Identification of vitamin D receptor as a target of p63

Vitamin D receptor cross-talk with p63 signaling promotes epidermal cell fate

The vitamin D receptor with its ligand 1,25 dihydroxy vitamin D3 (1,25D3) regulates epidermal stem cell fate, such that VDR removal from Krt14 expressing keratinocytes delays re-epithelialization of epidermis after wound injury in mice. In this study we deleted Vdr from Lrig1 expressing stem cells in the isthmus of the hair follicle then used lineage tracing to evaluate the impact on re-epithelialization following injury. We showed that Vdr deletion from these cells prevents their migration to and regeneration of the interfollicular epidermis without impairing their ability to repopulate the sebaceous gland. To pursue the molecular basis for these effects of VDR, we performed genome wide transcriptional analysis of keratinocytes from Vdr cKO and control littermate mice. Ingenuity Pathway analysis (IPA) pointed us to the TP53 family including p63 as a partner with VDR, a transcriptional factor that is essential for proliferation and differentiation of epidermal keratinocytes. Epigenetic studies on epidermal keratinocytes derived from interfollicular epidermis showed that VDR is colocalized with p63 within the specific regulatory region of MED1 containing super-enhancers of epidermal fate driven transcription factor genes such as Fos and Jun. Gene ontology analysis further implicated that Vdr and p63 associated genomic regions regulate genes involving stem cell fate and epidermal differentiation. To demonstrate the functional interaction between VDR and p63, we evaluated the response to 1,25(OH)2D3 of keratinocytes lacking p63 and noted a reduction in epidermal cell fate determining transcription factors such as Fos, Jun. We conclude that VDR is required for the epidermal stem cell fate orientation towards interfollicular epidermis. We propose that this role of VDR involves cross-talk with the epidermal master regulator p63 through super-enhancer mediated epigenetic dynamics.

Regulation of XPC Binding Dynamics and Global Nucleotide Excision Repair by p63 and Vitamin D Receptor

p63 and the vitamin D receptor (VDR) play important roles in epidermal development and differentiation, but their roles and relationship in the response to ultraviolet (UV) radiation are less clear. Using TERT-immortalized human keratinocytes expressing shRNA targeting p63 in concert with exogenously applied siRNA targeting VDR, we assessed p63 and VDR's separate and combined effect on nucleotide excision repair (NER) of UV-induced 6-4 photoproducts (6-4PP). Knockdown of p63 reduced VDR and XPC expression relative to nontargeting controls, while knockdown of VDR had no effect on p63 and XPC protein expression, though alone it modestly reduced XPC mRNA. Upon UV irradiation through filters with 3 μm pores to create spatially discrete spots of DNA damage, keratinocytes depleted of p63 or VDR exhibited slower removal of 6-4PP than control cells over the first 30 min. Costaining of control cells with antibodies to XPC revealed that XPC accumulated at DNA damage foci, peaking within 15 min and gradually fading over 90 min as NER proceeded. In either p63- or VDR-depleted keratinocytes, XPC overaccumulated at spots of DNA damage so that 50% more XPC was retained at 15 min and 100% more XPC was retained at 30 min than in control cells, suggesting dissociation of XPC after binding was also delayed. Concurrent knockdown of VDR and p63 resulted in similar impairment of 6-4PP repair and XPC overaccumulation but even slower release of XPC from DNA damage sites such that 200% more XPC was retained relative to controls at 30 min post-UV. These results suggest that VDR accounts for some of p63's effects in delaying 6-4PP repair associated with overaccumulation and slower dissociation of XPC, though p63's regulation of basal XPC expression appears to be VDR-independent. The results are consistent with a model where XPC dissociation is an important step during NER and that failure to do so may inhibit subsequent repair steps. This work further links two important regulators of epidermal growth and differentiation to the DNA repair response to UV.

Role of vitamin D and calcium signaling in epidermal wound healing

PURPOSE

This review will discuss the role of vitamin D and calcium signaling in the epidermal wound response with particular focus on the stem cells of the epidermis and hair follicle that contribute to the wounding response.

METHODS

Selected publications relevant to the mechanisms of wound healing in general and the roles of calcium and vitamin D in wound healing in particular were reviewed.

RESULTS

Following wounding the stem cells of the hair follicle and interfollicular epidermis are activated to proliferate and migrate to the wound where they take on an epidermal fate to re-epithelialize the wound and regenerate the epidermis. The vitamin D and calcium sensing receptors (VDR and CaSR, respectively) are expressed in the stem cells of the hair follicle and epidermis where they play a critical role in enabling the stem cells to respond to wounding. Deletion of Vdr and/or Casr from these cells delays wound healing. The VDR is regulated by co-regulators such as the Med 1 complex and other transcription factors such as Ctnnb (beta-catenin) and p63. The formation of the Cdh1/Ctnn (E-cadherin/catenin) complex jointly stimulated by vitamin D and calcium plays a critical role in the activation, migration, and re-epithelialization processes.

CONCLUSION

Vitamin D and calcium signaling are critical for the ability of epidermal and hair follicle stem cells to respond to wounding. Vitamin D deficiency with the accompanying decrease in calcium signaling can result in delayed and/or chronic wounds, a major cause of morbidity, loss of productivity, and medical expense.

p63-related signaling at a glance

p63 (also known as TP63) is a transcription factor of the p53 family, along with p73. Multiple isoforms of p63 have been discovered and these have diverse functions encompassing a wide array of cell biology. p63 isoforms are implicated in lineage specification, proliferative potential, differentiation, cell death and survival, DNA damage response and metabolism. Furthermore, p63 is linked to human disease states including cancer. p63 is critical to many aspects of cell signaling, and in this Cell science at a glance article and the accompanying poster, we focus on the signaling cascades regulating TAp63 and ΔNp63 isoforms and those that are regulated by TAp63 and ΔNp63, as well the role of p63 in disease.

