Accelerated apoptosis in the Timp-3-deficient mammary gland

The proapoptotic proteinase inhibitor TIMP-3 is the only molecule of this f amily thought to influence cell death. We examined epithelial apoptosis in TIMP-3-deficient mice during mammary gland involution. Lactation was not af fected by the absence of TIMP-3, but glandular function, as measured by gla nd-to-body weight ratio and production of beta -casein, was suppressed earl ier during post-lactational involution than in controls. Histological exami nation revealed accelerated lumen collapse, alveolar-epithelial loss, and a dipose reconstitution in Timp-3(-/-) females. Epithelial apoptosis peaked o n the first day of involution in Timp-3-null glands but at day 3 in wild-ty pe littermates. Unscheduled activation of gelatinase-A was evident by zymog raphy and correlated with earlier fragmentation of fibronectin in Timp-3(-/ -) mammary. To obtain independent evidence of the proapoptotic effects of T IMP-3 deficiency, we introduced recombinant TIMP-3-releasing pellets into r egressing Timp-3(-/-) mammary tissue and showed that this treatment rescued lumen collapse and epithelial apoptosis. Ex vivo, involuting Timp-3(-/-) m ammary tissue demonstrated accelerated epithelial apoptosis that could be r educed by metalloproteinase inhibition. The physiological relevance of TIMP -3 became apparent as Timp-3(-/-) dams failed to reestablish lactation afte r brief cessation of suckling. Thus, TIMP-3 is a critical epithelial surviv al factor during mammary gland involution.