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 (s80HER4) that localizes to the nucleus; the physiologic relevance of s80HER4 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 HER4V675A mutant abolishing transmembrane cleavage, were expressed in HC11 cells. HC11 HER4kd or HER4V675A 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 s80HER4 formation. HC11 cells constitutively expressing s80HER4 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 s80HER4 production.