Phosphatidylinositol-4-phosphate 5-kinase 1alpha mediates extracellular calcium-induced keratinocyte differentiation

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.

The role of PIP5K1A in cancer development and progression

Malignant tumors are formed via a pathological process of uncontrolled cell division that seriously endangers human physical and mental health. The PI3K/AKT signaling pathway plays an important role in the occurrence and development of various cancers. As a lipid kinase, PIP5K1A acts on the upstream of the PI3K/AKT signaling pathway and has a variety of biological functions, including cell differentiation, cell migration, and sperm development. An increasing number of studies have shown that the overexpression of PIP5K1A promotes the growth, invasion, and migration of cancer cells, and the inhibition of PIP5K1A can effectively hinder tumor progression. These findings imply that PIP5K1A are potential markers and targets for cancers. The aim of this study was to systemically review the structure and function of PIP5K1A, the relationship between PIP5K1A and tumors and the potential therapeutic value of PIP5K1A inhibitors in cancer. PIP5K1A affects the occurrence and progression of many tumors and will provide new strategies for cancer diagnosis and treatment.

CircSPAG16 suppresses cadmium-induced transformation of human bronchial epithelial cells by decoying PIP5K1α to inactivate Akt

Circular RNAs (circRNAs) have been implicated to have important regulatory functions in chemical carcinogenesis via sponging microRNAs to regulate gene expression. Our study revealed a novel mechanism of circRNA in cadmium carcinogenesis through directly binding with protein. Here, we used cadmium-transformed human bronchial epithelial BEAS-2B cells to study the involvement and mechanism of circRNA in lung carcinogenesis caused by cadmium. By high-throughput sequencing, circSPAG16 was identified to be the most significantly downregulated circRNA in cadmium-transformed cells. CircSPAG16 was downregulated at Week 8, 12, 16, and 20 during cadmium-induced cell transformation. In addition, circSPAG16 overexpression prevented cadmium-induced transformation of BEAS-2B cells. Mechanistically, circSPAG16 inhibited the function of phosphatidylinositol 4-phosphate 5-kinase type-1 α (PIP5K1α) by binding with it. We demonstrated that PIP5K1α acted as an oncogene to activate Akt and promoted cancer hallmarks including proliferation, migration, invasion, and anchorage-independent growth in cadmium-transformed cells. CircSPAG16 overexpression inactivates PIP5K1α/Akt signaling in the transformed cells. Furthermore, PIP5K1α overexpression significantly rescued the inhibitory effects of circSPAG16 overexpression on pAkt and cancer hallmarks in cadmium-transformed cells. Collectively, our results revealed that circSPAG16 could prevent cadmium-induced transformation through binding with PIP5K1α to inactivate Akt. These results provide a novel regulatory mechanism of circRNA into carcinogenesis induced by cadmium.

Ultraviolet B irradiation-induced keratinocyte senescence and impaired development of 3D epidermal reconstruct

Ultraviolet B (UVB) induces morphological and functional changes of the skin. This study investigated the effect of UVB on keratinocyte senescence and the development of reconstructed human epidermis (RHE). Primary normal human keratinocytes (NHK) from juvenile foreskin were irradiated with UVB (30 mJ cm-2) and these effects were compared to NHK that underwent senescence in the late passage. UVB enhanced the accumulation of reactive oxygen species (ROS) and halted cell replication as detected by BrdU cell proliferation assay. The senescence phenotype was evaluated by beta-galactosidase (β-gal) staining and qPCR of genes related to senescent regulation, i.e. p16INK4a, cyclin D2, and IFI27. Senescence induced by high dose UVB resulted in morphological changes, enhanced β-gal activity, elevated cellular ROS levels and reduced DNA synthesis. qPCR revealed differential expression of the genes regulated senescence. p16INK4a expression was significantly increased in NHK exposed to UVB whereas enhanced IFI27 expression was observed only in cultural senescence. The levels of cyclin D2 expression were not significantly altered either by UVB or long culturing conditions. UVB significantly induced the aging phenotype in keratinocytes and impaired epidermal development. RHE generated from UVB-irradiated keratinocytes showed a thinner cross-sectional structure and the majority of keratinocytes in the lower epidermis were degenerated. The 3D epidermis model is useful in studying the skin aging process.

Study and detection of potential markers for predicting metastasis into lymph nodes in prostate cancer

Hormone-refractory prostate carcinoma has a different cell surface protein profile than hormone-sensitive prostate carcinoma, which provides migration ability and interactions with organs/tissues. Detection and association of these proteins with lymph node metastasis via lymphadenectomy might be beneficial for patients. Gene expression analysis in hormone-refractory and hormone-sensitive commercial cancer cell lines was performed and, after co-cultivation with osteoblasts or endothelial cells, knockdown experiments followed to validate potential biomarkers. "Myeloid-associated differentiation markers, myosin 1b and phosphatidylinositol-4-phosphate-5-kinase type 1 alpha are implicated in metastasis", their knockdown altered the expression of key regulators of endothelial-mesenchymal transition, invasion, motility and migration. In primary prostate tumors, these genes could be an indicator for future metastasis into lymph nodes.

Vitamin D and calcium signaling in epidermal stem cells and their regeneration

Epidermal stem cells (SCs) residing in the skin play an essential role for epidermal regeneration during cutaneous wound healing. Upon injury, distinct epidermal SCs residing in the interfollicular epidermis and/or hair follicles are activated to proliferate. Subsequently, SCs and progeny migrate, differentiate and restore the epidermis. We review a role of the vitamin D signaling through its receptor of vitamin D receptor (Vdr) in these processes. Vdr conditional knockout (cKO) mouse skin experiences a delay in wound re-epithelialization under low dietary calcium conditions, stimulating our efforts to examine a cooperative role of Vdr with calcium signaling through the calcium sensing receptor in the epidermis. We review the role of vitamin D and calcium signaling in different processes essential for injury induced epidermal regeneration during cutaneous wound repair. First, we discuss their roles in self-renewal of epidermal SCs through β-catenin signaling. Then, we describe epidermal remodeling, in which SCs and progeny migrate and differentiate to restore the epidermis, events controlled by the E-cadherin mediated adherens junction signaling. Finally, we discuss the potential mechanisms for vitamin D and calcium signaling to regulate injury induced epidermal regeneration mutually and interdependently.

