The intracellular domain of ErbB4 induces differentiation of mammary epithelial cells

Cell proliferation in the mammary epithelium is stimulated in part by EGF receptor activation, while differentiation requires ErbB4/HER4, prolactin and STAT5A [1]. Unlike other EGFR family members, HER4 undergoes ligand-dependent transmembrane domain cleavage, releasing a soluble 80 kDa tyrosine kinase (s80^HER4) that localizes to the nucleus; the physiologic relevance of s80^HER4 is unknown [2]. Using HC11 mouse mammary cells, we showed that EGF, HB-EGF and prolactin increased STAT5A phosphotyrosine and promoter transactivation, but only HB-EGF and prolactin induced differentiation markers and organization into polarized three-dimensional, lumen-containing structures in Matrigel. Heregulin did not stimulate lumen formation; rather, it increased and disorganized HC11 cell growth. Selective inhibition of ErbB1/ErbB2 activation unmasked heregulin-dependent differentiation and lumen formation. Kinase-dead HER4, or a HER4^V675A mutant abolishing transmembrane cleavage, were expressed in HC11 cells. HC11 HER4^kd or HER4^V675A cells exhibited impaired HB-EGF and prolactin-dependent STAT5A translocation, promoter activation and lactogenic marker induction, indicating that both differentiation pathways need ErbB4 kinase activity and s80^HER4 formation. HC11 cells constitutively expressing s80^HER4 exhibited basal expression of differentiation markers, increased basal STAT5A activity and three-dimensional lumen formation. These results demonstrate that mammary cell differentiation can be stimulated by HER4 through a process requiring s80^HER4 production.