Raffaele Ambrosio

Sonic hedgehog-induced type 3 deiodinase blocks thyroid hormone action enhancing proliferation of normal and malignant keratinocytes

The Sonic hedgehog (Shh) pathway plays a critical role in hair follicle physiology and is constitutively active in basal cell carcinomas (BCCs), the most common human malignancy. Type 3 iodothyronine deiodinase (D3), the thyroid hormone-inactivating enzyme, is frequently expressed in proliferating and neoplastic cells, but its role in this context is unknown. Here we show that Shh, through Gli2, directly induces D3 in proliferating keratinocytes and in mouse and human BCCs. We demonstrate that Gli-induced D3 reduces intracellular active thyroid hormone, thus resulting in increased cyclin D1 and keratinocyte proliferation. D3 knockdown caused a 5-fold reduction in the growth of BCC xenografts in nude mice. Shh-induced thyroid hormone degradation via D3 synergizes with the Shh-mediated reduction of the type 2 deiodinase, the thyroxine-activating enzyme, and both effects are reversed by cAMP. This previously unrecognized functional cross-talk between Shh/Gli2 and thyroid hormone in keratinocytes is a pathway by which Shh produces its proliferative effects and offers a potential therapeutic approach to BCC.

Mutant p63 causes defective expansion of ectodermal progenitor cells and impaired FGF signalling in AEC syndrome.

Ankyloblepharon‐ectodermal defects‐cleft lip/palate (AEC) syndrome, which is characterized by cleft palate and severe defects of the skin, is an autosomal dominant disorder caused by mutations in the gene encoding transcription factor p63. Here, we report the generation of a knock‐in mouse model for AEC syndrome (p63+/L514F) that recapitulates the human disorder. The AEC mutation exerts a selective dominant‐negative function on wild‐type p63 by affecting progenitor cell expansion during ectodermal development leading to a defective epidermal stem cell compartment. These phenotypes are associated with impairment of fibroblast growth factor (FGF) signalling resulting from reduced expression of Fgfr2 and Fgfr3, direct p63 target genes. In parallel, a defective stem cell compartment is observed in humans affected by AEC syndrome and in Fgfr2b−/− mice. Restoring Fgfr2b expression in p63+/L514F epithelial cells by treatment with FGF7 reactivates downstream mitogen‐activated protein kinase signalling and cell proliferation. These findings establish a functional link between FGF signalling and p63 in the expansion of epithelial progenitor cells and provide mechanistic insights into the pathogenesis of AEC syndrome.

p63 control of desmosome gene expression and adhesion is compromised in AEC syndrome

Ankyloblepharon, ectodermal defects, cleft lip/palate (AEC) syndrome is a rare autosomal dominant disorder caused by mutations in the p63 gene, essential for embryonic development of stratified epithelia. The most severe cutaneous manifestation of this disorder is the long-lasting skin fragility associated with severe skin erosions after birth. Using a knock-in mouse model for AEC syndrome, we found that skin fragility was associated with microscopic blistering between the basal and suprabasal compartments of the epidermis and reduced desmosomal contacts. Expression of desmosomal cadherins and desmoplakin was strongly reduced in AEC mutant keratinocytes and in newborn epidermis. A similar impairment in desmosome gene expression was observed in human keratinocytes isolated from AEC patients, in p63-depleted keratinocytes and in p63 null embryonic skin, indicating that p63 mutations causative of AEC syndrome have a dominant-negative effect on the wild-type p63 protein. Among the desmosomal components, desmocollin 3, desmoplakin and desmoglein 1 were the most significantly reduced by mutant p63 both at the RNA and protein levels. Chromatin immunoprecipitation experiments and transactivation assays revealed that p63 controls these genes at the transcriptional level. Consistent with reduced desmosome function, AEC mutant and p63-deficient keratinocytes had an impaired ability to withstand mechanical stress, which was alleviated by epidermal growth factor receptor inhibitors known to stabilize desmosomes. Our study reveals that p63 is a crucial regulator of a subset of desmosomal genes and that this function is impaired in AEC syndrome. Reduced mechanical strength resulting from p63 mutations can be alleviated pharmacologically by increasing desmosome adhesion with possible therapeutic implications.