PIP4Ks Suppress Insulin Signaling through a Catalytic-Independent Mechanism

Insulin stimulates the conversion of phosphatidylino-sitol-4,5-bisphosphate (PI(4,5)P₂) to phosphatidylinositol-3,4,5-trisphosphate (PI(3,4,5)P₃), which mediates downstream cellular responses. PI(4,5)P₂ is produced by phosphatidylinositol-4-phosphate 5-kinases (PIP5Ks) and by phosphatidylinositol-5-phos-phate 4-kinases (PIP4Ks). Here, we show that the loss of PIP4Ks (PIP4K2A, PIP4K2B, and PIP4K2C) in vitro results in a paradoxical increase in PI(4,5)P₂ and a concomitant increase in insulin-stimulated production of PI(3,4,5)P₃. The reintroduction of either wild-type or kinase-dead mutants of the PIP4Ks restored cellular PI(4,5)P₂ levels and insulin stimulation of the PI3K pathway, suggesting a catalytic-independent role of PIP4Ks in regulating PI(4,5)P₂ levels. These effects are explained by an increase in PIP5K activity upon the deletion of PIP4Ks, which normally suppresses PIP5K activity through a direct binding interaction mediated by the N-terminal motif VMLϕFPDD of PIP4K. Our work uncovers an allosteric function of PIP4Ks in suppressing PIP5K-mediated PI(4,5)P₂ synthesis and insulin-dependent conversion to PI(3,4,5)P₃ and suggests that the pharmacological depletion of PIP4K enzymes could represent a strategy for enhancing insulin signaling.

PI(4,5)P₂ is produced by both phosphatidylinositol-4-phosphate 5-kinases (PIP5Ks) and by phosphatidylinositol-5-phosphate 4-kinases (PIP4Ks). Wang et al. report an allosteric function of a conserved N-terminal motif of PIP4Ks in suppressing PIP5K-mediated PI(4,5)P₂ synthesis and insulin-dependent conversion to PI(3,4,5) P₃. This non-catalytic role has implications for the development of PIP4K targeted therapies.

Genomic DNA methylation in HLA-Cw*0602 carriers and non-carriers of psoriasis

BACKGROUND

HLA-Cw*0602 has long been established as one of the most important genetic biomarkers in psoriasis. However, the epigenetic and gene expression differences between HLA-Cw*0602 carriers and non-carriers has not yet been investigated.

OBJECTIVE

We aim to explore the whole-genome methylation and gene expression differences between HLA-Cw*0602 carriers and non-carriers.

METHODS

HLA imputation was performed to get landscape of variants in this region. Genome-wide DNA methylation was compared between positive and negative HLA-Cw*0602 groups. Eleven methylation loci were selected for further validation in additional 43 cases. For differentially methylated genes, GO and KEGG were used to annotate gene functions.

RESULTS

We imputed 29,948 variants based on the constructed HLA reference panels, and obtained 42 HLA-Cw*0602 carriers and 72 non-carriers. Significant methylation differences were detected at 4321 sites (811 hypo- and 3510 hypermethylated). The cg02607779 (KLF7, P = 0.001), cg06936779 (PIP5K1A, P = 0.002), cg03860400 (BTBD10, P = 0.017) and cg26112390 (GOLGA2P5, P = 0.019) were identified and validated to be the significant CpGs contributed to different HLA-C*0602 groups. Among the hypo- and hypermethylated sites, the top CpGs were in gene body and CpG island.

CONCLUSION

We performed the first whole-genome study on methylation differences between psoriatic individuals with or without HLA-Cw*0602, and found the key methylation sites which may contribute to the carrying status of HLA-Cw*0602. Methylation loci located in gene body and CpG island are more likely to affect the methylation levels in HLA-Cw*0602 carriers. This integrated analysis shed light on novel insights into the pathogenic mechanisms of genomic methylation in different HLA genotypes of psoriasis.

p120-catenin suppresses proliferation and tumor growth of oral squamous cell carcinoma via inhibiting nuclear phospholipase C-γ1 signaling

p120-catenin (p120) serves as a stabilizer of the calcium-dependent cadherin-catenin complex and loss of p120 expression has been observed in several types of human cancers. The p120-dependent E-cadherin-β-catenin complex has been shown to mediate calcium-induced keratinocyte differentiation via inducing activation of plasma membrane phospholipase C-γ1 (PLC-γ1). On the other hand, PLC-γ1 has been shown to interact with phosphatidylinositol 3-kinase enhancer in the nucleus and plays a critical role in epidermal growth factor-induced proliferation of oral squamous cell carcinoma (OSCC) cells. To determine whether p120 suppresses OSCC proliferation and tumor growth via inhibiting PLC-γ1, we examined effects of p120 knockdown or p120 and PLC-γ1 double knockdown on proliferation of cultured OSCC cells and tumor growth in xenograft OSCC in mice. The results showed that knockdown of p120 reduced levels of PLC-γ1 in the plasma membrane and increased levels of PLC-γ1 and its signaling in the nucleus in OSCC cells and OSCC cell proliferation as well as xenograft OSCC tumor growth. However, double knockdown of p120 and PLC-γ1 or knockdown of PLC-γ1 alone did not have any effect. Immunohistochemical analysis of OSCC tissue from patients showed a lower expression level of p120 and a higher expression level of PLC-γ1 compared with that of adjacent noncancerous tissue. These data indicate that p120 suppresses OSCC cell proliferation and tumor growth by inhibiting signaling mediated by nuclear PLC-γ1.

New aspects of vitamin D metabolism and action - addressing the skin as source and target

Vitamin D has a key role in stimulating calcium absorption from the gut and promoting skeletal health, as well as many other important physiological functions. Vitamin D is produced in the skin. It is subsequently metabolized to its hormonally active form, 1,25-dihydroxyvitamin D (1,25(OH)₂D), by the 1-hydroxylase and catabolized by the 24-hydroxylase. In this Review, we pay special attention to the effect of mutations in these enzymes and their clinical manifestations. We then discuss the role of vitamin D binding protein in transporting vitamin D and its metabolites from their source to their targets, the free hormone hypothesis for cell entry and HSP70 for intracellular transport. This is followed by discussion of the vitamin D receptor (VDR) that mediates the cellular actions of 1,25(OH)₂D. Cell-specific recruitment of co-regulatory complexes by liganded VDR leads to changes in gene expression that result in distinct physiological actions by 1,25(OH)₂D, which are disrupted by mutations in the VDR. We then discuss the epidermis and hair follicle, to provide a non-skeletal example of a tissue that expresses VDR that not only makes vitamin D but also can metabolize it to its hormonally active form. This enables vitamin D to regulate epidermal differentiation and hair follicle cycling and, in so doing, to promote barrier function, wound healing and hair growth, while limiting cancer development.

CD9 regulates keratinocyte differentiation and motility by recruiting E-cadherin to the plasma membrane and activating the PI3K/Akt pathway

During keratinocyte stratification and wound healing, keratinocytes undergo a switch between differentiation and motility. However, limited knowledge exists on the mechanisms of the switch. We have previously demonstrated that the expression of CD9 was changed in different wound stages and involved in the regulation of keratinocyte migration. In this study, we showed that CD9 expression was increased in both human and mouse keratinocytes undergoing differentiation. CD9 overexpression in keratinocytes stimulated terminal differentiation and reduced cell motility. CD9 silencing inhibited calcium-induced keratinocyte differentiation and increased cell motility. Furthermore, CD9 overexpression recruited E-cadherin to the plasma membrane and subsequently activated PI3K/Akt signaling, while CD9 knockdown inhibited the recruitment of E-cadherin to the plasma membrane and PI3K/Akt activation. Importantly, silencing E-cadherin expression or inhibiting PI3K/Akt signaling reversed CD9 overexpression-induced differentiation and -reduced motility. These results demonstrate that CD9 acts as an important node that regulates keratinocyte differentiation and motility. The recruitment of E-cadherin to the plasma membrane and activation of the PI3K/Akt signaling pathway mediated by CD9 play an important role in these processes.