KMT2D regulates p63 target enhancers to coordinate epithelial homeostasis

In this study, Lin-Shiao et al. identify a novel role for KMT2D, an epigenetic regulator, in coordinating self-renewal, proliferation, and differentiation, as depletion of KMT2D from undifferentiated epidermal keratinocytes results in reduced proliferation, premature spurious activation of terminal differentiation genes, and disorganized epidermal stratification. Their results reveal a critical role for KMT2D in the control of epithelial enhancers and p63 target gene expression, including the requirement of KMT2D for the maintenance of epithelial progenitor gene expression and the coordination of proper terminal differentiation.

Epithelial tissues rely on a highly coordinated balance between self-renewal, proliferation, and differentiation, disruption of which may drive carcinogenesis. The epigenetic regulator KMT2D (MLL4) is one of the most frequently mutated genes in all cancers, particularly epithelial cancers, yet its normal function in these tissues is unknown. Here, we identify a novel role for KMT2D in coordinating this fine balance, as depletion of KMT2D from undifferentiated epidermal keratinocytes results in reduced proliferation, premature spurious activation of terminal differentiation genes, and disorganized epidermal stratification. Genome-wide, KMT2D interacts with p63 and is enriched at its target enhancers. Depletion of KMT2D results in a broad loss of enhancer histone modifications H3 Lys 4 (H3K4) monomethylation (H3K4me1) and H3K27 acetylation (H3K27ac) as well as reduced expression of p63 target genes, including key genes involved in epithelial development and adhesion. Together, these results reveal a critical role for KMT2D in the control of epithelial enhancers and p63 target gene expression, including the requirement of KMT2D for the maintenance of epithelial progenitor gene expression and the coordination of proper terminal differentiation.

Pan-cancer analyses of the nuclear receptor superfamily

Nuclear receptors (NR) act as an integrated conduit for environmental and hormonal signals to govern genomic responses, which relate to cell fate decisions. We review how their integrated actions with each other, shared co-factors and other transcription factors are disrupted in cancer. Steroid hormone nuclear receptors are oncogenic drivers in breast and prostate cancer and blockade of signaling is a major therapeutic goal. By contrast to blockade of receptors, in other cancers enhanced receptor function is attractive, as illustrated initially with targeting of retinoic acid receptors in leukemia. In the post-genomic era large consortia, such as The Cancer Genome Atlas, have developed a remarkable volume of genomic data with which to examine multiple aspects of nuclear receptor status in a pan-cancer manner. Therefore to extend the review of NR function we have also undertaken bioinformatics analyses of NR expression in over 3000 tumors, spread across six different tumor types (bladder, breast, colon, head and neck, liver and prostate). Specifically, to ask how the NR expression was distorted (altered expression, mutation and CNV) we have applied bootstrapping approaches to simulate data for comparison, and also compared these NR findings to 12 other transcription factor families. Nuclear receptors were uniquely and uniformly downregulated across all six tumor types, more than predicted by chance. These approaches also revealed that each tumor type had a specific NR expression profile but these were most similar between breast and prostate cancer. Some NRs were down-regulated in at least five tumor types (e.g. NR3C2/MR and NR5A2/LRH-1)) whereas others were uniquely down-regulated in one tumor (e.g. NR1B3/RARG). The downregulation was not driven by copy number variation or mutation and epigenetic mechanisms maybe responsible for the altered nuclear receptor expression.

1α, 25-Dihydroxyvitamin D₃ and the vitamin D receptor regulates ΔNp63α levels and keratinocyte proliferation

1α, 25-dihydroxyvitamin D₃ (VD₃), a secosteriod that has been explored as an anti-cancer agent, was also shown to promote cell survival. Its receptor, the Vitamin D Receptor (VDR), is a direct target of the proto-oncogene ΔNp63α, which is overexpressed in non-melanoma skin cancers. The interconnection between VDR/VD₃ signaling and ΔNp63α, led us to examine whether VDR/VD₃ signaling promotes keratinocyte proliferation by regulating ΔNp63α levels. Our data demonstrate that VDR regulates ΔNp63α expression at both the transcript and protein level. Interestingly, although low doses of VD₃ led to an increase in ΔNp63α protein levels and keratinocyte proliferation, high doses of VD₃ failed to increase ΔNp63α protein levels and resulted in reduced proliferation. Increased expression of ΔNp63α by low dose VD₃ was shown to be dependent on VDR and critical for the proliferative effects of VD₃. VD₃-mediated increases in ΔNp63α protein levels occur via activation of both p38 MAPK and Akt kinases. Finally, analysis of samples from patients with squamous cell carcinoma (SCC), basal cell carcinoma and precursors to invasive SCC demonstrated a significant correlation between p63 and VDR levels when compared with healthy normal skin control samples. Delineation of the mechanisms by which VD₃ exerts its effect on ΔNp63α and cell proliferation is critical for determining the future of VD₃ in cancer therapies.

Transcription factor p63 bookmarks and regulates dynamic enhancers during epidermal differentiation

The transcription factor p63 plays a pivotal role in keratinocyte proliferation and differentiation in the epidermis. However, how p63 regulates epidermal genes during differentiation is not yet clear. Using epigenome profiling of differentiating human primary epidermal keratinocytes, we characterized a catalog of dynamically regulated genes and p63-bound regulatory elements that are relevant for epithelial development and related diseases. p63-bound regulatory elements occur as single or clustered enhancers, and remarkably, only a subset is active as defined by the co-presence of the active enhancer mark histone modification H3K27ac in epidermal keratinocytes. We show that the dynamics of gene expression correlates with the activity of p63-bound enhancers rather than with p63 binding itself. The activity of p63-bound enhancers is likely determined by other transcription factors that cooperate with p63. Our data show that inactive p63-bound enhancers in epidermal keratinocytes may be active during the development of other epithelial-related structures such as limbs and suggest that p63 bookmarks genomic loci during the commitment of the epithelial lineage and regulates genes through temporal- and spatial-specific active enhancers.

