The Expression of T-Box (Tbx3) in the Bovine Mammary Gland

Development of the bovine mammary gland is a complex process that is regulated by several hormones, growth factors and transcription factors including but not limited to growth hormone (GH), insulin-like growth factor (IGF)-I and T-box (Tbx)2 and 3. Tbx2 and Tbx3 are transcription factors required for mammary gland development in humans and known to regulate cell cycle. In addition, there is recent evidence that GH increases expression of Tbx3 in osteoblasts independent of IGF-I. Based on these findings, we hypothesized GH and IGF-I will increase Tbx2 and Tbx3 expression in bovine mammary epithelial cells (MEC), the cell responsible for milk production. To test our hypothesis, MAC-T cells (MEC line) were treated with GH at 100 (GH100) or 500 (GH500) ng/mL or IGF-I at 100 (IGF100) or 200 (IGF200) ng/mL for 24 and 48 hours. As determined by real-time RT-PCR, we did not observe a change in Tbx3 expression in cells treated with GH (P ≥ 0.74). However, both IGF-I treatments increased Tbx3 expression (P ≤ 0.03). Surprisingly, expression of Tbx2 was not detectable in MEC cells; however it was expressed in mammary fibroblast cells. In fibroblast cells treated with GH500 and IGF200, we did observe a change in Tbx2 or Tbx3 expression (P ≥ 0.76). In conclusion, IGF-I regulates Tbx3 expression in bovine MEC and Tbx2 and Tbx3 expression are cell type specific.