Calcium-Sensing Receptor Regulates Epidermal Intracellular Ca2+ Signaling and Re-Epithelialization after Wounding

Extracellular Ca2+ (Ca2+o) is a crucial regulator of epidermal homeostasis and its receptor, the Ca2+-sensing receptor (CaSR), conveys the Ca2+o signals to promote keratinocyte adhesion, differentiation, and survival via activation of intracellular Ca2+ (Ca2+i) and E-cadherin-mediated signaling. Here, we took genetic loss-of-function approaches to delineate the functions of CaSR in wound re-epithelialization. Cutaneous injury triggered a robust CaSR expression and a surge of Ca2+i in epidermis. CaSR and E-cadherin were co-expressed at the cell-cell membrane between migratory keratinocytes in the nascent epithelial tongues. Blocking the expression of CaSR or E-cadherin in cultured keratinocytes markedly inhibited the wound-induced Ca2+i propagation and their ability to migrate collectively. Depleting CaSR also suppressed keratinocyte proliferation by downregulating the E-cadherin/epidermal growth factor receptor/mitogen-activated protein kinase signaling axis. Blunted epidermal Ca2+i response to wounding and retarded wound healing were observed in the keratinocyte-specific CaSR knockout (EpidCasr-/-) mice, whose shortened neo-epithelia exhibited declined E-cadherin expression and diminished keratinocyte proliferation and differentiation. Conversely, stimulating endogenous CaSR with calcimimetic NPS-R568 accelerated wound re-epithelialization through enhancing the epidermal Ca2+i signals and E-cadherin membrane expression. These findings demonstrated a critical role for the CaSR in epidermal regeneration and its therapeutic potential for improving skin wound repair.

HPV8-E6 Interferes with Syntenin-2 Expression through Deregulation of Differentiation, Methylation and Phosphatidylinositide-Kinase Dependent Mechanisms

The E6 oncoproteins of high-risk human papillomaviruses (HPV) of genus alpha contain a short peptide sequence at the carboxy-terminus, the PDZ binding domain, with which they interact with the corresponding PDZ domain of cellular proteins. Interestingly, E6 proteins from papillomaviruses of genus beta (betaPV) do not encode a comparable PDZ binding domain. Irrespective of this fact, we previously showed that the E6 protein of HPV8 (betaPV type) could circumvent this deficit by targeting the PDZ protein Syntenin-2 through transcriptional repression (Lazic et al., 2012). Despite its high binding affinity to phosphatidylinositol-4,5-bisphosphate (PI(4,5)P₂), very little is known about Syntenin-2. This study aimed to extend the knowledge on Syntenin-2 and how its expression is controlled. We now identified that Syntenin-2 is expressed at high levels in differentiating and in lower amounts in keratinocytes cultured in serum-free media containing low calcium concentration. HPV8-E6 led to a further reduction of Syntenin-2 expression only in cells cultured in low calcium. In the skin of patients suffering from Epidermodysplasia verruciformis, who are predisposed to betaPV infection, Syntenin-2 was expressed in differentiating keratinocytes of non-lesional skin, but was absent in virus positive squamous tumors. Using 5-Aza-2′-deoxycytidine, which causes DNA demethylation, Syntenin-2 transcription was profoundly activated and fully restored in the absence and presence of HPV8-E6, implicating that E6 mediated repression of Syntenin-2 transcription is due to promoter hypermethylation. Since Syntenin-2 binds to PI(4,5)P₂, we further tested whether the PI(4,5)P₂ metabolic pathway might govern Syntenin-2 expression. PI(4,5)P₂ is generated by the activity of phosphatidylinositol-4-phosphate-5-kinase type I (PIP5KI) or phosphatidylinositol-5-phosphate-4-kinase type II (PIP4KII) isoforms α, β and γ. Phosphatidylinositide kinases have recently been identified as regulators of gene transcription. Surprisingly, transfection of siRNAs directed against PIP5KI and PIP4KII resulted in higher Syntenin-2 expression with the highest effect mediated by siPIP5KIα. HPV8-E6 was able to counteract siPIP4KIIα, siPIP4KIIβ and siPIP5KIγ mediated Syntenin-2 re-expression but not siPIP5KIα. Finally, we identified Syntenin-2 as a key factor regulating PIP5KIα expression. Collectively, our data demonstrates that Syntenin-2 is regulated through multiple mechanisms and that downregulation of Syntenin-2 expression may contribute to E6 mediated dedifferentiation of infected skin cells.

Combined Deletion of the Vitamin D Receptor and Calcium-Sensing Receptor Delays Wound Re-epithelialization

When the skin is injured, keratinocytes proliferate, migrate, and differentiate to regenerate the epidermis. We recently showed that ablation of the vitamin D receptor (Vdr) in keratinocytes delays wound re-epithelialization in mice also fed a low-calcium diet, implicating a cooperative role of Vdr and calcium signaling in this process. In this study, we examined the role of vitamin D and calcium signaling in wound healing by deleting their receptors, Vdr and the calcium-sensing receptor (Casr). Gene expression profiling of neonatal epidermis lacking both Vdr and Casr [Vdr and Casr double knockout (DKO)] specifically in keratinocytes revealed that DKO affects a number of pathways relevant to wound healing, including Vdr, β-catenin, and adherens junction (AJ) signaling. In adult skin, DKO caused a significant delay in wound closure and re-epithelialization, whereas myofibroblast numbers and matrix deposition were unaffected. The injury-induced proliferation of epidermal keratinocytes was blunted in both epidermis and hair follicles, and expression of β-catenin target genes was reduced in the DKO. Expression of E-cadherin and desmoglein 1 was reduced in the shortened leading edges of the epithelial tongues re-epithelializing the wounds, consistent with the decreased migration rate of DKO keratinocytes in vitro. These results demonstrate that Vdr and Casr are required for β-catenin-regulated cell proliferation and AJ formation essential for re-epithelialization after wounding. We conclude that vitamin D and calcium signaling in keratinocytes are required for a normal regenerative response of the skin to wounding.

