α2‐Adrenoceptor action on cell proliferation and mammary tumour growth in mice

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Breast cancer, the most common cancer in women in most countries, is a highly stressful disease. Catecholamines released during stress bind to adrenoceptors and we have recently described alpha(2)-adrenoceptors in human breast cell lines, linked to enhanced cell proliferation. The purpose was to assess the in vivo effects of compounds acting on alpha(2)-adrenoceptors in a reliable model of breast cancer. The expression of alpha(2)-adrenoceptors was confirmed by immunocytochemistry, immunofluorescence and reverse transcription-PCR in the mouse mammary tumour cell line MC4-L5. Proliferation was assessed by [(3)H]thymidine incorporation and tumours were measured daily. Apoptosis was assessed by terminal deoxynucleotidyl transferase-mediated dUTP digoxigenin nick-end labelling. Incubation for 2 days with alpha(2)-adrenoceptor agonists (clonidine and dexmedetomidine) significantly enhanced proliferation of the mouse mammary tumour cell line MC4-L5. These agonists also significantly stimulated tumour growth of the progestin-dependent tumour C4-HD even in the presence of medroxyprogesterone acetate (MPA). In every tumour tested (C4-HD, CC4-2-HD and CC4-3-HI), regardless of MPA sensitivity, clonidine significantly enhanced tumour growth in the absence of MPA. The alpha(2)-adrenoceptor antagonists, yohimbine and rauwolscine, completely reversed the effects of clonidine. However, the group receiving yohimbine alone showed a nonsignificant but constant increase in tumour growth, whereas rauwolscine alone diminished tumour growth significantly, behaving as a reverse agonist. In CC4-3-HI tumours, rauwolscine treatment enhanced apoptosis and diminished the mitotic index, whereas clonidine had the inverse effect. Alpha(2)-adrenoceptor agonists enhanced tumour growth and rauwolscine behaved in vivo as a reverse agonist, suggesting that it may be tested for adjuvant treatment.