Kick-Starting Wound Healing: A Review of Pro-Healing Drugs.

Cutaneous wound healing consists of four stages: hemostasis, inflammation, proliferation/repair, and remodeling. While healthy wounds normally heal in four to six weeks, a variety of underlying medical conditions can impair the progression through the stages of wound healing, resulting in the development of chronic, non-healing wounds. Great progress has been made in developing wound dressings and improving surgical techniques, yet challenges remain in finding effective therapeutics that directly promote healing. This review examines the current understanding of the pro-healing effects of targeted pharmaceuticals, re-purposed drugs, natural products, and cell-based therapies on the various cell types present in normal and chronic wounds. Overall, despite several promising studies, there remains only one therapeutic approved by the United States Food and Drug Administration (FDA), Becaplermin, shown to significantly improve wound closure in the clinic. This highlights the need for new approaches aimed at understanding and targeting the underlying mechanisms impeding wound closure and moving the field from the management of chronic wounds towards resolving wounds.

Tumor cell-intrinsic PD-1 promotes Merkel cell carcinoma growth by activating downstream mTOR-mitochondrial ROS signaling.

Merkel cell carcinoma (MCC) is a rare and aggressive skin cancer. Inhibitors targeting the programmed cell death 1 (PD-1) immune checkpoint have improved MCC patient outcomes by boosting antitumor T cell immunity. Here, we identify PD-1 as a growth-promoting receptor intrinsic to MCC cells. In human MCC lines and clinical tumors, RT-PCR-based sequencing, immunoblotting, flow cytometry, and immunofluorescence analyses demonstrated PD-1 gene and protein expression by MCC cells. MCC-PD-1 ligation enhanced, and its inhibition or silencing suppressed, in vitro proliferation and in vivo tumor xenograft growth. Consistently, MCC-PD-1 binding to PD-L1 or PD-L2 induced, while antibody-mediated PD-1 blockade inhibited, protumorigenic mTOR signaling, mitochondrial (mt) respiration, and ROS generation. Last, pharmacologic inhibition of mTOR or mtROS reversed MCC-PD-1:PD-L1-dependent proliferation and synergized with PD-1 checkpoint blockade in suppressing tumorigenesis. Our results identify an MCC-PD-1-mTOR-mtROS axis as a tumor growth-accelerating mechanism, the blockade of which might contribute to clinical response in patients with MCC.

PRMT1 Inhibition Selectively Targets BNC1-Dependent Proliferation, but not Migration in Squamous Cell Carcinoma.

Squamous Cell Carcinoma (SCC) develops in stratified epithelial tissues and demonstrates frequent alterations in transcriptional regulators. We sought to discover SCC-specific transcriptional programs and identified the transcription factor Basonuclin 1 (BNC1) as highly expressed in SCC compared to other tumor types. RNA-seq and ChIP-seq analysis identified pro-proliferative genes activated by BNC1 in SCC cells and keratinocytes. Inhibition of BNC1 in SCC cells suppressed proliferation and increased migration via FRA1. In contrast, BNC1 reduction in keratinocytes caused differentiation, which was abrogated by IRF6 knockdown, leading to increased migration. Protein interactome analysis identified PRMT1 as a co-activator of BNC1-dependent proliferative genes. Inhibition of PRMT1 resulted in a dose-dependent reduction in SCC cell proliferation without increasing migration. Importantly, therapeutic inhibition of PRMT1 in SCC xenografts significantly reduced tumor size, resembling functional effects of BNC1 knockdown. Together, we identify BNC1-PRMT1 as an SCC-lineage specific transcriptional axis that promotes cancer growth, which can be therapeutically targeted to inhibit SCC tumorigenesis.

Inhibition of Melanoma Cell-Intrinsic Tim-3 Stimulates MAPK-Dependent Tumorigenesis.