ISA-2011B, a Phosphatidylinositol 4-Phosphate 5-Kinase α Inhibitor, Impairs CD28-Dependent Costimulatory and Pro-inflammatory Signals in Human T Lymphocytes

Phosphatidylinositol 4,5-biphosphate (PIP2) is a membrane phospholipid that controls the activity of several proteins regulating cytoskeleton reorganization, cytokine gene expression, T cell survival, proliferation, and differentiation. Phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks) are the main enzymes involved in PIP2 biosynthesis by phosphorylating phosphatidylinositol 4-monophosphate (PI4P) at the D5 position of the inositol ring. In human T lymphocytes, we recently found that CD28 costimulatory molecule is pivotal for PIP2 turnover by recruiting and activating PIP5Kα. We also found that PIP5Kα is the main regulator of both CD28 costimulatory signals integrating those delivered by TCR as well as CD28 autonomous signals regulating the expression of pro-inflammatory genes. Given emerging studies linking alterations of PIP2 metabolism to immune-based diseases, PIP5Kα may represent a promising target to modulate immunity and inflammation. Herewith, we characterized a recently discovered inhibitor of PIP5Kα, ISA-2011B, for its inhibitory effects on T lymphocyte functions. We found that the inhibition of PIP5Kα lipid-kinase activity by ISA-2011B significantly impaired CD28 costimulatory signals necessary for TCR-mediated Ca²⁺ influx, NF-AT transcriptional activity, and IL-2 gene expression as well as CD28 autonomous signals regulating the activation of NF-κB and the transcription of pro-inflammatory cytokine and chemokine genes. Moreover, our data on the inhibitory effects of ISA-2011B on CD28-mediated upregulation of inflammatory cytokines related to Th17 cell phenotype in type 1 diabetes patients suggest ISA-2011B as a promising anti-inflammatory drug.

Phospholipase Cδ1 regulates p38 MAPK activity and skin barrier integrity

Keratinocytes undergo a unique type of programmed cell death known as cornification, which leads to the formation of the stratum corneum (SC), the main physical barrier of the epidermis. A defective epidermal barrier is a hallmark of the two most common inflammatory skin disorders, psoriasis, and atopic dermatitis. However, the detailed molecular mechanisms of skin barrier formation are not yet fully understood. Here, we showed that downregulation of phospholipase C (PLC) δ1, a Ca²⁺-mobilizing and phosphoinositide-metabolizing enzyme abundantly expressed in the epidermis, impairs the barrier functions of the SC. PLCδ1 downregulation also impairs localization of tight junction proteins. Loss of PLCδ1 leads to a decrease in intracellular Ca²⁺ concentrations and nuclear factor of activated T cells activity, along with hyperactivation of p38 mitogen-activated protein kinase (MAPK) and inactivation of RhoA. Treatment with a p38 MAPK inhibitor reverses the barrier defects caused by PLCδ1 downregulation. Interestingly, this treatment also attenuates psoriasis-like skin inflammation in imiquimod-treated mice. These findings demonstrate that PLCδ1 is essential for epidermal barrier integrity. This study also suggests a possible link between PLCδ1 downregulation, p38 MAPK hyperactivation, and barrier defects in psoriasis-like skin inflammation.

Disruption of Vitamin D and Calcium Signaling in Keratinocytes Predisposes to Skin Cancer

1,25 dihydroxyvitamin D (1,25(OH)₂D), the active metabolite of vitamin D, and calcium regulate epidermal differentiation. 1,25(OH)₂D exerts its effects through the vitamin D receptor (VDR), a transcription factor in the nuclear hormone receptor family, whereas calcium acts through the calcium sensing receptor (Casr), a membrane bound member of the G protein coupled receptor family. We have developed mouse models in which the Vdr and Casr have been deleted in the epidermis (^epidVdr^−∕− and ^epidCasr^−∕−). Both genotypes show abnormalities in calcium induced epidermal differentiation in vivo and in vitro, associated with altered hedgehog (HH) and β–catenin signaling that when abnormally expressed lead to basal cell carcinomas (BCC) and trichofolliculomas, respectively. The Vdr^−∕− mice are susceptible to tumor formation following UVB or chemical carcinogen exposure. More recently we found that the keratinocytes from these mice over express long non-coding RNA (lncRNA) oncogenes such as H19 and under express lncRNA tumor suppressors such as lincRNA-21. Spontaneous tumors have not been observed in either the ^epidVdr^−∕− or ^epidCasr^−∕−. But in mice with epidermal specific deletion of both Vdr and Casr (^epidVdr^−∕−/^epidCasr^−∕− [DKO]) tumor formation occurs spontaneously when the DKO mice are placed on a low calcium diet. These results demonstrate important interactions between vitamin D and calcium signaling through their respective receptors that lead to cancer when these signals are disrupted. The roles of the β–catenin, hedgehog, and lncRNA pathways in predisposing the epidermis to tumor formation when vitamin D and calcium signaling are disrupted will be discussed.

Control of diverse subcellular processes by a single multi-functional lipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]

Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] is a multi-functional lipid that regulates several essential subcellular processes in eukaryotic cells. In addition to its well-established function as a substrate for receptor-activated signalling at the plasma membrane (PM), it is now recognized that distinct PI(4,5)P2 pools are present at other organelle membranes. However, a long-standing question that remains unresolved is the mechanism by which a single lipid species, with an invariant functional head group, delivers numerous functions without loss of fidelity. In the present review, we summarize studies that have examined the molecular processes that shape the repertoire of PI(4,5)P2 pools in diverse eukaryotes. Collectively, these studies indicate a conserved role for lipid kinase isoforms in generating functionally distinct pools of PI(4,5)P2 in diverse metazoan species. The sophistication underlying the regulation of multiple functions by PI(4,5)P2 is also shaped by mechanisms that regulate its availability to enzymes involved in its metabolism as well as molecular processes that control its diffusion at nanoscales in the PM. Collectively, these mechanisms ensure the specificity of PI(4,5)P2 mediated signalling at eukaryotic membranes.

Phosphoprotein Phosphatase 1 Is Required for Extracellular Calcium-Induced Keratinocyte Differentiation

Extracellular calcium is a major regulator of keratinocyte differentiation in vitro and appears to play that role in vivo, but the mechanism is unclear. We have previously demonstrated that, following calcium stimulation, PIP5K1α is recruited by the E-cadherin-β-catenin complex to the plasma membrane where it provides the substrate PIP2 for both PI3K and PLC-γ1. This signaling pathway is critical for calcium-induced generation of second messengers including IP3 and intracellular calcium and keratinocyte differentiation. In this study, we explored the upstream regulatory mechanism by which calcium activates PIP5K1α and the role of this activation in calcium-induced keratinocyte differentiation. We found that treatment of human keratinocytes in culture with calcium resulted in an increase in serine dephosphorylation and PIP5K1α activation. PP1 knockdown blocked extracellular calcium-induced increase in serine dephosphorylation and activity of PIP5K1α and induction of keratinocyte differentiation markers. Knockdown of PLC-γ1, the downstream effector of PIP5K1α, blocked upstream dephosphorylation and PIP5K1α activation induced by calcium. Coimmunoprecipitation revealed calcium induced recruitment of PP1 to the E-cadherin-catenin-PIP5K1α complex in the plasma membrane. These results indicate that PP1 is recruited to the extracellular calcium-dependent E-cadherin-catenin-PIP5K1α complex in the plasma membrane to activate PIP5K1α, which is required for PLC-γ1 activation leading to keratinocyte differentiation.

