Accumulating evidence indicates that heregulins,
EGF (
epidermal growth factor)-like
ligands, promote
breast cancer cell proliferation and are involved in the progression of
breast cancer towards an aggressive and invasive phenotype. However, there is limited information regarding the molecular mechanisms that mediate these effects. We have recently established that
HRG (
heregulin beta1) promotes
breast cancer cell proliferation and migration via cross-talk with EGFR (
EGF receptor) that involves the activation of the
small GTPase Rac1. In the present paper we report that Rac1 is an essential player for mediating the induction of
cyclin D1 and p21(Cip1) by
HRG in
breast cancer cells. Inhibition of Rac function by expressing either the Rac-GAP (
GTPase-activating protein)
beta2-chimaerin or the dominant-negative Rac mutant N17Rac1, or Rac1 depletion using RNAi (RNA interference), abolished the
cyclin D1 and p21(Cip1) induction by
HRG. Interestingly, the proliferative effect of
HRG was impaired not only when the expression of Rac1 or
cyclin D1 was inhibited, but also when cells were depleted of p21(Cip1) using RNAi. Inhibition of EGFR, PI3K (
phosphoinositide 3-kinase;
kinases required for Rac activation by
HRG) or
MEK [MAPK (
mitogen-activated protein kinase)/ERK (
extracellular-signal-regulated kinase)
kinase] also blocked the up-regulation of
cyclin D1 and p21(Cip1) by
HRG. In addition, we found that
HRG activates
NF-kappaB (
nuclear factor kappaB) in a Rac1- and
MEK-dependent fashion, and inhibition of
NF-kappaB abrogates
cyclin D1/p21(Cip1) induction and proliferation by
HRG. Taken together, these findings establish a central role for Rac1 in the control of
HRG-induced
breast cancer cell-cycle progression and proliferation through up-regulating the expression of
cyclin D1 and p21(Cip1).