VHL deletion impairs mammary alveologenesis but is not sufficient for mammary tumorigenesis

Effects of hypoxia stress on the milk synthesis in bovine mammary epithelial cells

BACKGROUND

Milk synthesis is an energy-intensive process influenced by oxygen availability. This study investigates how hypoxia affects milk synthesis in BMECs, focusing on key genes involved in lactation and energy metabolism.

METHODS

BMECs were cultured in a normoxic environment and then transferred to a hypoxia chamber with 1% O2 for specified durations. The study evaluated cellular responses through various molecular experiments and RNA sequencing. Small interfering RNA was employed to knock down HIF-1α to investigate whether the lactation-related phenotype alteration depends on HIF-1α.

RESULTS

Hypoxia disrupted milk protein production by reducing mTOR/P70S6K/4EBP1 signaling and downregulating genes critical for amino acid transport and protein synthesis. Triglyceride synthesis increased due to enhanced fatty acid uptake and the upregulation of regulatory proteins, including FASN and PPARγ. Although glucose uptake was elevated under hypoxia, key enzymes for lactose synthesis were downregulated, suggesting a redirection of glucose toward energy production. Mitochondrial function was impaired under hypoxia, with reduced gene expression in TCA cycle, ETC, cytosol-mitochondrial transport, decreased ATP levels, increased ROS levels, and structural alterations. Additionally, lipid synthesis and glucose uptake depend on HIF-1α, while milk protein synthesis alterations occurred independently of HIF-1α.

CONCLUSIONS

Hypoxia alters milk synthesis in BMECs by disrupting milk protein synthesis, enhancing lipid metabolism, and impairing energy production. These findings provide valuable insights into the molecular mechanisms underlying the effect of oxygen deprivation on lactation efficiency, offering potential targets for mitigating hypoxic stress in the mammary glands of dairy animals.

Hypoxia inducible factor signaling in breast tumors controls spontaneous tumor dissemination in a site-specific manner

Hypoxia is a common feature in tumors and induces signaling that promotes tumor cell survival, invasion, and metastasis, but the impact of hypoxia inducible factor (HIF) signaling in the primary tumor on dissemination to bone in particular remains unclear. To better understand the contributions of hypoxia inducible factor 1 alpha (HIF1α), HIF2α, and general HIF pathway activation in metastasis, we employ a PyMT-driven spontaneous murine mammary carcinoma model with mammary specific deletion of Hif1α, Hif2α, or von Hippel-Lindau factor (Vhl) using the Cre-lox system. Here we show that Hif1α or Hif2α deletion in the primary tumor decreases metastatic tumor burden in the bone marrow, while Vhl deletion increases bone tumor burden, as hypothesized. Unexpectedly, Hif1α deletion increases metastatic tumor burden in the lung, while deletion of Hif2α or Vhl does not affect pulmonary metastasis. Mice with Hif1α deleted tumors also exhibit reduced bone volume as measured by micro computed tomography, suggesting that disruption of the osteogenic niche may be involved in the preference for lung dissemination observed in this group. Thus, we reveal that HIF signaling in breast tumors controls tumor dissemination in a site-specific manner.

Differential HIF-1α and HIF-2α Expression in Mammary Epithelial Cells during Fat Pad Invasion, Lactation, and Involution

The development and functional cycle of the mammary gland involves a number of processes that are caricatured by breast cancer cells during invasion and metastasis. Expression of the hypoxia-inducible transcription factors HIF-1 and HIF-2 has been associated with metastatic, poor prognosis, and high-grade breast cancers. Since hypoxia affects normal epithelial differentiation, we hypothesise that HIFs are important for normal breast epithelial development and regeneration as well as cancer initiation and progression. Here, we investigated the expression of the oxygen-sensitive HIF-alpha subunits during mouse mammary gland development, lactation, and involution. In breast epithelial cells, HIF-1α was expressed during early development, prior to cell polarisation. In contrast, expression of HIF-2α occurred later and was restricted to a subpopulation of luminal epithelial cells in the lactating gland. Mammary gland involution is a developmental stage that involves extensive tissue remodelling with cell death but survival of tissue stem/progenitor cells. At this stage, HIF-2α, but little HIF-1α, was expressed in CK14-positive epithelial cells. The temporal but differential expression of the HIF-alpha subunits during the mammary gland life cycle indicates that their expression is controlled by additional factors to hypoxia. Further functional studies of the roles of these proteins in the mammary gland and breast cancer are warranted.