Phosphatidylinositol 4-Phosphate 5-Kinases in the Regulation of T Cell Activation

Phosphatidylinositol 4,5-biphosphate kinases (PIP5Ks) are critical regulators of T cell activation being the main enzymes involved in the synthesis of phosphatidylinositol 4,5-biphosphate (PIP2). PIP2 is indeed a pivotal regulator of the actin cytoskeleton, thus controlling T cell polarization and migration, stable adhesion to antigen-presenting cells, spatial organization of the immunological synapse, and co-stimulation. Moreover, PIP2 also serves as a precursor for the second messengers inositol triphosphate, diacylglycerol, and phosphatidylinositol 3,4,5-triphosphate, which are essential for the activation of signaling pathways regulating cytokine production, cell cycle progression, survival, metabolism, and differentiation. Here, we discuss the impact of PIP5Ks on several T lymphocyte functions with a specific focus on the role of CD28 co-stimulation in PIP5K compartimentalization and activation.

Lipid kinases as therapeutic targets for chronic pain

Existing analgesics are not efficacious in treating all patients with chronic pain and have harmful side effects when used long term. A deeper understanding of pain signaling and sensitization could lead to the development of more efficacious analgesics. Nociceptor sensitization occurs under conditions of inflammation and nerve injury where diverse chemicals are released and signal through receptors to reduce the activation threshold of ion channels, leading to an overall increase in neuronal excitability. Drugs that inhibit specific receptors have so far been unsuccessful in alleviating pain, possibly because they do not simultaneously target the diverse receptors that contribute to nociceptor sensitization. Hence, the focus has shifted toward targeting downstream convergence points of nociceptive signaling. Lipid mediators, including phosphatidylinositol 4,5-bisphosphate (PIP2), are attractive targets, as these molecules are required for signaling downstream of G-protein-coupled receptors and receptor tyrosine kinases. Furthermore, PIP2 regulates the activity of various ion channels. Thus, PIP2 sits at a critical convergence point for multiple receptors, ion channels, and signaling pathways that promote and maintain chronic pain. Decreasing the amount of PIP2 in neurons was recently shown to attenuate pronociceptive signaling and could provide a novel approach for treating pain. Here, we review the lipid kinases that are known to regulate pain signaling and sensitization and speculate on which additional lipid kinases might regulate signaling in nociceptive neurons.

IQGAP1 is a phosphoinositide effector and kinase scaffold

Phosphatidylinositol 4,5-bisphosphate (PI4,5P2) is a lipid messenger that regulates a wide variety of cellular functions. The majority of cellular PI4,5P2 is generated by isoforms of the type I phosphatidylinositol phosphate kinases (PIPKI) that are generated from three genes, and each PIPKI isoform has a unique distribution and function in cells. It has been shown that the signaling specificity of PI4,5P2 can be determined by a physical association of PIPKs with PI4,5P2 effectors. IQGAP1 is newly identified as an interactor of multiple isoforms of PIPKs. Considering the versatile roles of IQGAP1 in cellular signaling pathways, IQGAP1 may confer isoform-specific roles of PIPKs in distinct cellular locations. In this mini review, the emerging roles of PIPKs that are regulated by an association with IQGAP1 will be summarized. Focuses will be on cell migration, vesicle trafficking, cell signaling, and nuclear events.

In serum, higher parathyroid hormone but not lower vitamin D is associated with oral squamous cell carcinoma

INTRODUCTION

Vitamin D and calcium are known to regulate differentiation and proliferation of keratinocytes; they might potentially have a role in suppressing carcinogenesis in squamous epithelium. Serum parathyroid hormone (pth) is a sensitive indicator of calcium and vitamin D deficiency, and 25-hydroxyvitamin D [25(OH)D] is an established marker of vitamin D status.

METHODS

To determine whether levels of 25(OH)D, calcium, or pth in serum are associated with oral squamous cell carcinoma (oscc), we examined those parameters in serum collected from 70 patients with oscc and from an equal number of matched control subjects.

RESULTS

The results showed that intact pth was significantly higher in serum from oscc patients than in serum from control subjects. However, we observed no significant differences in 25(OH)D or calcium in serum from oscc patients and from control subjects.

CONCLUSIONS

We conclude that higher serum pth, but not lower serum vitamin D or calcium, is associated with oscc.

Reduced expression of E-cadherin and p120-catenin and elevated expression of PLC-γ1 and PIKE are associated with aggressiveness of oral squamous cell carcinoma

Oral squamous cell carcinoma (OSCC) is one of the most lethal malignant tumors. The cadherin/catenin cell-cell adhesion complex plays a major role in cancer development and progression. p120-catenin (p120) is a cytoplasmic molecule closely associated with E-cadherin which activates phospholipase C-γ1 (PLC-γ1). Our previous studies indicate that activation of PLC-γ1 plays a critical role in epidermal growth factor (EGF)-induced migration and proliferation of squamous cell carcinoma (SCC) cells and phosphatidylinositol 3-kinase enhancer (PIKE) is highly expressed in SCC cells and mediates EGFR-dependent SCC cell proliferation. Our current study was to determine whether the expression of E-cadherin, p120, PLC-γ1, and PIKE, is associated with OSCC. To address this issue, we assessed levels and localization of E-cadherin, p120, PLC-γ1, and PIKE in specimen of 92 patients with OSCC by immunohistochemistry. The results showed that the expression of E-cadherin, and p120 negatively correlated with the tumor differentiation and the expression of PLC-γ1 and PIKE positively correlated with the tumor differentiation. The expression of PLC-γ1 and PIKE in OSCC stage T3 + T4 or in OSCC with lymph node metastasis was significantly higher than that in OSCC stage T1 + T2 or in OSCC without lymph node metastasis. The expression of p120 positively correlated with levels of E-cadherin but negatively correlated with levels of PLC-γ1 and PIKE in OSCC. These data indicate that increased expression of PLC-γ1 and PIKE and decreased expression of E-cadherin and p120 are associated with the aggressiveness of OSCC.

PIP kinases define PI4,5P₂signaling specificity by association with effectors

Phosphatidylinositol 4,5-bisphosphate (PI4,5P₂) is an essential lipid messenger with roles in all eukaryotes and most aspects of human physiology. By controlling the targeting and activity of its effectors, PI4,5P₂modulates processes, such as cell migration, vesicular trafficking, cellular morphogenesis, signaling and gene expression. In cells, PI4,5P₂has a much higher concentration than other phosphoinositide species and its total content is largely unchanged in response to extracellular stimuli. The discovery of a vast array of PI4,5P₂ binding proteins is consistent with data showing that the majority of cellular PI4,5P₂is sequestered. This supports a mechanism where PI4,5P₂functions as a localized and highly specific messenger. Further support of this mechanism comes from the de novo synthesis of PI4,5P₂which is often linked with PIP kinase interaction with PI4,5P₂effectors and is a mechanism to define specificity of PI4,5P₂signaling. The association of PI4,5P₂-generating enzymes with PI4,5P₂effectors regulate effector function both temporally and spatially in cells. In this review, the PI4,5P₂effectors whose functions are tightly regulated by associations with PI4,5P₂-generating enzymes will be discussed. This article is part of a Special Issue entitled Phosphoinositides.

