Hedgehog Signaling and Bmi-1 Regulate Self-renewal of Normal and Malignant Human Mammary Stem Cells

TLDR

Mammary epithelial cells arise from self‑renewing stem cells capable of multilineage differentiation, and these cells or their progeny may be susceptible to transformation. The authors used in vitro mammosphere cultures and a xenograft mouse model to study how hedgehog signaling and Bmi‑1 regulate self‑renewal of normal and malignant human mammary stem cells. Hedgehog components PTCH1, Gli1, and Gli2 are highly expressed in mammary stem/progenitor cells and are down‑regulated upon differentiation; activating the pathway increases mammosphere initiation and size, inhibition reduces these effects, and these actions are mediated by Bmi‑1; overexpression of Gli2 induces ductal hyperplasia and altering Bmi‑1 alters mammary development in a humanized mouse, with the pathway also active in CD44+CD24−/lowLin− breast cancer stem cells, supporting a model in which hedgehog signaling and Bmi.

Abstract

Abstract The epithelial components of the mammary gland are thought to arise from stem cells with a capacity for self-renewal and multilineage differentiation. Furthermore, these cells and/or their immediate progeny may be targets for transformation. We have used both in vitro cultivation and a xenograft mouse model to examine the role of hedgehog signaling and Bmi-1 in regulating self-renewal of normal and malignant human mammary stem cells. We show that hedgehog signaling components PTCH1, Gli1, and Gli2 are highly expressed in normal human mammary stem/progenitor cells cultured as mammospheres and that these genes are down-regulated when cells are induced to differentiate. Activation of hedgehog signaling increases mammosphere-initiating cell number and mammosphere size, whereas inhibition of the pathway results in a reduction of these effects. These effects are mediated by the polycomb gene Bmi-1. Overexpression of Gli2 in mammosphere-initiating cells results in the production of ductal hyperplasia, and modulation of Bmi-1 expression in mammosphere-initiating cells alters mammary development in a humanized nonobese diabetic-severe combined immunodeficient mouse model. Furthermore, we show that the hedgehog signaling pathway is activated in human breast “cancer stem cells” characterized as CD44+CD24−/lowLin−. These studies support a cancer stem cell model in which the hedgehog pathway and Bmi-1 play important roles in regulating self-renewal of normal and tumorigenic human mammary stem cells. (Cancer Res 2006; 66(12): 6063-71)