Transcription factor Nkx-2.5 induces sodium/iodide symporter gene expression and participates in retinoic acid- and lactation-induced transcription in mammary cells

ABSTRACT The sodium/iodide symporter (NIS) is a plasma membrane protein that mediates active iodide transport in thyroid and mammary cells. It is a prerequisite for radioiodide treatment of thyroid cancer and a promising diagnostic and therapeutic tool for breast cancer. We investigated the molecular mechanisms governing NIS expression in mammary cells. Here we report that Nkx-2.5, a cardiac homeobox transcription factor that is also expressed in the thyroid primordium, is a potent inducer of the NIS promoter. By binding to two specific promoter sites (N2 and W), Nkx-2.5 induced the rNIS promoter (about 50-fold over the basal level). Interestingly, coincident with NIS expression, Nkx-2.5 mRNA and protein were present in lactating, but not virgin, mammary glands in two human breast cancer samples and in all-trans retinoic acid (tRA)-stimulated MCF-7 breast cancer cells. A cotransfected dominant-negative Nkx-2.5 mutant abolished tRA-induced endogenous NIS induction, which shows that Nkx-2.5 activity is critical for this process. Remarkably, in MCF-7 cells, Nkx-2.5 overexpression alone was sufficient to induce NIS and iodide uptake. In conclusion, Nkx-2.5 is a novel relevant transcriptional regulator of mammary NIS and could thus be exploited to manipulate NIS expression in breast cancer treatment strategies.

Pendrin is a novel in vivo downstream target gene of the TTF-1/Nkx-2.1 homeodomain transcription factor in differentiated thyroid cells.

ABSTRACT Thyroid transcription factor gene 1 (TTF-1) is a homeobox-containing gene involved in thyroid organogenesis. During early thyroid development, the homeobox gene Nkx-2.5 is expressed in thyroid precursor cells coincident with the appearance of TTF-1. The aim of this study was to investigate the molecular mechanisms underlying thyroid-specific gene expression. We show that the Nkx-2.5 C terminus interacts with the TTF-1 homeodomain and, moreover, that the expression of a dominant-negative Nkx-2.5 isoform (N188K) in thyroid cells reduces TTF-1-driven transcription by titrating TTF-1 away from its target DNA. This process reduced the expression of several thyroid-specific genes, including pendrin and thyroglobulin. Similarly, down-regulation of TTF-1 by RNA interference reduced the expression of both genes, whose promoters are sensitive to and directly associate with TTF-1 in the chromatin context. In conclusion, we demonstrate that pendrin and thyroglobulin are downstream targets in vivo of TTF-1, whose action is a prime factor in controlling thyroid differentiation in vivo.

Intracellular Inactivation of Thyroid Hormone Is a Survival Mechanism for Muscle Stem Cell Proliferation and Lineage Progression

Summary Precise control of the thyroid hormone (T3)-dependent transcriptional program is required by multiple cell systems, including muscle stem cells. Deciphering how this is achieved and how the T3 signal is controlled in stem cell niches is essentially unknown. We report that in response to proliferative stimuli such as acute skeletal muscle injury, type 3 deiodinase (D3), the thyroid hormone-inactivating enzyme, is induced in satellite cells where it reduces intracellular thyroid signaling. Satellite cell-specific genetic ablation of dio3 severely impairs skeletal muscle regeneration. This impairment is due to massive satellite cell apoptosis caused by exposure of activated satellite cells to the circulating TH. The execution of this proapoptotic program requires an intact FoxO3/MyoD axis, both genes positively regulated by intracellular TH. Thus, D3 is dynamically exploited in vivo to chronically attenuate TH signaling under basal conditions while also being available to acutely increase gene programs required for satellite cell lineage progression.

Role of type 3 deiodinase in cancer

Thyroid hormone (TH) influences a wide variety of biological events in vertebrates. Among them, the balance between proliferation and differentiation is crucial in TH action in normal and pathological conditions, including cancer. Thyroid hormone signaling results from the interaction of T3 with nuclear receptors that, in concert with other transcription factors, stimulate or repress the expression of target genes, some of which are involved in the control of cellular proliferation. Ligand (T3) availability is under tight control at both extracellular and intracellular levels. Intracellular T3 concentrations are influenced by the action of selenodeiodinases. These enzymes can, within the single cell, enhance (D1 and D2) or reduce (D3) T3 concentrations, thereby constituting a potent mechanism of pre-receptoral control of TH action. Type 3 deiodinase, the major physiological inactivator of TH, is highly expressed in developing tissues and in some tumoral tissues, with a mostly unknown function. Recent studies suggest that D3 enzyme plays an important role in the control of TH metabolism and action during tumorigenesis. In this review, we focus on D3 and its potential as a novel tumoral marker and new molecular target in cancer treatment.