Inhibition of 4-nitroquinoline-1-oxide-induced oral carcinogenesis by dietary calcium

Calcium is a strong inducer of keratinocyte differentiation. We have previously demonstrated that extracellular calcium promotes keratinocyte differentiation via E-cadherin-catenin complex-mediated phospholipase C-γ1 (PLC-γ1) activation in the plasma membrane. However, it is unclear whether dietary calcium regulates keratinocyte proliferation, differentiation or carcinogenesis. To address this issue, the rates of oral tumor and levels of proliferation and differentiation in the oral epithelium were assessed in mice on different calcium diets and the carcinogen 4-nitroquinoline-1-oxide. The results showed that mice on the high calcium diet had lower rates of oral tumors, lower levels of proliferation and higher levels of differentiation in the normal oral epithelium than those on the normal calcium diet. Higher levels of E-cadherin, β-catenin, p120-catenin (p120), epidermal growth factor receptor (EGFR), and calcium and lower levels of PLC-γ1 were also noted in the normal oral epithelium in mice on high calcium diet than the control mice. In contrast, mice on low calcium diet had opposite effects. However, dietary calcium had no effect on the proliferation, differentiation or the levels of E-cadherin, β-catenin, p120, PLC-γ1 and EGFR in oral tumors. These data indicate that dietary calcium increases calcium levels in oral epithelium, suppresses oral carcinogenesis, inhibits proliferation and promotes differentiation of normal oral epithelium. Increased E-cadherin, β-catenin, p120 and EGFR and decreased PLC-γ1 may participate in the inhibitory effect of dietary calcium in oral carcinogenesis.

Novel mechanisms for the vitamin D receptor (VDR) in the skin and in skin cancer

The VDR acting with or without its principal ligand 1,25(OH)2D regulates two central processes in the skin, interfollicular epidermal (IFE) differentiation and hair follicle cycling (HFC). Calcium is an important co-regulator with 1,25(OH)2D at least of epidermal differentiation. Knockout of the calcium sensing receptor (CaSR) in addition to VDR accelerates the development of skin cancer in mice on a low calcium diet. Coactivators such as mediator 1 (aka DRIP205) and steroid receptor coactivator 3 (SRC3) regulate VDR function at different stages of the differentiation process, with Med 1 essential for hair follicle differentiation and early stages of epidermal differentiation and proliferation and SRC3 essential for the latter stages of differentiation including formation of the permeability barrier and innate immunity. The corepressor of VDR, hairless (HR), is essential for hair follicle cycling, although its effect on epidermal differentiation in vivo is minimal. In its regulation of HFC and IFE VDR controls two pathways-wnt/β-catenin and sonic hedgehog (SHH). In the absence of VDR these pathways are overexpressed leading to tumor formation. Whereas, VDR binding to β-catenin may block its activation of TCF/LEF1 sites, β-catenin binding to VDR may enhance its activation of VDREs. 1,25(OH)2D promotes but may not be required for these interactions. Suppression of SHH expression by VDR, on the other hand, requires 1,25(OH)2D. The major point of emphasis is that the role of VDR in the skin involves a number of novel mechanisms, both 1,25(OH)2D dependent and independent, that when disrupted interfere with IFE differentiation and HFC, predisposing to cancer formation. This article is part of a Special Issue entitled '17th Vitamin D Workshop'.

Vitamin D receptor, a tumor suppressor in skin

Vitamin D and calcium are well-established regulators of keratinocyte proliferation and differentiation. Therefore, it was not a great surprise that deletion of the vitamin D receptor (VDR) should predispose the skin to tumor formation, and that the combination of deleting both the VDR and calcium sensing receptor (CaSR) should be especially pro-oncogenic. In this review I have examined 4 mechanisms that appear to underlie the means by which VDR acts as a tumor suppressor in skin. First, DNA damage repair is curtailed in the absence of the VDR, allowing mutations in DNA to accumulate. Second and third involve the increased activation of the hedgehog and β-catenin pathways in the epidermis in the absence of the VDR, leading to poorly regulated proliferation with reduced differentiation. Finally, VDR deletion leads to a shift in the expression of long noncoding RNAs toward a more oncogenic profile. How these different mechanisms interact and their relative importance in the predisposition of the VDR null epidermis to tumor formation remain under active investigation.

Role of the calcium-sensing receptor in calcium regulation of epidermal differentiation and function

The epidermis is a stratified squamous epithelium composed of proliferating basal and differentiated suprabasal keratinocytes. It serves as the body's major physical and chemical barrier against infection and harsh environmental insults, as well as preventing excess water loss from the body into the atmosphere. Calcium is a key regulator of the proliferation and differentiation in keratinocytes. Elevated extracellular Ca(2+) concentration ([Ca(2+)]o) raises the levels of intracellular free calcium ([Ca(2+)]i), promotes cell-cell adhesion, and activates differentiation-related genes. Keratinocytes deficient in the calcium-sensing receptor fail to respond to [Ca(2+)]o stimulation and to differentiate, indicating a role for the calcium-sensing receptor in transducing the [Ca(2+)]o signal during differentiation. The concepts derived from in vitro gene knockdown experiments have been evaluated and confirmed in three mouse models in vivo.

Pulsed electric current induces the differentiation of human keratinocytes

Although normal human keratinocytes are known to migrate toward the cathode in a direct current (DC) electric field, other effects of the electric stimulation on keratinocyte activities are still unclear. We have investigated the keratinocyte differentiation under monodirectional pulsed electric stimulation which reduces the electrothermal and electrochemical hazards of a DC application. When cultured keratinocytes were exposed to the electric field of 3 V (ca. 100 mV/mm) or 5 V (ca. 166 mV/mm) at a frequency of 4,800 Hz for 5 min a day for 5 days, cell growth under the 5-V stimulation was significantly suppressed as compared with the control culture. Expression of mRNAs encoding keratinocyte differentiation markers such as keratin 10, involucrin, transglutaminase 1, and filaggrin was significantly increased in response to the 5-V stimulation, while the 3-V stimulation induced no significant change. After the 5-V stimulation, enhanced immunofluorescent stainings of involucrin and filaggrin were observed. These results indicate that monodirectional pulsed electric stimulation induces the keratinocyte differentiation with growth arrest.