T-cell immunoglobulin mucin family member 3 (Tim-3) is an immune checkpoint receptor that dampens effector functions and causes terminal exhaustion of cytotoxic T cells. Tim-3 inhibitors are under investigation in immuno-oncology (IO) trials, because blockade of T-cell-Tim-3 enhances antitumor immunity. Here, we identify an additional role for Tim-3 as a growth-suppressive receptor intrinsic to melanoma cells. Inhibition of melanoma cell-Tim-3 promoted tumor growth in both immunocompetent and immunocompromised mice, while melanoma-specific Tim-3 overexpression attenuated tumorigenesis. Ab-mediated Tim-3 blockade inhibited growth of immunogenic murine melanomas in T-cell-competent hosts, consistent with established antitumor effects of T-cell-Tim-3 inhibition. In contrast, Tim-3 Ab administration stimulated tumorigenesis of both highly and lesser immunogenic murine and human melanomas in T-cell-deficient mice, confirming growth-promoting effects of melanoma-Tim-3 antagonism. Melanoma-Tim-3 activation suppressed, while its blockade enhanced, phosphorylation of pro-proliferative downstream MAPK signaling mediators. Finally, pharmacologic MAPK inhibition reversed unwanted Tim-3 Ab-mediated tumorigenesis in T-cell-deficient mice and enhanced desired antitumor activity of Tim-3 interference in T-cell-competent hosts. These results identify melanoma-Tim-3 blockade as a mechanism that antagonizes T-cell-Tim-3-directed IO therapeutic efficacy. They further reveal MAPK targeting as a combination strategy for circumventing adverse consequences of unintended melanoma-Tim-3 inhibition. SIGNIFICANCE: Tim-3 is a growth-suppressive receptor intrinsic to melanoma cells, the blockade of which promotes MAPK-dependent tumorigenesis and thus counteracts antitumor activity of T-cell-directed Tim-3 inhibition.

Regulation of 5-Hydroxymethylcytosine by TET2 Contributes to Squamous Cell Carcinoma Tumorigenesis.

DNA methylation is a key regulatory event controlling a variety of physiological processes and can have dramatic effects on gene transcription. Methylated cytosine (5-methylcytosine) can be oxidized by the TET family of enzymes to 5-hydroxymethylcytosine (5-hmC), a key intermediate in the demethylation cycle, and 5-hmC levels are reduced in malignancies such as acute myeloid leukemia and melanoma. We constructed a tissue microarray of human cutaneous squamous cell carcinoma tumors and found a global reduction in 5-hmC levels compared with that in the adjacent skin. Using a murine K14-CreER system, we have found that loss of Tet2 promotes carcinogen-induced squamous cell carcinoma and cooperates with loss of Tp53 to drive spontaneous squamous cell carcinoma tumors in epithelial tissues. Analysis of changes in 5-hmC and gene expression after loss of Tet2 in the epidermis revealed focal alterations in 5-hmC levels and an increase in hair follicle transient amplifying cell genes along with a reduction in epidermal differentiation genes. These results show a role for TET2 in epidermal lineage specification, consistent with reported roles for TET enzymes in controlling lineage commitment in hematopoietic stem cells and embryonic stem cells and establishing TET2 as a bone fide tumor suppressor in squamous cell carcinoma.

Lysyl hydroxylase 2-induced collagen cross-link switching promotes metastasis in head and neck squamous cell carcinomas.

Head and neck squamous cell carcinoma (HNSCC) is the 6th most common cancer worldwide and incidence rates are continuing to rise globally. HNSCC patient prognosis is closely related to the occurrence of tumor metastases, and collagen within the tumor microenvironment (TME) plays a key role in this process. Lysyl hydroxylase 2 (LH2), encoded by the Procollagen-Lysine,2-Oxoglutarate 5-Dioxygenase 2 (PLOD2) gene, catalyzes hydroxylation of telopeptidyl lysine (Lys) residues of fibrillar collagens which then undergo subsequent modifications to form stable intermolecular cross-links that change the biomechanical properties (i.e. quality) of the TME. While LH2-catalyzed collagen modification has been implicated in driving tumor progression and metastasis in diverse cancers, little is known about its role in HNSCC progression. Thus, using gain- and loss-of-function studies, we examined the effects of LH2 expression levels on collagen cross-linking and cell behavior in vitro and in vivo using a tractable bioluminescent imaging-based orthotopic xenograft model. We found that LH2 overexpression dramatically increases HNSCC cell migratory and invasive abilities in vitro and that LH2-driven changes in collagen cross-linking robustly induces metastasis in vivo. Specifically, the amount of LH2-mediated collagen cross-links increased significantly with PLOD2 overexpression, without affecting the total quantity of collagen cross-links. Conversely, LH2 knockdown significantly blunted HNSCC cells invasive capacity in vitro and metastatic potential in vivo. Thus, regardless of the total "quantity" of collagen crosslinks, it is the "quality" of these cross-links that is the key driver of HNSCC tumor metastatic dissemination. These data implicate LH2 as a key regulator of HNSCC tumor invasion and metastasis by modulating collagen cross-link quality and suggest that therapeutic strategies targeting LH2-mediated collagen cross-linking in the TME may be effective in controlling tumor progression and improving disease outcomes.