Activated Thyroid Hormone Promotes Differentiation and Chemotherapeutic Sensitization of Colorectal Cancer Stem Cells by Regulating Wnt and BMP4 Signaling.

Thyroid hormone is a pleiotropic factor that controls many cellular processes in multiple cell types such as cancer stem cells (CSC). Thyroid hormone concentrations in the blood are stable, but the action of the deiodinases (D2-D3) provides cell-specific regulation of thyroid hormone activity. Deregulation of deiodinase function and thyroid hormone status has been implicated in tumorigenesis. Therefore, we investigated the role of thyroid hormone metabolism and signaling in colorectal CSCs (CR-CSC), where deiodinases control cell division and chemosensitivity. We found that increased intracellular thyroid hormone concentration through D3 depletion induced cell differentiation and sharply mitigated tumor formation. Upregulated BMP4 expression and concomitantly attenuated Wnt signaling accompanied these effects. Furthermore, we demonstrate that BMP4 is a direct thyroid hormone target and is involved in a positive autoregulatory feedback loop that modulates thyroid hormone signaling. Collectively, our findings highlight a cell-autonomous metabolic mechanism by which CR-CSCs exploit thyroid hormone signaling to facilitate their self-renewal potential and suggest that drug-induced cell differentiation may represent a promising therapy for preventing CSC expansion and tumor progression.

Epigenetic control of type 2 and 3 deiodinases in myogenesis: role of Lysine-specific Demethylase enzyme and FoxO3

The proliferation and differentiation of muscle precursor cells require myogenic regulatory factors and chromatin modifiers whose concerted action dynamically regulates access to DNA and allows reprogramming of cells towards terminal differentiation. Type 2 deiodinase (D2), the thyroid hormone (TH)-activating enzyme, is sharply upregulated during myoblast differentiation, whereas type 3 deiodinase (D3), the TH-inactivating enzyme, is downregulated. The molecular determinants controlling synchronized D2 and D3 expression in muscle differentiation are completely unknown. Here, we report that the histone H3 demethylating enzyme (LSD-1) is essential for transcriptional induction of D2 and repression of D3. LSD-1 relieves the repressive marks (H3-K9me2-3) on the Dio2 promoter and the activation marks (H3-K4me2-3) on the Dio3 promoter. LSD-1 silencing impairs the D2 surge in skeletal muscle differentiation while inducing D3 expression thereby leading to a global decrease in intracellular TH production. Furthermore, endogenous LSD-1 interacts with FoxO3a, and abrogation of FoxO3-DNA binding compromises the ability of LSD-1 to induce D2. Our data reveal a novel epigenetic control of reciprocal deiodinases expression and provide a molecular mechanism by which LSD-1, through the opposite regulation of D2 and D3 expression, acts as a molecular switch that dynamically finely tunes the cellular needs of active TH during myogenesis.

The selective loss of the type 2 iodothyronine deiodinase in mouse thyrotrophs increases basal TSH but blunts the thyrotropin response to hypothyroidism

The type 2 iodothyronine deiodinase (D2) is essential for feedback regulation of TSH by T4. We genetically inactivated in vivo D2 in thyrotrophs using a mouse model of Cga-driven cre recombinase. Pituitary D2 activity was reduced 90% in the Cga-cre D2 knockout (KO) mice compared with control Dio2(fl/fl) mice. There was no growth or reproductive phenotype. Basal TSH levels were increased 1.5- to 1.8-fold, but serum T4 and T3 were not different from the controls in adult mice. In hypothyroid adult mice, suppression of TSH by T4, but not T3, was impaired. Despite mild basal TSH elevation, the TSH increase in response to hypothyroidism was 4-fold reduced in the Cga-cre D2KO compared with control mice despite an identical level of pituitary TSH α- and β-subunit mRNAs. In neonatal Cga-cre D2KO mice, TSH was also 2-fold higher than in the controls, but serum T4 was elevated. Despite a constant TSH, serum T4 increased 2-3-fold between postnatal day (P) 5 and P15 in both genotypes. The pituitary, but not cerebrocortical, D2 activity was markedly elevated in P5 mice decreasing towards adult levels by P17. In conclusion, a congenital severe reduction of thyrotroph D2 causes a major impairment of the TSH response to hypothyroidism. This would be deleterious to the compensatory adaptation of the thyroid gland to iodine deficiency.