Vitamin D receptor and RXR in the post-genomic era

Following the elucidation of the human genome and components of the epigenome, it is timely to revisit what is known of vitamin D receptor (VDR) function. Early transcriptomic studies using microarray approaches focused on the protein coding mRNA that were regulated by the VDR, usually following treatment with ligand. These studies quickly established the approximate size and surprising diversity of the VDR transcriptome, revealing it to be highly heterogenous and cell type and time dependent. Investigators also considered VDR regulation of non-protein coding RNA and again, cell and time dependency was observed. Attempts to integrate mRNA and miRNA regulation patterns are beginning to reveal patterns of co-regulation and interaction that allow for greater control of mRNA expression, and the capacity to govern more complex cellular events. Alternative splicing in the trasncriptome has emerged as a critical process in transcriptional control and there is evidence of the VDR interacting with components of the splicesome. ChIP-Seq approaches have proved to be pivotal to reveal the diversity of the VDR binding choices across cell types and following treatment, and have revealed that the majority of these are non-canonical in nature. The underlying causes driving the diversity of VDR binding choices remain enigmatic. Finally, genetic variation has emerged as important to impact the transcription factor affinity towards genomic binding sites, and recently the impact of this on VDR function has begun to be considered.

TAp63γ and ΔNp63β promote osteoblastic differentiation of human mesenchymal stem cells: regulation by vitamin D3 Metabolites

The transcription factor p63 is required for skeletal formation, and is important for the regulation of 1α,25(OH)₂D₃ receptor (VDR) in human mesenchymal stem cells (hMSC). Herein we report that TAp63γ and ΔNp63β appear to be an integral part of the osteoblastic differentiation of hMSC and are differentially regulated by the vitamin D₃ metabolites 1α,25(OH)₂D₃ and 24R,25(OH)₂D₃. We compared the endogenous expression of p63 isoforms (TA- and ΔNp63) and splice variants (p63α, -β, -γ), in naive hMSC and during osteoblastic differentiation of hMSC. TAp63α and -β were the predominant p63 variants in naive, proliferating hMSC. In contrast, under osteoblastic differentiation conditions, expression of p63 changed from the TAp63α and -β to the TAp63γ and ΔNp63β variants. Transient overexpression of the p63 variants demonstrated that TAp63β, ΔNp63β, and ΔNp63γ increased alkaline phosphatase activity and ΔNp63α and -γ increased the expression of mRNA for osteocalcin and osterix. Our results support the hypothesis that TAp63α and -β promote a naive state in hMSC. Moreover, TAp63γ is increased during and promotes early osteoblastic differentiation through the expression of pro-osteogenic genes; VDR, Osterix, Runx2 and Osteopontin. ΔNp63β also appears to support osteogenic maturation through increased alkaline phosphatase activity. Treatment with 1α,25(OH)₂D₃ increased the expression of mRNA for ΔNp63, while addition of 24R,25(OH)₂D₃ increased the expression of TA- and ΔNp63γ variants. These novel findings demonstrate for the first time that p63 variants are differentially expressed in naive hMSC (TAp63α,β), are important during the osteoblastic differentiation of hMSC (TAp63γ and ΔNp63β), and are differentially regulated by the vitamin D3 metabolites, 1α,25(OH)₂D₃ and 24R,25(OH)₂D₃. The molecular nuances and mechanisms of osteoblastic differentiation presented here will hopefully improve our understanding of bone development, complications in bone repair (mal- and non-union fractures), osteoporosis and possibly lead to new modalities of treatment.

Cutting edge: progesterone directly upregulates vitamin d receptor gene expression for efficient regulation of T cells by calcitriol

The two nuclear hormone receptor ligands progesterone and vitamin D (vit.D) play important roles in regulating T cells. The mechanism that connects these two hormones in regulating T cells has not been established. In this study, we report that progesterone is a novel inducer of vit.D receptor (VDR) in T cells and makes T cells highly sensitive to calcitriol. At the molecular level, the induction by progesterone is mediated by two progesterone receptor-binding elements in the intron region after the first noncoding exon of the human VDR gene. Increased expression of VDR by progesterone allows highly sensitive regulation of T cells by vit.D even when vit.D levels are suboptimal. This novel regulatory pathway allows enhanced induction of regulatory T cells but suppression of Th1 and Th17 cells by the two nuclear hormones. The results have significant ramifications in effective regulation of T cells to prevent adverse immune responses during pregnancy.

Role of vitamin D3 in modulation of ΔNp63α expression during UVB induced tumor formation in SKH-1 mice

ΔNp63α, a proto-oncogene, is up-regulated in non-melanoma skin cancers and directly regulates the expression of both Vitamin D receptor (VDR) and phosphatase and tensin homologue deleted on chromosome ten (PTEN). Since ΔNp63α has been shown to inhibit cell invasion via regulation of VDR, we wanted to determine whether dietary Vitamin D₃ protected against UVB induced tumor formation in SKH-1 mice, a model for squamous cell carcinoma development. We examined whether there was a correlation between dietary Vitamin D₃ and ΔNp63α, VDR or PTEN expression in vivo in SKH-1 mice chronically exposed to UVB radiation and fed chow containing increasing concentrations of dietary Vitamin D₃. Although we observed differential effects of the Vitamin D₃ diet on ΔNp63α and VDR expression in chronically irradiated normal mouse skin as well as UVB induced tumors, Vitamin D₃ had little effect on PTEN expression in vivo. While low-grade papillomas in mice exposed to UV and fed normal chow displayed increased levels of ΔNp63α, expression of both ΔNp63α and VDR was reduced in invasive tumors. Interestingly, in mice fed high Vitamin D₃ chow, elevated levels of ΔNp63α were observed in both local and invasive tumors but not in normal skin suggesting that oral supplementation with Vitamin D₃ may increase the proliferative potential of skin tumors by increasing ΔNp63α levels.