Understanding Transcriptional Networks Regulating Initiation of Cutaneous Wound Healing.

The epidermis has an essential function in creating a barrier against the external environment to retain proper fluid balance and block the entry of pathogens. When damage occurs to this barrier, the wound must quickly be sealed to avoid fluid loss, cleared of invading pathogens, and then keratinocytes must re-form an intact barrier. This requires complex integration of temporally and spatially distinct signals to execute orderly closure of the wound, and failure of this process can lead to chronic ulceration. Transcription factors serve as a key integration point for the myriad of information coming from the external environment, allowing for an orderly process of re-epithelialization. Importantly, transcription factors engage with and alter the chromatin structure around key target genes through association with different chromatin-modifying complexes. In this review, we will discuss the current understanding of how transcription is regulated during the initiation of re-epithelialization, and the exciting technological advances that will allow for a more refined mechanistic understanding of the re-epithelialization process.

Loss of the Epigenetic Mark 5-hmC in Psoriasis: Implications for Epidermal Stem Cell Dysregulation.

Epigenetic regulation has a profound influence on stem cell fate during normal development in maintenance of physiologic tissue homeostasis. Here we report diminished ten-eleven translocation (TET) methylcytosine dioxygenase expression and loss of the DNA hydroxymethylation mark 5-hydroxymethylcytosine (5-hmC) in keratinocyte stem cells and transit amplifying cells in human psoriasis and in imiquimod-induced murine psoriasis. Loss of 5-hmC was associated with dysregulated keratinocyte stem cell kinetics, resulting in accumulation of nestin and FABP5-expressing transit amplifying cells to produce classic psoriatic epidermal architecture. Moreover, 5-hmC loss was accompanied by diminished TET1 and TET2 mRNA expression. Genome-wide mapping of epidermal 5-hmC in murine psoriasis revealed loci-specific loss of 5-hmC in genes regulating stem cell homeostasis, including MBD1, RTN1, STRN4, PRKD2, AKT1, and MAPKAP2, as well as those associated with RAR and Wnt/ß-catenin signaling pathways. In vitro restoration of TET expression by ascorbic acid was accomplished in cultured human keratinocyte stem cells to show similar Ca++-induced differentiation, resulting in increased 5-hmC levels and reduced nestin expression. To our knowledge, an epigenetic deficiency in psoriasis with relevance to stem cell dysregulation has not been previously reported. This observation raises the possibility that epigenetic modifiers that impact on the TET-5-hmC pathway may be a relevant approach of heretofore unappreciated therapeutic utility.

Biological significance of 5-hydroxymethylcytosine in oral epithelial dysplasia and oral squamous cell carcinoma.

OBJECTIVES: The aim of this study was to determine the levels of 5-hydroxylmethylcytosine (5-hmC) in oral epithelial dysplasia (OED) and oral squamous cell carcinoma (OSCC) compared with those in benign, reactive inflammatory lesions and to explore whether DNA hydroxymethylation may serve as a novel biomarker for early diagnosis and prognosis of OSCC. STUDY DESIGN: The study included normal mucosa from uninvolved margins of 9 fibromas, 10 oral lichen planus, 15 OED, and 23 OSCC. Cultured human keratinocyte lines from benign oral mucosa, OED, and OSCC, as well as a murine model in which OSCC was induced with 4-nitroquinoline-1-oxide, were also evaluated. RESULTS: Progressive loss of 5-hmC from benign oral mucosal lesions to OED and OSCC was documented in patient samples. Decreased levels in 5-hmC that typify OED and OSCC were also detectable in human cell lines. Moreover, we characterized similar alterations in 5-hmC in an animal model of OED/OSCC. CONCLUSIONS: This study demonstrated that 5-hmC distinguishes OED and OSCC from benign lesions with high sensitivity and specificity. Consequently, loss of 5-hmC may be useful for the diagnosis of OED with potential implications for therapy of OSCC.