PIKE mediates EGFR proliferative signaling in squamous cell carcinoma cells

One of the key drivers for squamous cell carcinoma (SCC) proliferation is activation of the epidermal growth factor receptor (EGFR), a known proto-oncogene. However, the mechanism of EGFR-dependent SCC proliferation remains unclear. Our previous studies indicate that epidermal growth factor (EGF)-induced SCC cell proliferation requires the SH3 domain of phospholipase C-γ1 (PLC-γ1), but not its catalytic activity. The SH3 domain of PLC-γ1 is known to activate the short form of nuclear phosphatidylinositol 3-kinase enhancer (PIKE) that enhances the activity of nuclear class Ia phosphatidylinositol 3-kinase (PI3K) required for proliferation. However, PIKE has been described for more than a decade to be present exclusively in neuronal cells. In the present study, we found that PIKE was highly expressed in malignant human keratinocytes (SCC4 and SCC12B2) but had low expression in normal human keratinocytes. Immunohistochemical analysis showed strong nuclear staining of PIKE in human epidermal and tongue SCC specimens but little staining in the adjacent non-cancerous epithelium. Treatment of SCC4 cells with EGF-induced translocation of PLC-γ1 to the nucleus and binding of PLC-γ1 to the nuclear PIKE. Knockdown of PLC-γ1 or PIKE blocked EGF-induced activation of class Ia PI3K and protein kinase C-ζ and phosphorylation of nucleolin in the nucleus as well as EGF-induced SCC cell proliferation. However, inhibition of the catalytic activity of PLC-γ1 had little effect. These data suggest that PIKE has a critical role in EGF-induced SCC cell proliferation and may function as a proto-oncogene in SCC.

Calcium regulation of keratinocyte differentiation

Calcium is the major regulator of keratinocyte differentiation in vivo and in vitro. A calcium gradient within the epidermis promotes the sequential differentiation of keratinocytes as they traverse the different layers of the epidermis to form the permeability barrier of the stratum corneum. Calcium promotes differentiation by both outside-in and inside-out signaling. A number of signaling pathways involved with differentiation are regulated by calcium, including the formation of desmosomes, adherens junctions and tight junctions, which maintain cell-cell adhesion and play an important intracellular signaling role through their activation of various kinases and phospholipases that produce second messengers that regulate intracellular free calcium and PKC activity, critical for the differentiation process. The calcium receptor plays a central role by initiating the intracellular signaling events that drive differentiation in response to extracellular calcium. This review will discuss these mechanisms.

The Hsp90 inhibitor 17-AAG represses calcium-induced cytokeratin 1 and 10 expression in HaCaT keratinocytes

Hsp90 is essential for maintaining the activity of numerous signaling factors, and plays a key role in cellular signal transduction networks. 17-Allylamino-17-demethoxygeldanamycin (17-AAG) is an ansamycin antibiotic that binds to Hsp90 and inhibits its function. HaCaT human keratinocytes were used to investigate the cellular and molecular functions of Hsp90 in keratinocyte differentiation. Inhibition of Hsp90 by 17-AAG leads to downregulation of the differentiation markers cytokeratin 1 and cytokeratin 10 at the protein and mRNA levels.

Vitamin D and the skin: Physiology and pathophysiology

The keratinocytes of the skin are unique in being not only the primary source of vitamin D for the body, but in possessing both the enzymatic machinery to metabolize the vitamin D produced to active metabolites (in particular 1,25(OH)(2)D) and the vitamin D receptor (VDR) that enables the keratinocytes to respond to the 1,25(OH)(2)D thus generated. Numerous functions of the skin are regulated by vitamin D and/or its receptor. These include inhibition of proliferation, stimulation of differentiation including formation of the permeability barrier, promotion of innate immunity, regulation of the hair follicle cycle, and suppression of tumor formation. Regulation of these actions is exerted by a number of different coregulator complexes including the coactivators vitamin D receptor interacting protein (DRIP) complex also known as Mediator and the steroid receptor coactivator (SRC) family (of which SRC 2 and 3 are found in keratincytes), the inhibitor hairless (Hr), and β-catenin whose impact on VDR function is complex. Different coregulators appear to be involved in different VDR regulated functions. This review will examine the various functions of vitamin D and its receptor in the skin, and explore the mechanisms by which these functions are regulated.

SERCA2-controlled Ca²+-dependent keratinocyte adhesion and differentiation is mediated via the sphingolipid pathway: a therapeutic target for Darier's disease

Darier’s Disease (DD), caused by mutations in the endoplasmic reticulum (ER) Ca²⁺ ATPase ATP2A2 (SERCA2b), is a skin disease that exhibits impaired epidermal cell-to-cell adhesion and altered differentiation. Although previous studies have shown that keratinocyte Ca²⁺ sequestration and fluxes are controlled by sphingolipid signaling, the role of this signaling pathway in DD previously has not been investigated. We show here that sphingosine levels increase and sphingosine kinase (SPHK1) expression decreases after inactivating SERCA2b with the specific SERCA2 inhibitors thapsigargin (TG) or siRNA to SERCA2b. Conversely, inhibiting sphingosine lyase rescues the defects in keratinocyte differentiation, E-cadherin localization, Desmoplakin (DP) translocation, and ER Ca²⁺ sequestration seen in TG-treated keratinocytes. To our knowledge, it was previously unreported that the keratinocyte sphingolipid and Ca²⁺ signaling pathways intersect in ATP2A2- controlled ER Ca²⁺ sequestration, E-cadherin and desmoplakin localization and Ca²⁺ - controlled differentiation, and thus may be important mediators in DD.

Phosphatidylinositol 4, 5 bisphosphate and the actin cytoskeleton

Dynamic changes in PM PIP(2) have been implicated in the regulation of many processes that are dependent on actin polymerization and remodeling. PIP(2) is synthesized primarily by the type I phosphatidylinositol 4 phosphate 5 kinases (PIP5Ks), and there are three major isoforms, called a, b and g. There is emerging evidence that these PIP5Ks have unique as well as overlapping functions. This review will focus on the isoform-specific roles of individual PIP5K as they relate to the regulation of the actin cytoskeleton. We will review recent advances that establish PIP(2) as a critical regulator of actin polymerization and cytoskeleton/membrane linkages, and show how binding of cytoskeletal proteins to membrane PIP(2) might alter lateral or transverse movement of lipids to affect raft formation or lipid asymmetry. The mechanisms for specifying localized increase in PIP(2) to regulate dynamic actin remodeling will also be discussed.

The receptor tyrosine kinase FGFR2b/KGFR controls early differentiation of human keratinocytes

The FGFRs trigger divergent responses, such as proliferation and differentiation, and the cell type as well as the context-dependent signaling are crucial for the functional outcome. The FGFR2b/KGFR is expressed exclusively on epithelial cells and plays a key role in skin homeostasis. Here we analyzed in vitro the role of KGFR in the early differentiation of keratinocytes modulating its expression by KGFR cDNA transient transfection or KGFR siRNA microinjection and inducing a synchronous wave of differentiation in pre-confluent cells. Immunofluorescence, biochemical and molecular approaches demonstrated that KGFR overexpression increased the early differentiation marker keratin 1 at both transcriptional and translational levels, while receptor depletion reduced it. Ligand-dependent receptor activation and signaling were required for this differentiative effect. Overexpression of kinase negative KGFR mutant or Tyr769 KGFR signaling mutant, which is not able to recruit and activate PLC-γ, showed that the receptor kinase activity, but not its PLCγ-mediated signaling, is required for differentiation. Reduction of K1 expression, obtained by AKT inhibition, demonstrated that the PI3K/Akt signaling pathway is involved in the control of KGFR-mediated keratinocyte differentiation. This in vitro experimental model indicates that FGFR2b/KGFR expression represents a key event regulating keratinocyte early differentiation during the switch from undifferentiated to differentiating cells.