Vitamin D and the RNA transcriptome: more than mRNA regulation

The GRCh37.p13 primary assembly of the human genome contains 20805 protein coding mRNA, and 37147 non-protein coding genes and pseudogenes that as a result of RNA processing and editing generate 196501 gene transcripts. Given the size and diversity of the human transcriptome, it is timely to revisit what is known of VDR function in the regulation and targeting of transcription. Early transcriptomic studies using microarray approaches focused on the protein coding mRNA that were regulated by the VDR, usually following treatment with ligand. These studies quickly established the approximate size, and surprising diversity of the VDR transcriptome, revealing it to be highly heterogenous and cell type and time dependent. With the discovery of microRNA, investigators also considered VDR regulation of these non-protein coding RNA. Again, cell and time dependency has emerged. Attempts to integrate mRNA and miRNA regulation patterns are beginning to reveal patterns of co-regulation and interaction that allow for greater control of mRNA expression, and the capacity to govern more complex cellular events. As the awareness of the diversity of non-coding RNA increases, it is increasingly likely it will be revealed that VDR actions are mediated through these molecules also. Key knowledge gaps remain over the VDR transcriptome. The causes for the cell and type dependent transcriptional heterogenetiy remain enigmatic. ChIP-Seq approaches have confirmed that VDR binding choices differ very significantly by cell type, but as yet the underlying causes distilling VDR binding choices are unclear. Similarly, it is clear that many of the VDR binding sites are non-canonical in nature but again the mechanisms underlying these interactions are unclear. Finally, although alternative splicing is clearly a very significant process in cellular transcriptional control, the lack of RNA-Seq data centered on VDR function are currently limiting the global assessment of the VDR transcriptome. VDR focused research that complements publically available data (e.g., ENCODE Birney et al., 2007; Birney, 2012), TCGA (Strausberg et al., 2002), GTEx (Consortium, 2013) will enable these questions to be addressed through large-scale data integration efforts.

What is the relationship between ultraviolet B and global incidence rates of colorectal cancer?

The purpose of this study is to examine the relationship between ultraviolet B and global incidence of colorectal cancer, while controlling for relevant covariates. Linear regression was used to assess the relationship between latitude and incidence rates of colon cancer in 173 countries. Multiple linear regression was employed to investigate the relationship between ultraviolet B dose and colorectal cancer rates while controlling for per capita intake of energy from animal sources, per capita health expenditure, pigmentation, and life expectancy. Data on all variables were available for 139 countries. Incidence of colon cancer was highest in countries distant from the equator (R² = 0.50, p < 0.0001). UV B dose (p < 0.0001) was independently, inversely associated with incidence rates of colorectal cancer after controlling for intake of energy from animal sources, per capita health expenditure, pigmentation, and life expectancy (R² for overall model = 0.76, p < 0.0001). Consistent with previous research, UVB was inversely associated with incidence of colon cancer. Further research on vitamin D and prevention of colon cancer in individuals should be conducted, including studies of higher serum 25-hydroxyvitamin D concentrations than have been studied to date.

Delineating Molecular Mechanisms of Squamous Tissue Homeostasis and Neoplasia: Focus on p63

Mouse models have informed us that p63 is critical for normal epidermal development and homeostasis. The p53/p63/p73 family is expressed as multiple protein isoforms due to a combination of alternative promoter usage and C-terminal alternative splicing. These isoforms can mimic or interfere with one another, and their balance ultimately determines biological outcome in a context-dependent manner. While not frequently mutated, p63, and in particular the ΔNp63 subclass, is commonly overexpressed in human squamous cell cancers. In vitro keratinocytes and murine transgenic and transplantation models have been invaluable in elucidating the contribution of altered p63 levels to cancer development, and studies have identified the roles for ΔNp63 isoforms in keratinocyte survival and malignant progression, likely due in part to their transcriptional regulatory function. These findings can be extended to human cancers; for example, the novel recognition of NFκB/c-Rel as a downstream effector of p63 has identified a role for NFκB/c-Rel in human squamous cell cancers. These models will be critical in enhancing the understanding of the specific molecular mechanisms of cancer development and progression.

Vitamin D metabolism and effects on pluripotency genes and cell differentiation in testicular germ cell tumors in vitro and in vivo

Testicular germ cell tumors (TGCTs) are classified as either seminomas or nonseminomas. Both tumors originate from carcinoma in situ (CIS) cells, which are derived from transformed fetal gonocytes. CIS, seminoma, and the undifferentiated embryonal carcinoma (EC) retain an embryonic phenotype and express pluripotency factors (NANOG/OCT4). Vitamin D (VD) is metabolized in the testes, and here, we examined VD metabolism in TGCT differentiation and pluripotency regulation. We established that the VD receptor (VDR) and VD-metabolizing enzymes are expressed in human fetal germ cells, CIS, and invasive TGCTs. VD metabolism diminished markedly during the malignant transformation from CIS to EC but was reestablished in differentiated components of nonseminomas, distinguished by coexpression of mesodermal markers and loss of OCT4. Subsequent in vitro studies confirmed that 1,25(OH)(2)D(3) (active VD) downregulated NANOG and OCT4 through genomic VDR activation in EC-derived NTera2 cells and, to a lesser extent, in seminoma-derived TCam-2 cells, and up-regulated brachyury, SNAI1, osteocalcin, osteopontin, and fibroblast growth factor 23. To test for a possible therapeutic effect in vivo, NTera2 cells were xenografted into nude mice and treated with 1,25(OH)(2)D(3), which induced down-regulation of pluripotency factors but caused no significant reduction of tumor growth. During NTera2 tumor formation, down-regulation of VDR was observed, resulting in limited responsiveness to cholecalciferol and 1,25(OH)(2)D(3) treatment in vivo. These novel findings show that VD metabolism is involved in the mesodermal transition during differentiation of cancer cells with embryonic stem cell characteristics, which points to a function for VD during early embryonic development and possibly in the pathogenesis of TGCTs.