CCAR2 Is Required for Proliferation and Tumor Maintenance in Human Squamous Cell Carcinoma.

CCAR2 is a widely expressed protein involved in the regulation of a variety of transcriptional complexes. High expression of CCAR2 correlates with poor outcomes in many human tumor types such as squamous cell carcinoma (SCC). Paradoxically, loss of Ccar2 in the mouse results in an increased tumor burden, suggesting that CCAR2 may in fact function as a tumor suppressor. This tumor suppressor function is dependent on p53, a protein that is inactivated in the vast majority of SCC tumors, leaving the role of CCAR2 in p53-null tumors unclear. We sought to identify p53-independent CCAR2 functions in SCC and to examine its role in tumorigenesis. We found that CCAR2 is highly overexpressed in p53-deficient SCC cell lines compared with normal primary keratinocytes due to increased protein stability. We identify a role for CCAR2 in promoting the stability of the transcription factors RFX1 and CREB1, which are both required for proliferation. Finally, we show that CCAR2 is required for proliferation in vitro and in established SCC tumors in vivo. Our data suggest an important role for CCAR2 in maintaining cell cycle progression and promoting SCC tumorigenesis.

FGFR2 signaling underlies p63 oncogenic function in squamous cell carcinoma.

Oncogenic transcription factors drive many human cancers, yet identifying and therapeutically targeting the resulting deregulated pathways has proven difficult. Squamous cell carcinoma (SCC) is a common and lethal human cancer, and relatively little progress has been made in improving outcomes for SCC due to a poor understanding of its underlying molecular pathogenesis. While SCCs typically lack somatic oncogene-activating mutations, they exhibit frequent overexpression of the p53-related transcription factor p63. We developed an in vivo murine tumor model to investigate the function and key transcriptional programs of p63 in SCC. Here, we show that established SCCs are exquisitely dependent on p63, as acute genetic ablation of p63 in advanced, invasive SCC induced rapid and dramatic apoptosis and tumor regression. In vivo genome-wide gene expression analysis identified a tumor-survival program involving p63-regulated FGFR2 signaling that was activated by ligand emanating from abundant tumor-associated stroma. Correspondingly, we demonstrate the therapeutic efficacy of extinguishing this signaling axis in endogenous SCCs using the clinical FGFR2 inhibitor AZD4547. Collectively, these results reveal an unanticipated role for p63-driven paracrine FGFR2 signaling as an addicting pathway in human cancer and suggest a new approach for the treatment of SCC.

ΔNp63α represses anti-proliferative genes via H2A.Z deposition.

ΔNp63α is a member of the p53 family of transcription factors that functions as an oncogene in squamous cell carcinomas (SCCs). Because ΔNp63α and p53 bind virtually identical DNA sequence motifs, it has been proposed that ΔNp63α functions as a dominant-negative inhibitor of p53 to promote proliferation and block apoptosis. However, most SCCs concurrently overexpress ΔNp63α and inactivate p53, suggesting the autonomous action of these oncogenic events. Here we report the discovery of a novel mechanism of transcriptional repression by ΔNp63α that reconciles these observations. We found that although both proteins bind the same genomic sites, they regulate largely nonoverlapping gene sets. Upon activation, p53 binds all enhancers regardless of ΔNp63α status but fails to transactivate genes repressed by ΔNp63α. We found that ΔNp63α associates with the SRCAP chromatin regulatory complex involved in H2A/H2A.Z exchange and mediates H2A.Z deposition at its target loci. Interestingly, knockdown of SRCAP subunits or H2A.Z leads to specific induction of ΔNp63α-repressed genes. We identified SAMD9L as a key anti-proliferative gene repressed by ΔNp63α and H2A.Z whose depletion suffices to reverse the arrest phenotype caused by ΔNp63α knockdown. Collectively, these results illuminate a molecular pathway contributing to the autonomous oncogenic effects of ΔNp63α.

Physical association of HDAC1 and HDAC2 with p63 mediates transcriptional repression and tumor maintenance in squamous cell carcinoma.