HIF targets in bone remodeling and metastatic disease

The bone marrow is a hypoxic microenvironment that is rich in growth factors and blood vessels and is readily colonized by tumor cells disseminated from numerous cancers including tumors of the breast, prostate, lung, and skin. The origin of metastatic growth promoting factors for tumor cells disseminated to the bone marrow is derived from multiple sources: the bone matrix, which is a reservoir for growth factors, and cells residing in the marrow and along bone surfaces, such as osteoblasts, osteoclasts, macrophages, and T cells, which secrete cytokines and chemokines. Low oxygen levels within the bone marrow induce hypoxia signaling pathways such as hypoxia inducible factor (HIF), which is regulated by oxygen requiring prolyl hydroxylases (PHDs) and von Hippel-Lindau (VHL) tumor suppressor. These hypoxia signaling pathways have profound effects on bone development and homeostasis. Likewise, hypoxic conditions observed in local breast and prostate tumors point to a role for hypoxia-inducible genes in metastasis to and colonization of the bone marrow. This review will explore the role of hypoxia-regulated factors in bone development and remodeling, and how these elements may contribute to solid tumor metastasis to the bone.

Loss of VHL promotes progerin expression, leading to impaired p14/ARF function and suppression of p53 activity

Renal cell carcinomas (RCCs) are frequently occurring genitourinary malignancies in the aged population. A morphological characteristic of RCCs is an irregular nuclear shape, which is used to index cancer grades. Other features of RCCs include the genetic inactivation of the von Hippel-Lindau gene, VHL, and p53 genetic-independent inactivation. An aberrant nuclear shape or p53 suppression has not yet been demonstrated. We examined the effect of progerin (an altered splicing product of the LMNA gene linked to Hutchinson Gilford progeria syndrome; HGPS) on the nuclear deformation of RCCs in comparison to that of HGPS cells. In this study, we showed that progerin was suppressed by pVHL and was responsible for nuclear irregularities as well as p53 inactivation. Thus, progerin suppression can ameliorate nuclear abnormalities and reactivate p53 in response to genotoxic addition. Furthermore, we found that progerin was a target of pVHL E3 ligase and suppressed p53 activity by p14/ARF inhibition. Our findings indicate that the elevated expression of progerin in RCCs results from the loss of pVHL and leads to p53 inactivation through p14/ARF suppression. Interestingly, we showed that progerin was expressed in human leukemia and primary cell lines, raising the possibility that the expression of this LMNA variant may be a common event in age-related cancer progression.

Normal mammary development and function in mice with Ift88 deleted in MMTV- and K14-Cre expressing cells

Background

Primary cilia (PC) are non-motile microtubule based organelles present on almost every cell type and are known to serve as critical organizing centers for several signaling pathways crucial to embryonic and postnatal development. Alterations in the Hh pathway, the most studied signaling pathway regulated by PC, affect mammary gland development as well as maintenance of the stem and progenitor cell populations.

Results

We developed mouse models with deletion of PC in mammary luminal epithelial, basal epithelial, and stromal cells for evaluation of the function of PC in mammary development via MMTV-Cre, K14-Cre, and Prx1-Cre mediated deletion, respectively. The activity of Cre was confirmed using ROSA26 reporters. Mammary stem and progenitor cells were enriched through growth as mammospheres. Adenovirus-Cre mediated deletion of Ift88 was used to determine a role for PC in this population of cells. Disruption of Ift88 and PC were confirmed in using PCR and immunofluorescent methods. Prx1-Cre; Ift88^Del mice demonstrated defects in terminal end buds during puberty. However, these Ift88^Del glands exhibited typical terminal end bud formation as well as normal ductal histology when transplanted into wild type hosts, indicating that the phenotype observed was not intrinsic to the mammary gland. Furthermore, no discernable alterations to mammary development were observed in MMTV-Cre- or K14-Cre; Ift88^Del lines. These mice were able to feed and support several litters of pups even though wide spread depletion of PC was confirmed. Cells grown in mammosphere culture were enriched for PC containing cells suggesting PC are preferentially expressed on mammary stem and progenitor cells. Deletion of Ift88 in mammary epithelial cells resulted in a significant reduction in the number of primary mammospheres established; however, there was no effect on outgrowth of secondary mammospheres in PC-depleted cells.

Conclusions

PC regulate systemic factors that can affect mammary development in early puberty. PC on MMTV- or K14-expressing epithelial cells are not required for normal mammary development or function. PC are expressed at high levels on cells in mammosphere cultures. PC may be required for cells to establish mammospheres in culture; however, PC are not required for renewal of the cultures.

