Breast cancer (BCa) is the most common cancer affecting women worldwide. 
Overexpression of human epidermal growth factor receptor 2 (HER2) occurs in 
~20-25% of invasive ductal breast carcinomas and is associated with the more 
aggressive phenotype. Herceptin, a humanized antibody against HER2, is a 
standard therapy in HER2-overexpressing cases. Approximately one-third of 
patients relapse despite treatment. Therefore numerous studies have investigated 
the molecular mechanisms associated with Herceptin resistance. An interaction 
between HER2 signalling and steroid hormone receptor signalling pathways has 
been previously investigated, but the effect of this relationship on Herceptin 
resistance has never been studied. The present study analysed an impact of the 
steroid hormone, progesterone (PG), on Herceptin-dependent cell growth 
inhibition. Results indicated that Herceptin-inhibited proliferation of breast 
cancer cell lines overexpressing HER2 (BT474 and MCF/HER2) in 3D culture is 
abolished by PG. Furthermore, results demonstrated that PG led to the activation 
of HER2/HER3-mediated signalling. Moreover, PG treatment induced a shift of 
Herceptin-dependent cell cycle arrest in G1 phase towards S and G2 phases with 
concomitant upregulation of cyclin-dependent kinase 2 (CDK2) and downregulation 
of CDK inhibitor p27Kip1. These results demonstrate for the first time PG 
involvement in the failure of Herceptin treatment in vitro. The present 
observations suggest that cross-talk between PG- and HRG/HER2-initiated 
signalling pathways may lead to the acquisition of resistance to Herceptin in 
patients with BCa.