Squamous cell carcinoma (SCC) is a treatment-refractory subtype of human cancer arising from stratified epithelium of the skin, lung, esophagus, oropharynx, and other tissues. A unifying feature of SCC is high-level expression of the p53-related protein p63 (TP63) in 80% of cases. The major protein isoform of p63 expressed in SCC is ΔNp63α, an N-terminally truncated form which functions as a key SCC cell survival factor by mechanisms that are unclear. In this study, we show that ΔNp63α associates with histone deacetylase 1 (HDAC1) and HDAC2 to form an active transcriptional repressor complex that can be targeted to therapeutic advantage. Repression of proapoptotic Bcl-2 family member genes including p53 upregulated modulator of apoptosis (PUMA) by p63/HDAC is required for survival of SCC cells. Cisplatin chemotherapy, a mainstay of SCC treatment, promotes dissociation of p63 and HDAC from the PUMA promoter, leading to increased histone acetylation, PUMA activation, and apoptosis. These effects are recapitulated upon targeting the p63/HDAC complex selectively with class I/II HDAC inhibitors using both in vitro and in vivo models. Sensitivity to HDAC inhibition is directly correlated with p63 expression and is abrogated in tumor cells that overexpress endogenous Bcl-2. Together, our results elucidate a mechanism of p63-mediated transcriptional repression and they identify the ΔNp63α/HDAC complex as an essential tumor maintenance factor in SCC. In addition, our findings offer a rationale to apply HDAC inhibitors for SCC treatment.

A microRNA-dependent program controls p53-independent survival and chemosensitivity in human and murine squamous cell carcinoma.

The p53 tumor suppressor, a central mediator of chemosensitivity in normal cells, is functionally inactivated in many human cancers. Therefore, a central challenge in human cancer therapy is the identification of pathways that control tumor cell survival and chemosensitivity in the absence of functional p53. The p53-related transcription factors p63 and p73 exhibit distinct functions­p73 mediates chemosensitivity while p63 promotes proliferation and cell survival­and are both overexpressed in squamous cell carcinomas (SCCs). However, how p63 and p73 interact functionally and govern the balance between prosurvival and proapoptotic programs in SCC remains elusive. Here, we identify a microRNA-dependent mechanism of p63/p73 crosstalk that regulates p53-independent survival of both human and murine SCC. We first discovered that a subset of p63-regulated microRNAs target p73 for inhibition. One of these, miR-193a-5p, expression of which was repressed by p63, was activated by proapoptotic p73 isoforms in both normal cells and tumor cells in vivo. Chemotherapy caused p63/p73-dependent induction of this microRNA, thereby limiting chemosensitivity due to microRNA-mediated feedback inhibition of p73. Importantly, inhibiting miR-193a interrupted this feedback and thereby suppressed tumor cell viability and induced dramatic chemosensitivity both in vitro and in vivo. Thus, we have identified a direct, microRNA-dependent regulatory circuit mediating inducible chemoresistance, whose inhibition may provide a new therapeutic opportunity in p53-deficient tumors.

Mitigation of hematologic radiation toxicity in mice through pharmacological quiescence induced by CDK4/6 inhibition.

Total body irradiation (TBI) can induce lethal myelosuppression, due to the sensitivity of proliferating hematopoietic stem/progenitor cells (HSPCs) to ionizing radiation (IR). No effective therapy exists to mitigate the hematologic toxicities of TBI. Here, using selective and structurally distinct small molecule inhibitors of cyclin-dependent kinase 4 (CDK4) and CDK6, we have demonstrated that selective cellular quiescence increases radioresistance of human cell lines in vitro and mice in vivo. Cell lines dependent on CDK4/6 were resistant to IR and other DNA-damaging agents when treated with CDK4/6 inhibitors. In contrast, CDK4/6 inhibitors did not protect cell lines that proliferated independently of CDK4/6 activity. Treatment of wild-type mice with CDK4/6 inhibitors induced reversible pharmacological quiescence (PQ) of early HSPCs but not most other cycling cells in the bone marrow or other tissues. Selective PQ of HSPCs decreased the hematopoietic toxicity of TBI, even when the CDK4/6 inhibitor was administered several hours after TBI. Moreover, PQ at the time of administration of therapeutic IR to mice harboring autochthonous cancers reduced treatment toxicity without compromising the therapeutic tumor response. These results demonstrate an effective method to mitigate the hematopoietic toxicity of IR in mammals, which may be potentially useful after radiological disaster or as an adjuvant to anticancer therapy.

