The presence of hormone receptors is related to survival outcome in breast cancer. Previous results from our laboratory established a correlation between the presence of nitric oxide synthase II (NOSII) and nitric oxide (NO) production with progesterone receptors in a series of human breast tumours. Furthermore, this was directly related to a lower tumour grade and a lower proliferation rate of the tumour cells. To examine these results in further detail, the effect of progesterone (Pg) and 17b-oestradiol (E2) on NOSII expression was analysed in the human breast cancer cell line MCF-7. By Northern blot and promoter activity, we show that a cytokine mix (TNF-a, IL-b, and IFN-c) induces NOSII transcription after 6 h stimulation. In the absence of cytokines, neither hormone affects NOSII expression. However, Pg but not E2, enhances cytokine-induced NOSII transcription as well as NO synthesis, mainly by cooperation with gamma-interferon. The increase in NO accumulation in the media induced by addition of Pg to the cytokine treatment significantly increases cell death, mainly accounted for by apoptosis, as compared to the effect of cytokines alone. Our findings help clarify the role of steroid hormones in NOSII expression as well as the effect on cell viability and may suggest novel approaches towards hormonotherapy and the treatment of cancer. Keywords: NOSII; progesterone; transcription; breast cancer; apoptosis; nitric oxide Endogenous nitric oxide (NO) is produced in cells by the nitric oxide synthases (NOS), which exist as three isoforms. The calcium-dependent endothelial (eNOS or NOSIII) and neuronal (nNOS or NOSI) enzymes are constitutively present in various cell types and produce low levels of short acting NO. 1 The third isoform (iNOS or NOSII) is induced in most cell types only in response to external stimuli such as bacterial lipopolysaccharides (LPS) or diverse cytokines, and generates high sustained levels of NO. 2 NO is a transcellular messenger that plays an important role in diverse physiological processes such as vascular homeostasis, immunity and neurotransmission. The effect of endogenous NO on cellular homeostasis is highly dependent on the level of its synthesis. Thus, at low levels it can promote cell proliferation and angiogenesis whereas at high concentrations, such as those produced by NOSII, it can lead to cell damage and death. 3 Therefore, it is important to understand the regulation of the NOS enzymes in order to unravel the role of NO in physiological and pathological conditions. So far it is known that expression of NOSII is regulated mainly at the transcriptional level, although additional post-transcriptional and posttranslational controls have been documented. 4 The most important transcription factors required for NOSII expression are nuclear factor kappa B (NFkB) 5 and activator protein 1 (AP-1). 6 These transcription factors are induced or activated by cytokines such as tumour necrosis alpha (TNF-a), interleukin-1beta (IL-1b), gamma interferon (IFN-g) or LPS. 7 However, differenc...