Von Hippel-Lindau protein and respiratory diseases

Von Hippel-Lindau protein (pVHL) was first identified as a tumor suppressor gene as mutations in the VHL gene predispose individuals to systemic benign or malignant tumors and cysts in many organs, including renal cell carcinoma of the clear-cell type and hemangioblastoma. Although pVHL is best known to act as a component of ubiquitin protein ligase for the proteasomal degradation of hypoxia inducible factor (HIF)-α, pVHL also interacts with extracellular matrix proteins and cytoskeleton, regulating extracellular matrix assembly, cell signaling, and many other cellular functions. Recent studies suggest that pVHL contributes to many lung diseases, including pulmonary arterial hypertension, lung cancer, pulmonary fibrosis, and acute respiratory distress syndrome. Mutation or loss of function of pVHL activates HIF and induced expression of vascular endothelial growth factor, endothelin-1, and FoxM1, leading to pulmonary arterial hypertension. Loss of pVHL in lung cancer cells promotes epithelial-mesenchymal transition and cancer migration and invasion while decreasing lung cancer cell proliferation and colonization. In patients of idiopathic pulmonary fibrosis, elevated expression of pVHL induces expression of fibronectin/integrin α5β1/focal adhesion kinase signaling, resulting in fibroproliferation and fibrosis. In alveolar epithelial cells, pVHL mediates Na-K-ATPase degradation in an HIF independent pathway, causing decreased edema clearance during hypoxia. These studies suggest that pVHL plays key roles in the pathogenesis of many lung diseases, and further investigations are warranted to elucidate the underlying molecular mechanisms.

p63, Sharp1, and HIFs: master regulators of metastasis in triple-negative breast cancer

Metastasis is the most significant cause of cancer-associated morbidity and mortality but remains poorly understood. Recent work revealed that metastasis of aggressive triple-negative breast cancers is suppressed by Sharp1, a factor that promotes degradation of hypoxia-inducible factors (HIF) and blunts HIF-induced malignant cell behavior.

Estrogen receptor α is a novel target of the Von Hippel-Lindau protein and is responsible for the proliferation of VHL-deficient cells under hypoxic conditions

The Von Hippel-Lindau gene (VHL) is frequently deleted or mutated in human renal cell carcinoma (RCC) at the early stage. According to the well-established theory, pVHL acts as a tumor suppressor through its E3 ligase activity, which targets hypoxia-inducing factor-1α (HIF-1α). However, the elevated expression of HIF-1α did not promote cell proliferation, indicating that there would be another target, which could promote cell proliferation at the early cancer stage of RCC. In this study, we show that estrogen receptor-α (ER-α) is a novel proteasomal degradation target of the pVHL E3 ligase. Indeed, the overexpression of VHL suppresses exo- and endogenous ER-α expression, whereas si-pVHL can increase ER-α expression. The negative regulation of pVHL on ER-α expression is achieved by its E3 ligase activity. Thus, pVHL can promote the ER-α ubiquitinylation. In addition, we revealed that ER-α and HIF-1α are competitive substrates of pVHL. Thus, under normal conditions, ER-α overexpression can increase the transcription factor activity of HIF-1α. Under the hypoxic condition, where HIF-1α is not a suitable target of pVHL, ER-α is more rapidly degraded by pVHL. However, in VHL-deficient cells, the expression of ER-α and HIF-1α is retained, so that the hypoxic condition did not suppress cell proliferation obviously compared with cells that are expressing pVHL. Thus, blocking of ER-α using its inhibitor could suppress the proliferation of VHL-deficient cells as effectively as hypoxia-induced growth suppression. Considering our results, blocking of ER-α signaling in VHL-deficient cancer cells would be beneficial for cancer suppression. Indeed, we showed the anti-proliferative effect of Faslodex in VHL-deficient cells.

Von Hippel-Lindau disease (VHL): a need for a murine model with retinal hemangioblastoma

Von Hippel-Lindau (VHL) disease is a highly penetrant autosomal dominant systemic malignancy that gives rise to cystic and highly vascularized tumors in a constellation of organs. Patients with VHL disease commonly present with hemangioblastomas in the central nervous system and the eye while other manifestations include pheochromocytoma, clear cell renal cell carcinoma, endolymphatic sac tumors of the middle ear, pancreatic cystadenomas, epididymal and broad ligament cystadenomas. Animal models inactivating the VHL gene product in various organ tissues have been constructed over the past 15 years to parse its HIF-associated mechanisms and its link to tumorigenesis. These models, despite advancing our understanding the molecular role of VHL, are by and large unable to recapitulate the more common features of human VHL disease. Up to date, no model exists that develop retinal hemangioblastomas, the most common clinical manifestation. The purpose of this review is: (1) to discuss the need for an ocular VHL model, (2) to review the animal models that recapitulate clinical VHL disease and (3) to propose potential mechanisms of tumorigenesis for the development of ocular VHL.

