ERBB3 and breast cancer: Conversely, unlike breast cancer cells, mTOR inhibition downregulates NRG1 expression as well as pERBB3 activation, and treatment with MM-121 was unable to block activation of pAkt T308 induced by mTOR kinase inhibition (Fig. 3C), which was consistent with our RNA-seq results where we noted a decrease in ERBB3 and ERBB4 expression.