Down-regulation of vitamin D receptor in mammospheres: implications for vitamin D resistance in breast cancer and potential for combination therapy

Vitamin D signaling in mammary cancer stem cells (MCSCs), which are implicated in the initiation and progression of breast cancer, is poorly understood. In this study, we examined vitamin D signaling in mammospheres which are enriched in MCSCs from established breast cancer cell lines. Breast cancer cells positive for aldehyde dehydrogenase (ALDH(+)) had increased ability to form mammospheres compared to ALDH(-) cells. These mammospheres expressed MCSC-specific markers and generated transplantable xenografts in nude mice. Vitamin D receptor (VDR) was significantly down-regulated in mammospheres, as well as in ALDH(+) breast cancer cells. TN aggressive human breast tumors as well as transplantable xenografts obtained from SKBR3 expressed significantly lower levels of VDR but higher levels of CD44 expression. Snail was up-regulated in mammospheres isolated from breast cancer cells. Inhibition of VDR expression by siRNA led to a significant change in key EMT-specific transcription factors and increased the ability of these cells to form mammospheres. On the other hand, over-expression of VDR led to a down-regulation of Snail but increased expression of E-cad and significantly compromised the ability of cells to form mammospheres. Mammospheres were relatively insensitive to treatment with 1,25-dihydroxyvitamin D (1,25D), the active form of vitamin D, compared to more differentiated cancer cells grown in presence of serum. Treatment of H-Ras transformed HMLE(HRas) cells with DETA NONOate, a nitric oxide (NO)-donor led to induction of MAP-kinase phosphatase -1 (MKP-1) and dephosphorylation of ERK1/2 in the mammospheres. Combined treatment of these cells with 1,25D and a low-concentration of DETA NONOate led to a significant decrease in the overall size of mammospheres and reduced tumor volume in nude mice. Our findings therefore, suggest that combination therapy using 1,25D with drugs specifically targeting key survival pathways in MCSCs warrant testing in prospective clinical trial for treatment of aggressive breast cancer.

Pirh2 E3 ubiquitin ligase modulates keratinocyte differentiation through p63

p63, a homologue of the tumor suppressor p53, is essential for the development of epidermis and limb. p63 is highly expressed in epithelial cell layer and acts as a molecular switch that initiates epithelial stratification. However, the mechanisms controlling p63 protein level is still far from fully understood. Here, we demonstrate a regulatory protein for the p63 activity. We found that Pirh2 E3 ubiquitin ligase physically interacts with p63 and targets p63 for polyubiquitination and subsequently proteasomal degradation. We also found that ectopic expression of Pirh2 in HaCaT cells suppresses cell proliferation. Consistent with this, we found that along with altered expression of ΔNp63 protein, ectopic expression of Pirh2 promotes, whereas knockdown of Pirh2 inhibits, keratinocyte differentiation. Collectively, our data suggest that Pirh2 plays a physiologically relevant role in keratinocyte differentiation through posttranslational modification of p63 protein.

Hepatocyte growth factor (HGF) and 1,25-dihydroxyvitamin D together stimulate human bone marrow-derived stem cells toward the osteogenic phenotype by HGF-induced up-regulation of VDR

Bone formation and remodeling require generation of osteoprogenitors from bone marrow stem cells (MSC), which are regulated by growth factors and hormones, with putative roles in mesenchymal cell differentiation. Hepatocyte growth factor (HGF) is a pleiotropic growth factor, and together with its high affinity receptor cMet are widely expressed in normal tissues. 1,25-dihydroxyvitamin D (1,25OHD) is the most active metabolite of vitamin D; produced mainly in the kidney, but also by osteoblasts. We previously reported that HGF and 1,25OHD act together to increase osteogenic differentiation of human MSC (hMSC) potentially through increasing p53. Although p53 does not induce the vitamin D receptor (VDR), p63, a member of the p53 family of transcription factors has been reported to up-regulate VDR expression in some tumor cell lines, and thus might play a part in HGF-regulated VDR expression. Our hypothesis is that the combination of HGF and 1,25OHD can induce hMSC differentiation by up-regulation of 1,25OHD and/or VDR expression to increase cell response(s) to 1,25OHD. Using real-time RT-qPCR, Western blots, luciferase reporter assays, and siRNAs, as well as antibodies to specific signaling molecules we showed that HGF induced VDR gene expression, as well as up-regulated p63 gene expression. p63 gene knockdown by siRNA eliminated the effects of HGF on VDR gene expression as measured by RT-qPCR, Western blots and luciferase reporter assay, and downstream on osteogenic differentiation markers, including alkaline phosphatase staining. Differentiation is a coordinated process of cell cycle exit and tissue-specific gene expression. These results suggest HGF might be a good candidate to coordinate the regulation of these two processes during hMSC osteogenic differentiation. p63 could be a key connecting molecule on the pathway of HGF-induced VDR expression. Understanding the role of these factors and their actions could have important clinical implications for the use of hMSC in the development of novel stem cell therapies.

Matrix metalloproteinase-10 promotes Kras-mediated bronchio-alveolar stem cell expansion and lung cancer formation

Matrix metalloproteinase 10 (MMP-10; stromelysin 2) is a member of a large family of structurally related matrix metalloproteinases, many of which have been implicated in tumor progression, invasion and metastasis. We recently identified Mmp10 as a gene that is highly induced in tumor-initiating lung bronchioalveolar stem cells (BASCs) upon activation of oncogenic Kras in a mouse model of lung adenocarcinoma. However, the potential role of Mmp10 in lung tumorigenesis has not been addressed. Here, we demonstrate that Mmp10 is overexpressed in lung tumors induced by either the smoke carcinogen urethane or oncogenic Kras. In addition, we report a significant reduction in lung tumor number and size after urethane exposure or genetic activation of oncogenic Kras in Mmp10 null (Mmp10^−/−) mice. This inhibitory effect is reflected in a defect in the ability of Mmp10-deficient BASCs to expand and undergo transformation in response to urethane or oncogenic Kras in vivo and in vitro, demonstrating a role for Mmp10 in the tumor-initiating activity of Kras-transformed lung stem cells. To determine the potential relevance of MMP10 in human cancer we analyzed Mmp10 expression in publicly-available gene expression profiles of human cancers. Our analysis reveals that MMP10 is highly overexpressed in human lung tumors. Gene set enhancement analysis (GSEA) demonstrates that elevated MMP10 expression correlates with both cancer stem cell and tumor metastasis genomic signatures in human lung cancer. Finally, Mmp10 is elevated in many human tumor types suggesting a widespread role for Mmp10 in human malignancy. We conclude that Mmp10 plays an important role in lung tumor initiation via maintenance of a highly tumorigenic, cancer-initiating, stem-like cell population, and that Mmp10 expression is associated with stem-like, highly metastatic genotypes in human lung cancers. These results indicate that Mmp10 may represent a novel therapeutic approach to target lung cancer stem cells.

