Impaired cell death and mammary gland involution in the absence of Dock1 and Rac1 signaling

Abstract

Throughout life, the tight equilibrium between cell death and the prompt clearance of dead corpses is required to maintain a proper tissue homeostasis and prevent inflammation. Following lactation, mammary gland involution is triggered and results in the death of excessive epithelial cells that are rapidly cleared by phagocytes to ensure that the gland returns to its prepregnant state. Orthologs of Dock1 (dedicator of cytokinesis 1), Elmo and Rac1 (ras-related C3 botulinum toxin substrate 1) in Caenorhabditis elegans are part of a signaling module in phagocytes that is linking apoptotic cell recognition to cytoskeletal reorganization required for engulfment. In mammals, Elmo1 was shown to interact with the phosphatidylserine receptor Bai1 and relay signals to promote phagocytosis of apoptotic cells. Still, the role of the RacGEF Dock1 in the clearance of dying cells in mammals was never directly addressed. We generated two mouse models with conditional inactivation of Dock1 and Rac1 and revealed that the expression of these genes is not essential in the mammary gland during puberty, pregnancy and lactation. We induced mammary gland involution in these mice to investigate the role of Dock1/Rac1 signaling in the engulfment of cell corpses. Unpredictably, activation of Stat3 (signal transducer and activator of transcription 3), a key regulator of mammary gland involution, was impaired in the absence of Rac1 and Dock1 expression. Likewise, failure to activate properly Stat3 was coinciding with a significant delay in the initiation and progression of mammary gland involution in mutant animals. By using an in vitro phagocytosis assay, we observed that Dock1 and Rac1 are essential to mediate engulfment in epithelial phagocytes. In vivo, cell corpses accumulated at late time points of involution in Dock1 and Rac1 mutant mammary glands. Overall, our study demonstrated an unsuspected role for Dock1/Rac1 signaling in the initiation of mammary gland involution, and also suggested a role for this pathway in the clearance of dead cells by epithelial phagocytes.

Minireview: Mouse Models of Rho GTPase Function in Mammary Gland Development, Tumorigenesis, and Metastasis

“…In support of the idea that Rac1 function is not essential for mammary gland development, deletion of the Rac1 GEFs Tiam1 or Dock1 also did not significantly affect mammary gland development (15,28). However, Dock1 deletion delayed involution.…”

“…This delay was greater than after Rac1 deletion, perhaps reflecting the ability of Dedicator of cytokinesis 1 (Dock1) to activate Rac3 as well as Rac1. Similar to Rac1 deletion, loss of Dock1 delayed Stat3 activation and prevented epithelial cell apoptosis within the alveoli (15). Because Tiam1 and Dock1 function as GEFs for Rac1 subfamily GTPases, these observations indicate that Rac signaling is mainly important during involution.…”

“…Whole body deletion of RhoB or RhoC did not elicit large changes in mammary gland development, as these animals were able to nurse their young (12)(13)(14). Mammary gland-specific deletion of Rac1 also did not affect gland development during puberty or pregnancy (15). This was unexpected, as treatment of organotypic cultures of mammary tissue with the Rac1 inhibitor NSC23766 prevented ductal branching (16).…”

Uterine RAC1 via Pak1-ERM signaling directs normal luminal epithelial integrity conducive to on-time embryo implantation in mice

“…In fact, it has been reported that Rac1 is required for apoptotic cell clearance in bronchial epithelial cells and cell death in mammary gland epithelium. 18,19 Besides, we found that Rac1 mediates TNFα-induced uterine epithelial apoptosis via P38 MAPK signaling. TNFα has been demonstrated to be essential for epithelial cell apoptosis and it could upregulate the level of Rac1-GTP in intestinal epithelial cells.…”

“…Because Ras-related C3 botulinum toxin substrate 1 (Rac1), a small GTPase belonging to the 21 kDa Rho-GTPase family, 16,17 is a multifunctional switch involved in epithelial development, differentiation and apoptotic cell clearance in various systems, [18][19][20][21][22] and Rac1 is also involved in regulating human endometrial stromal cell migration in culture, 23 we speculated that Rac1 and its driving signaling cascade might be an important player directing normal luminal epithelial integrity conducive to on-time embryo implantation. To address this issue, in the present study, we used a uterine Rac1 conditional deletion mouse model and demonstrated that Rac1 depletion in the uterus induces premature decrease of epithelial apical-basal polarity and defective junction remodeling, hampering normal uterine receptivity and thus on-time embryo implantation.…”