Differential response to 1α,25-dihydroxyvitamin D₃ (1α,25(OH) ₂D₃) in non-small cell lung cancer cells with distinct oncogene mutations

We previously demonstrated that non-small cell lung cancer (NSCLC) cells and primary human lung tumors aberrantly express the vitamin D ₃-catabolizing enzyme, CYP24, and that CYP24 restricts transcriptional regulation and growth control by 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) in NSCLC cells. To ascertain the basis for CYP24 dysregulation, we assembled a panel of cell lines that represent distinct molecular classes of lung cancer: cell lines were selected which harbored mutually exclusive mutations in either the K-ras or the Epidermal Growth Factor Receptor (EGFR) genes. We observed that K-ras mutant lines displayed a basal vitamin D receptor (VDR)^lowCYP24^high phenotype, whereas EGFR mutant lines had a VDR^highCYP24^low phenotype. A mutation-associated difference in CYP24 expression was also observed in clinical specimens. Specifically, K-ras mutation was associated with a median 4.2-fold increase in CYP24 mRNA expression (p = 4.8×10 ^-7) compared to EGFR mutation in a series of 147 primary lung adenocarcinoma cases. Because of their differential basal expression of VDR and CYP24, we hypothesized that NSCLC cells with an EGFR mutation would be more responsive to 1,25(OH)₂D₃ treatment than those with a K-ras mutation. To test this, we measured the ability of 1,25(OH) ₂D₃ to increase reporter gene activity, induce transcription of endogenous target genes, and suppress colony formation. In each assay, the extent of 1,25(OH)₂D₃ response was greater in EGFR mutation-positive HCC827 and H1975 cells than in K-ras mutation-positive A549 and 128.88T cells. We subsequently examined the effect of combining 1,25(OH)₂D₃ with erlotinib, which is used clinically in the treatment of EGFR mutation-positive NSCLC. 1,25(OH)₂D ₃/erlotinib combination resulted in significantly greater growth inhibition than either single agent in both the erlotinib-sensitive HCC827 cell line and the erlotinib-resistant H1975 cell line. These data are the first to suggest that EGFR mutations may identify a lung cancer subset which remains responsive to and is likely to benefit from 1,25(OH)₂D₃ administration.