Vitamin D3 receptor (VDR) expression in HC-11 mammary cells:: regulation by growth-modulatory agents, differentiation, and Ha-ras transformation

HC-11 mammary epithelial cells which originate from midpregnant BALB/c mice are able to differentiate in culture after epidermal (EGF) or basic fibroblast (FGF) growth factor pretreatment followed by lactogenic hormone stimulation (Dexamethasone, Insulin, and Prolactin - DIP). In our study, HC-11 cells exhibited specific vitamin D-3 receptors (VDR) determined by Northern analysis or flow cytometry and responded to 10 nM vitamin D-3 treatment displaying strong growth inhibition, arrest in G0/G1 phase without evidence of apoptosis, and VDR mRNA reduction, although the percentage of cells expressing VDR protein remained unchanged. In an attempt to verify if there was a correlation between the growth state of the cells and VDR levels, we have examined the effects of growth modulators such as EGF/bFGF and confluency and transformation by Ha-ras. A down-regulation of VDR expression was observed after Ha-ras transformation of HC-11 cells which desensitized the cells to the growth inhibitory effects of vitamin D-3. EGF or bFGF decreased VDR in parental cells and EGF antagonized the antiproliferative activity of vitamin D-3. As well, transition from proliferating to confluent state significantly reduced VDR levels only in parental cells. DIP-induced HC-11 cell differentiation (monitored by beta-casein transcripts), although leading to cell cycle arrest, increased VDR mRNA content, which seems to be rather related to lactogenic hormone induction than to differentiation itself. In fact, DIP-stimulated HC-11 cells in the absence of EGF pretreatment, or DIP-treated HC-11ras cultures, also displayed up-regulated VDR level even in the absence of differentiation. Concluding, mammary VDR levels might be regulated by growth modulating agents, by physiological conditions of the gland, and by the ras-mediated malignant transformation.