In this study, we examined the expression and functions of serum amyloid A (SAA) isoforms during apoptosis of HC11 mammary gland epithelial cells. Expression of SAA mRNAs and apoptosis were increased in HC11 cells by serum withdrawal and gradually decreased upon the addition of serum, or epidermal growth factor (EGF). TNFalpha treatment of HC11 cells also induced expression of SAA genes, and the effect on SAA1 and SAA2 expression was suppressed by treatment with MG132, and in cells transfected with a dominant negative mutant form of IkappaBalpha. Similar results were observed in response to interleukin-1 (IL-1), IL-6 and interferon gamma (IFNgamma). Furthermore, overexpression of the SAA1 and SAA2 isoforms suppressed growth and accelerated apoptosis of HC11 cells by increasing caspase 3/7 and caspase 8 activities, but the apoptotic effect of tumor necrosis factor alpha (TNFalpha) on HC11 cells was not enhanced. We found that expression of SAA1 and SAA2, but not SAA3, was regulated by an NFkappaB-dependent pathway, and that overexpression of SAA isoforms accelerated the apoptosis of HC11 cells.