Epigenetic control of a VDR-governed feed-forward loop that regulates p21(waf1/cip1) expression and function in non-malignant prostate cells

In non-malignant RWPE-1 prostate epithelial cells signaling by the nuclear receptor Vitamin D Receptor (VDR, NR1I1) induces cell cycle arrest through targets including CDKN1A (encodes p21^(waf1/cip1)). VDR dynamically induced individual histone modification patterns at three VDR binding sites (R1, 2, 3) on the CDKN1A promoter. The magnitude of these modifications was specific to each phase of the cell cycle. For example, H3K9ac enrichment occurred rapidly only at R2, whereas parallel accumulation of H3K27me3 occurred at R1; these events were significantly enriched in G₁ and S phase cells, respectively. The epigenetic events appeared to allow VDR actions to combine with p53 to enhance p21^(waf1/cip1) activation further. In parallel, VDR binding to the MCM7 gene induced H3K9ac enrichment associated with rapid mRNA up-regulation to generate miR-106b and consequently regulate p21^(waf1/cip1) expression. We conclude that VDR binding site- and promoter-specific patterns of histone modifications combine with miRNA co-regulation to form a VDR-regulated feed-forward loop to control p21^(waf1/cip1) expression and cell cycle arrest. Dissection of this feed-forward loop in a non-malignant prostate cell system illuminates mechanisms of sensitivity and therefore possible resistance in prostate and other VDR responsive cancers.

Modulation of the vitamin D3 response by cancer-associated mutant p53

The p53 gene is mutated in many human tumors. Cells of such tumors often contain abundant mutant p53 (mutp53) protein, which may contribute actively to tumor progression via a gain-of-function mechanism. We applied ChIP-on-chip analysis and identified the vitamin D receptor (VDR) response element as overrepresented in promoter sequences bound by mutp53. We report that mutp53 can interact functionally and physically with VDR. Mutp53 is recruited to VDR-regulated genes and modulates their expression, augmenting the transactivation of some genes and relieving the repression of others. Furthermore, mutp53 increases the nuclear accumulation of VDR. Importantly, mutp53 converts vitamin D into an antiapoptotic agent. Thus, p53 status can determine the biological impact of vitamin D on tumor cells.

p73 is essential for vitamin D-mediated osteoblastic differentiation

The secosteroid hormone vitamin D3 (VD3) exerts its biological actions through its cognate receptor, the vitamin D receptor (VDR). Vitamin D3 and VDR have a key function in bone formation and keratinocyte differentiation, exert antiproliferative actions in human cancer, and is widely used as a chemotherapeutic agent for cancer. In addition, VD3 promotes differentiation of human osteosarcoma cells by up-regulating genes involved in cell cycle arrest and osteoblastic differentiation. Although considerable work has been carried out in understanding the molecular mechanisms underlying the VD3-mediated differentiation of human osteosarcoma cells, the upstream regulation of VD3 signaling pathway is still unclear. In this study, we show that p73 acts as an upstream regulator of VD3-mediated osteoblastic differentiation. Transcription factor p73, a p53 homolog, has been shown to have a function in development and recently been termed as a tumor suppressor. Silencing p73 results in a significant reduction of VD3-mediated osteoblastic differentiation; although DNA damage induced p73 leads to an increase in VD3-mediated differentiation of osteosarcoma cells. Together, our data implicate a novel function for p73 in vitamin D-mediated differentiation of human osteosarcoma cells.

Regulation of VDR by deltaNp63alpha is associated with inhibition of cell invasion

The p63 transcription factor has a pivotal role in epithelial morphogenesis. Multiple transcripts of the TP63 gene are generated because of alternative promoter usage and splicing. DeltaNp63alpha is the predominant isoform of p63 observed during epithelial morphogenesis and in human cancers. Loss of DeltaNp63alpha expression has been shown to promote invasiveness in a subset of human cancer cell lines. Here, we studied whether the regulation of VDR by DeltaNp63alpha controls the invasiveness of an epidermoid cancer cell line. We demonstrate that VDR expression is induced by all p63 isoforms, including DeltaNp63alpha. Endogenous DeltaNp63alpha protein was observed to bind to the VDR promoter, and silencing of endogenous DeltaNp63alpha resulted in diminished VDR expression. Although silencing of p63 inhibits VDR expression leading to an increase in cell migration, overexpression of p63 or VDR results in reduced cell migration as a result of increased VDR expression. Therefore, it is conceivable that p63 inhibits cell invasion by regulating VDR expression. Finally, we observed that expression of p63 and VDR overlaps in the wild-type mouse skin, but a reduced or complete absence of VDR expression was observed in skin from p63-null mice and in p63-null mouse embryonic fibroblasts. In conclusion, we demonstrate a direct transcriptional regulation of VDR by DeltaNp63alpha. Our results highlight a crucial role for VDR in p63-mediated biological functions.

