Transcriptional regulation of vascular endothelial growth factor expression in epithelial and stromal cells during mouse mammary gland development.

Accompanying changes in the development and function of the mammary gland is the establishment of a vascular network of critical importance for lactogenesis and tumorigenesis. A potent angiogenic and permeability factor that regulates vascular development in association with epithelial-stromal interactions is vascular endothelial growth factor (VEGF). Analysis of VEGF transcription by RT-PCR revealed mRNA for all three VEGF isoforms (VEGF120, 164, 188) within the mammary gland of nulliparous females. During pregnancy the level of VEGF188 declined and became undetectable during lactation in association with the increased abundance of VEGF120 and VEGF164 mRNAS: All three isoforms were expressed at consistent levels within the cleared mammary fat pad throughout development. Furthermore, the presence of VEGF188 mRNA in omental adipose tissue at various stages established that VEGF188 is expressed specifically in adipose tissue within the mammary gland. Using 3T3-L1 preadipocytes it was demonstrated that VEGF188 mRNA transcription occurs as a late event during lipogenesis distinct from earlier induction of VEGF120 and VEGF164 mRNA during differentiation. In contrast, HC11 mammary epithelial cells only expressed mRNA for VEGF120 and VEGF164. Localization of VEGF mRNA and protein revealed that VEGF is expressed in stromal cells of the mammary gland in nulliparous females and then undergoes a transition to epithelial expression during lactation. By contrast, mRNA for the VEGF receptors, Flk-1 and Flt-1, localized to stromal cells within the mammary fat pad during virgin and gestational development and was expressed in the interstitial tissue basal to epithelial cells during lactation. Taken together, these results support the conclusion that VEGF is differentially transcribed by specific cell types within the mammary gland, and that under hormonal regulation it functions in an autocrine/paracrine manner.