Knockdown of von Hippel-Lindau protein decreases lung cancer cell proliferation and colonization

Although von Hippel-Lindau protein (pVHL) is known as a tumor suppressor in kidney and other organs, it remains unclear whether pVHL plays a role in lung cancer development. We investigated the role of pVHL in lung cancer cell proliferation, migration, and colonization using stable A549 cells with knockdown of pVHL. We found that knockdown of pVHL promotes epithelial-mesenchymal transition (EMT) in lung cancer cells. Knockdown of pVHL decreased tumor colonization in a tail-vein injection model and decreased cell proliferation, whereas overexpression of constitutive active HIF increased tumor colonization, suggesting a HIF-independent function of pVHL in lung. Knockdown of pVHL decreased phosphorylation of FAK and expression of integrin, suggesting that pVHL regulates lung cancer development via integrin/FAK signaling pathway.

Novel insight into KLF4 proteolytic regulation in estrogen receptor signaling and breast carcinogenesis

Krüppel-like factor 4 (KLF4), a zinc finger-containing transcriptional factor, is a pivotal regulator of cellular fate. KLF4 has attracted considerable attention for its opposing effect in carcinogenesis as tumor suppressor (e.g. colorectal cancer) or oncoprotein (e.g. breast cancer), depending on tissue context, with the underlying mechanism remaining largely unknown. Here we report that KLF4 mediates estrogen signaling in breast cancer formation. Accumulation of KLF4 by inhibiting its turnover triggers estrogen-induced transactivation. We identified Von Hippel-Lindau, pVHL, as the protein that governs KLF4 turnover in breast cancer cells and demonstrated that estrogen-induced down-regulation of pVHL facilitates accumulation of KLF4. We provide mechanistic insights into KLF4 steady-state degradation as well as its elevation in the presence of estrogen and show that elevated levels of pVHL or depletion of KLF4 attenuates the estrogen-induced transactivation and cell growth. Finally, immunohistochemical staining revealed reduced concentration of pVHL and accumulation of KLF4 in breast cancer tissues. We thus propose that suppression of pVHL in response to estrogen signaling results in elevation of KLF4, which mediates estrogen-induced mitogenic effect.

Hypoxia-inducible factor 1α promotes primary tumor growth and tumor-initiating cell activity in breast cancer

Introduction

Overexpression of the oxygen-responsive transcription factor hypoxia-inducible factor 1α (HIF-1α) correlates with poor prognosis in breast cancer patients. The mouse mammary tumor virus polyoma virus middle T (MMTV-PyMT) mouse is a widely utilized preclinical mouse model that resembles human luminal breast cancer and is highly metastatic. Prior studies in which the PyMT model was used demonstrated that HIF-1α is essential to promoting carcinoma onset and lung metastasis, although no differences in primary tumor end point size were observed. Using a refined model system, we investigated whether HIF-1α is directly implicated in the regulation of tumor-initiating cells (TICs) in breast cancer.

Methods

Mammary tumor epithelial cells were created from MMTV-PyMT mice harboring conditional alleles of Hif1a, followed by transduction ex vivo with either adenovirus β-galactosidase or adenovirus Cre to generate wild-type (WT) and HIF-1α-null (KO) cells, respectively. The impact of HIF-1α deletion on tumor-initiating potential was investigated using tumorsphere assays, limiting dilution transplantation and gene expression analysis.

Results

Efficient deletion of HIF-1α reduced primary tumor growth and suppressed lung metastases, prolonging survival. Loss of HIF-1α led to reduced expression of markers of the basal lineage (K5/K14) in cells and tumors and of multiple genes involved in the epithelial-to-mesenchymal transition. HIF-1α also enhanced tumorsphere formation at normoxia and hypoxia. Decreased expression of several genes in the Notch pathway as well as Vegf and Prominin-1 (CD133)was observed in response to Hif1a deletion. Immunohistochemistry confirmed that CD133 expression was reduced in KO cells and in tumorspheres. Tumorsphere formation was enhanced in CD133^hi versus CD133^neg cells sorted from PyMT tumors. Limiting dilution transplantation of WT and KO tumor cells into immunocompetent recipients revealed > 30-fold enrichment of TICs in WT cells.

Conclusion

These results demonstrate that HIF-1α plays a key role in promoting primary mammary tumor growth and metastasis, in part through regulation of TICs. HIF-1α regulates expression of several members of the Notch pathway, CD133 and markers of the basal lineage in mammary tumors. Our results suggest that CD133, which has not been profiled extensively in breast cancer, may be a useful marker of TICs in the PyMT mouse model. These data reveal for the first time that HIF-1α directly regulates breast TIC activity in vivo.