Vitamin D: an ancient hormone

Vitamin D has been produced by plants and animals almost from the time life began. The ability to transport and metabolize vitamin D to more active forms evolved as the structures of plants and animals became more complex, and the cells within these organisms took on more specialized functions. In higher-order animals, the vitamin D receptor (VDR) is found in nearly every cell, and the ability of the cell to produce the active hormone, 1,25(OH)2D, is also widely distributed. Furthermore, the physiological functions with which vitamin D signalling is now associated are as diverse as the tissues in which the VDR is located. Why is this, and is there a common theme? This viewpoint article argues that there is. All cells maintain a fairly constant and submicromolar concentration of free calcium. Calcium is an important regulator of many processes within the cell. The ebb and flow of calcium within cells is controlled by calcium pumps, antiporters and channels. Animals with calcified exo- or endoskeletons have an additional need for calcium, a need that changes during the life cycle of the organism. In this article, I make the case that vitamin D signalling evolved to enable the organism to effectively regulate calcium flux, storage and signalling and that such regulation is critical for the evolutionary process.

Critical role for the catalytic activity of phospholipase C-gamma1 in epidermal growth factor-induced cell migration

Phospholipase C-gamma1 (PLC-gamma1), a tyrosine kinase substrate, has been implicated in the pathway for the epidermal growth factor receptor (EGFR)-induced cell migration. However, the underlying mechanism by which PLC-gamma1 mediates EGFR-induced cell migration remains elusive. In the present study, we sought to determine whether the lipase activity of PLC-gamma1 is required for EGFR-induced cell migration. We found that overexpression of PLC-gamma1 in squamous cell carcinoma SCC4 cells markedly enhanced EGF-induced PLC-gamma1 activation, intracellular calcium rise, and cell migration. This enhancement was abolished by mutational inactivation of the catalytic domain of PLC-gamma1. Inhibition of the downstream signaling processes mediated by the activity of phospholipase C (PLC) using IP(3) receptor inhibitor or intracellular calcium chelator blocked EGF-induced cell migration. These data indicate that EGF-induced cell migration is mediated by the lipase domain of PLC-gamma1 and the subsequent IP(3) generation and intracellular calcium mobilization.

The SH3 domain, but not the catalytic domain, is required for phospholipase C-gamma1 to mediate epidermal growth factor-induced mitogenesis

Phospholipase C-gamma1 (PLC-gamma1) is a multiple-domain protein and plays an important role in epidermal growth factor (EGF)-induced cell mitogenesis, but the underlying mechanism is unclear. We have previously demonstrated that PLC-gamma1 is required for EGF-induced mitogenesis of squamous cell carcinoma (SCC) cells, but the mitogenic function of PLC-gamma1 is independent of its lipase activity. Earlier studies suggest that the Src homology 3 (SH3) domain of PLC-gamma1 possesses mitogenic activity. In the present study, we sought to determine the role of the SH3 domain of PLC-gamma1 in EGF-induced SCC cell mitogenesis. We examined the effect of overexpression of PLC-gamma1, a catalytically active PLC-gamma1 mutant lacking the SH3 domain or a catalytically inactive PLC-gamma1 mutant lacking the X domain on EGF-induced SCC4 (tongue squamous cell carcinoma) cell mitogenesis. We found that overexpression of PLC-gamma1 enhanced EGF-induced SCC4 cell mitogenesis. This enhancement was abolished by deletion of the SH3 domain but not by deletion of the X catalytic domain. These data suggest that the SH3 domain, but not the catalytic domain, is required for PLC-gamma1 to mediate EGF-induced SCC4 cell mitogenesis.

Phospholipase C-gamma1 is required for the epidermal growth factor receptor-induced squamous cell carcinoma cell mitogenesis

The epidermal growth factor receptor (EGFR) is a key driver in the process of squamous cell carcinoma (SCC) cell mitogenesis. Phospholipase C-gamma1 (PLC-gamma1) is a downstream target of EGFR signaling, but the role and necessity of PLC-gamma1 in EGFR-induced cell mitogenesis remain unclear. In the present study, we report an elevated expression of PLC-gamma1 in human SCC biopsies relative to adjacent normal epidermis, and in human SCC cell lines compared to normal human keratinocytes. EGFR-induced SCC cell mitogenesis was blocked by small interfering RNA knockdown of PLC-gamma1. However, inhibition of the catalytic activity of phospholipase C had no effect on EGFR-induced SCC cell mitogenesis. In response to the EGFR ligand epidermal growth factor (EGF), PLC-gamma1 was translocated not only to the plasma membrane but also to the nucleus. These data suggest that PLC-gamma1 is required for EGFR-induced SCC cell mitogenesis and the mitogenic function of PLC-gamma1 is independent of its lipase activity.

Activation of the inositol (1,4,5)-triphosphate calcium gate receptor is required for HIV-1 Gag release

The structural precursor polyprotein, Gag, encoded by all retroviruses, including the human immunodeficiency virus type 1 (HIV-1), is necessary and sufficient for the assembly and release of particles that morphologically resemble immature virus particles. Previous studies have shown that the addition of Ca(2+) to cells expressing Gag enhances virus particle production. However, no specific cellular factor has been implicated as mediator of Ca(2+) provision. The inositol (1,4,5)-triphosphate receptor (IP3R) gates intracellular Ca(2+) stores. Following activation by binding of its ligand, IP3, it releases Ca(2+) from the stores. We demonstrate here that IP3R function is required for efficient release of HIV-1 virus particles. Depletion of IP3R by small interfering RNA, sequestration of its activating ligand by expression of a mutated fragment of IP3R that binds IP3 with very high affinity, or blocking formation of the ligand by inhibiting phospholipase C-mediated hydrolysis of the precursor, phosphatidylinositol-4,5-biphosphate, inhibited Gag particle release. These disruptions, as well as interference with ligand-receptor interaction using antibody targeted to the ligand-binding site on IP3R, blocked plasma membrane accumulation of Gag. These findings identify IP3R as a new determinant in HIV-1 trafficking during Gag assembly and introduce IP3R-regulated Ca(2+) signaling as a potential novel cofactor in viral particle release.

PIP5K-driven PtdIns(4,5)P2 synthesis: regulation and cellular functions

It has long been known that phosphoinositides are present in cellular membranes, but only in the past four decades has our understanding of their importance for proper cell function advanced significantly. Key to determining the biological roles of phosphoinositides is understanding the enzymes involved in their metabolism. Although many such enzymes have now been identified, there is still much to learn about their cellular functions. Phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks) are a group of kinases that catalyse the production of phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P2]. As well as being a substrate for the enzymes phospholipase C (PLC) and phosphatidylinositol 3-kinase (PI3K), PtdIns(4,5)P2 acts as a second messenger in its own right, influencing a variety of cellular processes. In this Commentary, we review how PIP5Ks are modulated to achieve regulated PtdIns(4,5)P2 production, and discuss the role of these proteins in different cellular processes.