D Signaling from Nephrogenesis to Neoplasia: Spatial Protein Expression in Fetal Kidney and Transcriptomic Dysregulation in Renal Tumors

Background and Objectives: Vitamin D signaling plays critical roles in immune regulation, bone metabolism, and cellular differentiation across multiple tissues. However, the spatial and temporal expression patterns of key vitamin D signaling components—the vitamin D receptor (VDR) and the enzyme 1α-hydroxylase (encoded by CYP27B1)—during human nephrogenesis have not been mapped at the protein level. The primary objective of this study was to characterize VDR and 1α-hydroxylase expression across critical stages of human kidney development, complementing prior transcriptomic and single-cell descriptions, and to contextualize these developmental observations against the dysregulation of vitamin D pathway genes in adult renal and urothelial malignancies. Materials and Methods: Immunofluorescence analysis was performed on FFPE kidney tissue from 12 specimens (3 per stage) at 10, 22 and 38 gestational weeks and postnatally at 1.5 years. For each specimen, at least three non-adjacent sections were stained and 6 non-overlapping cortical fields were imaged at ×40 (18 fields per stage). Fluorescence-area percentages were quantified in ImageJ 1.54g, and group differences were assessed by one-way ANOVA with Tukey’s post hoc test at both field- and specimen-level. An accompanying bioinformatic analysis evaluated the differential expression of VDR, CYP27B1, and CYP24A1 in adult renal and urothelial malignancies (TCGA cohorts: KICH, KIRC, KIRP, BLCA) using unpaired Welch’s t-test, with Benjamini–Hochberg FDR correction applied across all 16 tumor-versus-normal comparisons (12 gene-wise + 4 post hoc log2(CYP24A1/CYP27B1) ratios). Results: VDR showed its highest mean fluorescence area at 10 weeks (3.40% (95% CI 3.24–3.56); field-level Tukey p < 0.0001 versus other stages) and its lowest at 22 weeks (0.69% (0.64–0.74)). 1α-hydroxylase was also highest at 10 weeks (5.44% (5.29–5.60); p < 0.0001) and stabilized at lower levels thereafter (3.04–4.26%). Co-expression of both proteins was observed throughout development except in 22-week glomeruli. In TCGA, all 12 significant gene-wise comparisons retained significance after BH FDR correction (q < 0.05). VDR showed cohort-specific dysregulation: reduced in KICH (q = 2 × 10−4) but increased in KIRC and KIRP (q = 2 × 10−4 for both). CYP24A1 was reduced in all three renal cohorts (q ≤ 0.029) and unchanged in BLCA. CYP27B1 showed cohort-specific direction (reduced in KIRC; increased in KICH, KIRP, and BLCA). Conclusions: This study provides an initial immunofluorescence-based spatial description of VDR and 1α-hydroxylase across human kidney development, revealing a coordinated redistribution from immature glomeruli at 10 weeks to mature tubular segments at later stages. The TCGA analysis demonstrates that vitamin D pathway dysregulation in renal carcinoma is cohort-specific and is not abolished by multiple-testing correction. Together, these results indicate that the developmentally engaged vitamin D pathway retains kidney-specific functional relevance in adult renal pathology and provide a baseline reference for future mechanistic studies.