Sustained activation of the HER1–ERK1/2–RSK signaling pathway controls myoepithelial cell fate in human mammary tissue

Abstract

Human mammary glands arise from multipotent progenitor cells, which likely respond both to cell-autonomous and to extrinsic cues. However, the identity of these cues and how they might act remain unclear. We analyzed HER1 ligand effects on mammary morphogenesis using a three-dimensional organoid model generated from human breast tissue that recapitulates both qualitatively and quantitatively the normal ductal network in situ. Strikingly, different HER1 ligands generate distinct patterns of cell fate. Epidermal growth factor (EGF) causes a massive expansion of the myoepithelial lineage. Amphiregulin, in contrast, enables normal ductal development. These differences cannot be ascribed to preferential apoptosis or proliferation of differentiated cell populations, but are dependent on HER1 signal intensity. Inhibition of the extracellular signal-regulated kinase 1/2 (ERK1/2) effector RSK prevents the EGF-induced myoepithelial expansion. Notably, mouse mammary organoids are much less responsive to HER1 ligands. Little is known about the myoepithelial lineage or about growth factor effects on mammary progenitor differentiation, and our studies provide an important window into human mammary development that reveals unexpected differences from the mouse model.

An essential role of CBL and CBL-B ubiquitin ligases in mammary stem cell maintenance

“…Among the upstream pathways that might add to increased mTOR activity, ERK is a likely candidate as it is hyperphosphorylated when Cbl and Cbl-b are genetically deleted in HSCs or depleted by shRNA in human MECs (Duan et al, 2011). Culture of human mammary organoids with amphiregulin has been found to maintain MECs in a less differentiated state, whereas growth in EGF led to myoepithelial differentiation; this dichotomy was shown to be related to sustained ERK/RSK signaling by EGF (Pasic et al, 2011).…”

“…Staining of paraffin-embedded MG sections and mammary organoids with fluorescently labeled antibodies was carried out using standard protocols as previously described (Pasic et al, 2011). WB of freshly isolated MG and organoids was performed as described in the supplementary Materials and Methods.…”

The mammary myoepithelial cell

“…On the one hand, Notch signaling directly activates the transcription of smooth muscle-specific genes, whereas, on the other hand, its downstream targets, acting in concert with other signaling pathways, can inhibit the expression of smooth muscle markers inducing dedifferentiation (Morrow et al, 2008). A recent report has suggested a role for the EGFR-ERK1/2 signaling pathway in the control of the propagation of the mammary basal cell population (Pasic et al, 2011) . However, the effects of EGFR ligands on the expression of smooth musclespecific proteins has not been examined in this study.…”

An epigenetically distinct breast cancer cell subpopulation promotes collective invasion

“…The ECM used was a mixture of collagen I and Matrigel (largely collagen IV and laminin), which are constituents of the ECM that surrounds normal mammary epithelial tissue and breast tumors (17,18). This composition of ECM induces mammary epithelial cells to form spherical duct-like structures (19) and sustains the ductal architecture of mouse (12) and human (20) mammary epithelial tissue explants. In addition, basal-type and KRT14-expressing breast cancer cells are not capable of leading invasion into this ECM (13,14).…”