Mammary epithelial cell transformation is associated with deregulation of the vitamin D pathway

The vitamin D endocrine system mediates anti-proliferative and pro-differentiating signaling in multiple epithelial tissues, including mammary gland and breast tumors. The vitamin D metabolite 1alpha,25(OH)2D3 mediates growth inhibitory signaling via activation of the vitamin D receptor (VDR), a ligand dependent transcription factor. 1alpha,25(OH)(2)D3 is synthesized from 25(OH)D3 (the major circulating form of the vitamin) by the mitochondrial enzyme CYP27b1 in renal and other tissues. Human mammary epithelial (HME) cells express VDR and CYP27b1 and undergo growth inhibition when exposed to physiological concentrations of 25(OH)D3, suggesting that autocrine or paracrine vitamin D signaling contributes to maintenance of differentiation and quiescence in the mammary epithelium. In the current studies we tested the hypothesis that cancer cells would exhibit reduced sensitivity to vitamin D mediated negative growth regulation. We used a series of progressively transformed HME cell lines expressing known oncogenic manipulations to study the effects of transformation per se on the vitamin D pathway. We report that mRNA and protein levels of VDR and CYP27b1 were reduced greater than 70% upon stable introduction of known oncogenes (SV40 T antigens and H-rasV12) into HME cells. Oncogenic transformation was also associated with reduced 1alpha,25(OH)2D3 synthesis, and cellular sensitivity to growth inhibition by 1alpha,25(OH)2D3 and 25(OH)D3 was decreased approximately 100-fold in transformed cells. These studies provide evidence that disruption of the vitamin D signaling pathway occurs early in the cancer development process.

Differential effects of p63 mutants on transactivation of p53 and/or p63 responsive genes

p63, known to play a role in development, has more recently also been implicated in cancer progression. Mutations in p63 have been shown to be responsible for several human developmental diseases. Differential splicing of the p63 gene gives rise to p63 isoforms, which can act either as tumor suppressors or as oncogene. In this report, we studied the effects of naturally occurring TAp63gamma mutants on the regulation of p53/p63 and p63 specific target genes. We observed significant differences among p63 mutants to regulate the p53/p63 and p63 specific target genes. Additionally, we observed a differential effect of p63 mutants on wildtype-p63-mediated induction of p53/p63 and p63 specific target genes. We also demonstrated that these mutants differentially regulate the binding of wildtype p63 to the promoter of target genes. Furthermore, the effects of these mutants on cell death and survival were consistent with their ability to regulate the downstream targets when compared to wildtype TAp63gamma. In summary, our data demonstrate that p63 mutants exhibit differential effects on p63 and p53/p63 specific target genes and on the induction of apoptosis, and provide further insight into the function of p63.

Gc-globulin (vitamin D binding protein) is synthesized and secreted by hepatocytes and internalized by hepatic stellate cells through Ca(2+)-dependent interaction with the megalin/gp330 receptor

BACKGROUND

Gc-globulin or vitamin D binding protein is a highly expressed, multifunctional and polymorphic serum protein, which also serves as the major transporter for vitamin D metabolites in the circulation. The present study was performed to analyze the interaction between gc-globulin of hepatocytes and hepatic stellate cells, the most important fat-/retinol-storing cell type in the liver, which spontaneously transdifferentiates to myofibroblasts in culture.

METHODS

Hepatic stellate cells and hepatocytes were isolated by the pronase/collagenase reperfusion method, hepatocytes by collagenase reperfusion of the organ. Gc-globulin expression was monitored by immunocytochemistry, immunoblotting, RT-PCR, metabolic labelling with [(35)S]-methionine, and its intracellular binding to alpha-smooth-muscle actin was investigated by co-immunoprecipitation. Cytoskeletal stainings of gc-globulin and alpha-smooth-muscle actin in hepatic stellate cells and the identification of the receptors megalin/gp330, HCAM/CD44, cubilin and annexin A2 were performed with confocal immunocytochemistry, immunoblotting and/or FACS-analysis.

RESULTS

Hepatocytes synthesize and secrete gc-globulin as shown by RT-PCR and [(35)S]-methionine labelling, which could be suppressed by cycloheximide. Also, a strong signal for gc-globulin was detected in the immunoblot of native hepatic stellate cell lysates. However, no mRNA for gc-globulin was found in this cell type, which suggests no active synthesis by hepatic stellate cells. Hepatic stellate cells were tested positively for the presence of known gc-globulin interacting receptors megalin/gp330, HCAM/CD44, cubilin and annexin A2. Inhibition of the megalin/gp330 receptor by a competitive, neutralizing antibody resulted in decreased intracellular availability of gc-globulin in hepatic stellate cells. The latter effect was enhanced by additional incubation of hepatic stellate cells with EDTA for complexing Ca(2+), suggesting a Ca(2+)-dependent internalization of gc-globulin into hepatic stellate cells via the megalin/gp300 receptor. This was supported by confocal microscopy which showed a co-localization of gc-globulin with the multifunctional megalin/gp330 receptor on this cell type. Inside hepatic stellate cells, a linkage between gc-globulin and alpha-smooth muscle actin filaments of hepatic stellate cells was detected by immunocytochemistry. Intracellular binding of gc-globulin to alpha-smooth-muscle actin filaments was confirmed by co-immunoprecipitation.

CONCLUSION

We give evidence to the expression of the megalin/gp330 receptor on hepatic stellate cells and that this receptor is involved in the Ca(2+)-dependent internalization of gc-globulin into hepatic stellate cells, a protein synthesized and secreted into the extracellular space and circulation by hepatocytes. Inside hepatic stellate cells, it co-localizes with and binds to alpha-smooth muscle actin filaments. Under consideration of the available literature, these findings propose a participation of gc-globulin in hepatic vitamin D metabolism as well as in hepatic stellate cell stability and apoptosis as important mechanisms of liver regeneration.

Reciprocal regulation of p63 by C/EBP delta in human keratinocytes

Background

Genetic experiments have clarified that p63 is a key transcription factor governing the establishment and maintenance of multilayered epithelia. Key to our understanding of p63 strategy is the identification of target genes. We perfomed an RNAi screening in keratinocytes for p63, followed by profiling analysis.