FOXO-regulated transcription restricts overgrowth of Tsc mutant organs.

FOXO is thought to function as a repressor of growth that is, in turn, inhibited by insulin signaling. However, inactivating mutations in Drosophila melanogaster FOXO result in viable flies of normal size, which raises a question over the involvement of FOXO in growth regulation. Previously, a growth-suppressive role for FOXO under conditions of increased target of rapamycin (TOR) pathway activity was described. Here, we further characterize this phenomenon. We show that tuberous sclerosis complex 1 mutations cause increased FOXO levels, resulting in elevated expression of FOXO-regulated genes, some of which are known to antagonize growth-promoting pathways. Analogous transcriptional changes are observed in mammalian cells, which implies that FOXO attenuates TOR-driven growth in diverse species.

LKB1 modulates lung cancer differentiation and metastasis.

Germline mutation in serine/threonine kinase 11 (STK11, also called LKB1) results in Peutz-Jeghers syndrome, characterized by intestinal hamartomas and increased incidence of epithelial cancers. Although uncommon in most sporadic cancers, inactivating somatic mutations of LKB1 have been reported in primary human lung adenocarcinomas and derivative cell lines. Here we used a somatically activatable mutant Kras-driven model of mouse lung cancer to compare the role of Lkb1 to other tumour suppressors in lung cancer. Although Kras mutation cooperated with loss of p53 or Ink4a/Arf (also known as Cdkn2a) in this system, the strongest cooperation was seen with homozygous inactivation of Lkb1. Lkb1-deficient tumours demonstrated shorter latency, an expanded histological spectrum (adeno-, squamous and large-cell carcinoma) and more frequent metastasis compared to tumours lacking p53 or Ink4a/Arf. Pulmonary tumorigenesis was also accelerated by hemizygous inactivation of Lkb1. Consistent with these findings, inactivation of LKB1 was found in 34% and 19% of 144 analysed human lung adenocarcinomas and squamous cell carcinomas, respectively. Expression profiling in human lung cancer cell lines and mouse lung tumours identified a variety of metastasis-promoting genes, such as NEDD9, VEGFC and CD24, as targets of LKB1 repression in lung cancer. These studies establish LKB1 as a critical barrier to pulmonary tumorigenesis, controlling initiation, differentiation and metastasis.

Expression of p16Ink4a compensates for p18Ink4c loss in cyclin-dependent kinase 4/6-dependent tumors and tissues.

Cell cycle progression from G(1) to S phase depends on phosphorylation of pRb by complexes containing a cyclin (D type or E type) and cyclin-dependent kinase (e.g., cdk2, cdk4, or cdk6). Ink4 proteins function to oppose the action of cdk4/6-cyclin D complexes by inhibiting cdk4/6. We employed genetic and pharmacologic approaches to study the interplay among Ink4 proteins and cdk4/6 activity in vivo. Mouse embryo fibroblasts (MEF) lacking p16(Ink4a) and p18(Ink4c) showed similar growth kinetics as wild-type MEFs despite increased cdk4 activity. In vivo, germline deficiency of p16(Ink4a) and p18(Ink4c) resulted in increased proliferation in the intermediate pituitary and pancreatic islets of adult mice, and survival of p16(Ink4a-/-);p18(Ink4c-/-) mice was significantly reduced due to aggressive pituitary tumors. Compensation among the Ink4 proteins was observed both in vivo in p18(Ink4c-/-) mice and in MEFs from p16(Ink4a-/-), p18(Ink4c-/-), or p16(Ink4a-/-);p18(Ink4c-/-) mice. Treatment with PD 0332991, a specific cdk4/6 kinase inhibitor, abrogated proliferation in those compartments where Ink4 deficiency was associated with enhanced proliferation (i.e., islets, pituitary, and B lymphocytes) but had no effect on proliferation in other tissues such as the small bowel. These data suggest that p16(Ink4a) and p18(Ink4c) coordinately regulate the in vivo catalytic activity of cdk4/6 in specific compartments of adult mice.