Autophagy regulates functional differentiation of mammary epithelial cells.

Mitochondria operate as a central hub for many metabolic processes by sensing and responding to the cellular environment. Developmental cues from the environment have been implicated in selective autophagy, or mitophagy, of mitochondria during cell differentiation and tissue development. Mitophagy occurring in this context, termed programmed mitophagy, responds to cell state rather than mitochondrial damage and is often accompanied by a metabolic transition. However, little is known about the mechanisms that engage and execute mitophagy under physiological or developmental conditions. As the mammary gland undergoes post-natal development and lactation challenges mitochondrial homeostasis, we investigated the contribution of mitochondria to differentiation of mammary epithelial cells (MECs). Using lactogenic differentiation of the HC11 mouse MEC line, we demonstrated that HC11 cells transition to a highly energetic state during differentiation by engaging both oxidative phosphorylation and glycolysis. Interestingly, this transition was lost when autophagy was inhibited with bafilomycin A1 or knockdown of Atg7 (autophagy related 7). To evaluate the specific targeting of mitochondria, we traced mitochondrial oxidation and turnover in vitro with the fluorescent probe, pMitoTimer. Indeed, we found that differentiation engaged mitophagy. To further evaluate the requirement of mitophagy during differentiation, we knocked down the expression of Prkn/parkin in HC11 cells. We found that MEC differentiation was impaired in shPrkn cells, implying that PRKN is required for MEC differentiation. These studies suggest a novel regulation of MEC differentiation through programmed mitophagy and provide a foundation for future studies of development and disease associated with mitochondrial function in the mammary gland.Abbreviations: AA: antimycin A; ATG5: autophagy related 5; BAF: bafilomycin A1; BNIP3: BCL2 interacting protein 3; BNIP3L/NIX: BCL2 interacting protein 3 like; COX8A: cytochrome c oxidase subunit 8A; CQ: chloroquine; CSN2: casein beta; ECAR: extracellular acidification rate; FCCP: trifluoromethoxy carbonylcyanide phenylhydrazone; FUNDC1: FUN14 domain containing 1; HIF1A: hypoxia inducible factor 1 subunit alpha; L1: lactation day 1; MAP1LC3B: microtubule associated protein 1 light chain 3 beta; MEC: mammary epithelial cell; mitoQ: mitoquinol; mROS: mitochondrial reactive oxygen species; OCR: oxygen consumption rate; P: priming; P16: pregnancy day 16; PARP1: poly(ADP-ribose) polymerase 1; PINK1: PTEN induced kinase 1; PPARGC1A: PPARG coactivator 1 alpha; PRKN: parkin RBR E3 ubiquitin protein ligase; shNT: short hairpin non-targeting control; SQSTM1: sequestosome 1; STAT3: signal transducer and activator of transcription 3; TEM: transmission electron microscopy; TFAM: transcription factor A, mitochondrial; U: undifferentiated.

Loss of SIM2s inhibits RAD51 binding and leads to unresolved replication stress.

BACKGROUND: Mutations in genes associated with homologous recombination (HR) increase an individual's risk of developing triple-negative breast cancer (TNBC). Although known for their role in repairing dsDNA breaks, HR repair elements also stabilize and restart stalled replication forks. Essential to these functions are RAD51 and its paralogs, each of which has a unique role in preventing replication fork collapse and restart. However, progress toward understanding the regulation of these factors has been slow. With such a pivotal role in the maintenance of genomic integrity, furthering our understanding of this pathway through the discovery of new factors involved in HR is important. Recently, we showed that singleminded-2s (SIM2s) is stabilized in response to dsDNA breaks and is required for effective HR. METHODS: Initial analysis of the effect loss of SIM2s has on replication stress resolution was conducted using DNA combing assays in established breast cancer cell lines. Further analysis was conducted via immunostaining to determine the effect loss of SIM2s has on factor recruitment. In vivo confirmation was achieved through the use of a mammary epithelial cell conditional knockout mouse model before SIM2s' role in RAD51 recruitment was determined by immunoblotting. RESULTS: Here, we show loss of SIM2s decreases replication fork stability, leading to fork collapse in response to genotoxic stress. Furthermore, loss of SIM2s results in aberrant separation of sister chromatids during mitosis, which has been previously shown to result in chromosomal fragmentation and aneuploidy. Interestingly, loss of SIM2s was shown to result in failure of RAD51 to localize to sites of replication stress in both breast cancer cell lines and primary mammary epithelial cells. Finally, we observed SIM2 is stabilized in response to genotoxic stress and interacts with RAD51, which is necessary for RAD51-DNA binding. CONCLUSIONS: Together, these results show a role for SIM2s in the resolution of replication stress and further characterize the necessity of SIM2s for effective RAD51 loading in response to DNA damage or stress, ultimately promoting genomic integrity and thus preventing the accumulation of cancer-promoting mutations.

ATM-dependent activation of SIM2s regulates homologous recombination and epithelial-mesenchymal transition.

There is increasing evidence that genomic instability is a prerequisite for cancer progression. Here we show that SIM2s, a member of the bHLH/PAS family of transcription factors, regulates DNA damage repair through enhancement of homologous recombination (HR), and prevents epithelial-mesenchymal transitions (EMT) in an Ataxia-telangiectasia mutated (ATM)-dependent manner. Mechanistically, we found that SIM2s interacts with ATM and is stabilized through ATM-dependent phosphorylation in response to IR. Once stabilized, SIM2s interacts with BRCA1 and supports RAD51 recruitment to the site of DNA damage. Loss of SIM2s through the introduction of shSIM2 or the mutation of SIM2s at one of the predicted ATM phosphorylation sites (S115) reduces HR efficiency through disruption of RAD51 recruitment, resulting in genomic instability and induction of EMT. The EMT induced by the mutation of S115 is characterized by a decrease in E-cadherin and an induction of the basal marker, K14, resulting in increased invasion and metastasis. Together, these results identify a novel player in the DNA damage repair pathway and provides a link in ductal carcinoma in situ progression to invasive ductal carcinoma through loss of SIM2s, increased genomic instability, EMT, and metastasis.

PER2 regulation of mammary gland development.

The molecular clock plays key roles in daily physiological functions, development and cancer. Period 2 (PER2) is a repressive element, which inhibits transcription activated by positive clock elements, resulting in diurnal cycling of genes. However, there are gaps in our understanding of the role of the clock in normal development outside of its time-keeping function. Here, we show that PER2 has a noncircadian function that is crucial to mammalian mammary gland development. Virgin Per2-deficient mice, Per2-/- , have underdeveloped glands, containing fewer bifurcations and terminal ducts than glands of wild-type mice. Using a transplantation model, we show that these changes are intrinsic to the gland and further identify changes in cell fate commitment. Per2-/- mouse mammary glands have a dual luminal/basal phenotypic character in cells of the ductal epithelium. We identified colocalization of E-cadherin and keratin 14 in luminal cells. Similar results were demonstrated using MCF10A and shPER2 MCF10A human cell lines. Collectively this study reveals a crucial noncircadian function of PER2 in mammalian mammary gland development, validates the Per2-/- model, and describes a potential role for PER2 in breast cancer.