Results

C/EBPδ, member of a family with known roles in differentiation pathways, emerged as a gene repressed by p63. We validated C/EBPδ as a primary target of ΔNp63α by RT-PCR and ChIP location analysis in HaCaT and primary cells. C/EBPδ is differentially expressed in stratification of human skin and it is up-regulated upon differentiation of HaCaT and primary keratinocytes. It is bound to and activates the ΔNp63 promoter. Overexpression of C/EBPδ leads to alteration in the normal profile of p63 isoforms, with the emergence of ΔNp63β and γ, and of the TA isoforms, with different kinetics. In addition, there are changes in the expression of most p63 targets. Inactivation of C/EBPδ leads to gene expression modifications, in part due to the concomitant repression of ΔNp63α. Finally, C/EBPδ is found on the p63 targets in vivo by ChIP analysis, indicating that coregulation is direct.

Conclusion

Our data highlight a coherent cross-talk between these two transcription factors in keratinocytes and a large sharing of common transcriptional targets.

Vitamin D and prevention of breast cancer

Epidemiologic data have demonstrated that breast cancer incidence is inversely correlated with indices of vitamin D status, including ultraviolet exposure, which enhances epidermal vitamin D synthesis. The vitamin D receptor (VDR) is expressed in mammary epithelial cells, suggesting that vitamin D may directly influence sensitivity of the gland to transformation. Consistent with this concept, in vitro studies have demonstrated that the VDR ligand, 1,25-dihydroxyvitamin D (1, 25D), exerts negative growth regulatory effects on mammary epithelial cells that contribute to maintenance of the differentiated phenotype. Furthermore, deletion of the VDR gene in mice alters the balance between proliferation and apoptosis in the mammary gland, which ultimately enhances its susceptibility to carcinogenesis. In addition, dietary supplementation with vitamin D, or chronic treatment with synthetic VDR agonists, reduces the incidence of carcinogen-induced mammary tumors in rodents. Collectively, these observations have reinforced the need to further define the human requirement for vitamin D and the molecular actions of the VDR in relation to prevention of breast cancer.

Differential regulation of vitamin D receptor (VDR) by the p53 Family: p73-dependent induction of VDR upon DNA damage

p63 and p73, members of the p53 family, have been shown to be functionally distinct from p53. Vitamin D receptor (VDR) is a ligand (vitamin D(3))-dependent transcription factor, which is shown to play a major role in calcium homeostasis and keratinocyte differentiation. Vitamin D and its analogues in combination with DNA-damaging agents are extensively used for cancer chemotherapy. In this report, we examined whether p53 affects p63-mediated induction of VDR and studied the effect of DNA damage on VDR induction in p53 null cell lines. Our results demonstrate that p53 itself does not induce VDR expression, nor does it affect p63-mediated VDR induction in the cell lines tested in this study. Furthermore, we observed p53-independent activation of VDR upon DNA damage and associated the induction of VDR to p73. We have demonstrated that ectopic expression of various p73 isoforms can induce VDR expression. Inhibition of p73 in cells treated with DNA-damaging agents exhibited decreased VDR expression. Finally, we show that upon DNA damage, induction of VDR sensitizes the cells to vitamin D treatment. In conclusion, our results indicate that VDR is regulated by p63 and p73 and that the induction of VDR expression upon DNA damage is p73-dependent.

Cellular Expression Levels of the Vitamin D Receptor Are Critical to Its Transcriptional Regulation by the Pituitary Transcription Factor Pit-1

The biological role of 1,25-dihydroxyvitamin D3 has generally been related to calcium homeostasis, but this hormone also has fundamental effects on processes of cellular proliferation and differentiation. The genomic actions of 1,25-dihydroxyvitamin D3 are mediated by the vitamin D receptor (VDR) present in target cells. However, VDR transcriptional regulation is not well understood, probably attributable to the complexity of the VDR gene and its promoter. In the present study, it is demonstrated that administration of the pituitary transcription factor Pit-1 (originally found in the pituitary gland but also present in other nonpituitary cell types and tissues) to the MCF-7 (human breast adenocarcinoma) cell line induces a significant increase in VDR mRNA and protein levels. Conversely, Pit-1-targeted small interference RNA markedly reduced expression of VDR in MCF-7 cells. Reporter gene assays demonstrated that the effect of Pit-1 is mediated by its binding to a region located between −254 and −246 bp from the VDR transcription start site. Selective mutations of this site completely abolished VDR transcription. Chromatin immunoprecipitation analysis showed that binding of Pit-1 to the VDR promoter leads additionally to recruitment of cAMP response element-binding protein binding protein, acetylated histone H4, and RNA polymerase II. Surprisingly, Pit-1 binding also recruits VDR protein to the VDR promoter. Using several cell lines with different levels of VDR expression, it was demonstrated that up-regulation of VDR transcription by Pit-1 is dependent on the presence of VDR protein, suggesting that transcriptional expression of VDR in a given cell type is dependent on, among other factors, its own expression levels.

p63 overexpression induces the expression of Sonic Hedgehog

p63 and p73 are members of the p53 protein family and have been shown to play an important role in cell death, development, and tumorigenesis. In particular, p63 has been shown to be involved in the maintenance of epidermal stem cells and in the stratification of the epidermis. Sonic Hedgehog (Shh) is a morphogen that has also been implicated to play a role in epithelial stem cell proliferation and in the development of organs. Recently, Shh has also been shown to play an important role in the progression of a variety of cancers. In this report, we show that p63 and p73 but not p53 overexpression induces Shh expression. In particular, p63gamma and p63beta (both TA and DeltaN isoforms) and TAp73beta isoform induce Shh. Expression of Shh was found to be significantly reduced in mouse embryo fibroblasts obtained from p63-/- mice. The naturally occurring p63 mutant TAp63gamma(R279H) and the tumor suppressor protein p14(ARF) inhibited the TAp63gamma-mediated transactivation of Shh. The region -228 to -102 bp of Shh promoter was found to be responsive to TAp63gamma-induced transactivation and TAp63gamma binds to regions within the Shh promoter in vivo. The results presented in this study implicate p63 in the regulation of the Shh signaling pathway.