Pharmacologic inhibition of ALK5 causes selective induction of terminal differentiation in mouse keratinocytes expressing oncogenic HRAS

TGFβ1 regulates HRas-mediated activation of IRE1α through the PERK-RPAP2 axis in keratinocytes

Transforming Growth Factor β1 (TGFβ1) is a critical regulator of tumor progression in response to HRas. Recently, TGFβ1 has been shown to trigger ER stress in many disease models; however, its role in oncogene-induced ER stress is unclear. Oncogenic HRas induces the unfolded protein response (UPR) predominantly via the Inositol-requiring enzyme 1α (IRE1α) pathway to initiate the adaptative responses to ER stress, with importance for both proliferation and senescence. Here, we show a role of the UPR sensor proteins IRE1α and (PKR)-like endoplasmic reticulum kinase (PERK) to mediate the tumor-suppressive roles of TGFβ1 in mouse keratinocytes expressing mutant forms of HRas. TGFβ1 suppressed IRE1α phosphorylation and activation by HRas both in in vitro and in vivo models while simultaneously activating the PERK pathway. However, the increase in ER stress indicated an uncoupling of ER stress and IRE1α activation by TGFβ1. Pharmacological and genetic approaches demonstrated that TGFβ1-dependent dephosphorylation of IRE1α was mediated by PERK through RNA Polymerase II Associated Protein 2 (RPAP2), a PERK-dependent IRE1α phosphatase. In addition, TGFβ1-mediated growth arrest in oncogenic HRas keratinocytes was partially dependent on PERK-induced IRE1α dephosphorylation and inactivation. Together, these results demonstrate a critical cross-talk between UPR proteins that is important for TGFβ1-mediated tumor suppressive responses.

mTORC1 loss impairs epidermal adhesion via TGF-β/Rho kinase activation

Despite its central position in oncogenic intracellular signaling networks, the role of mTORC1 in epithelial development has not been studied extensively in vivo. Here, we have used the epidermis as a model system to elucidate the cellular effects and signaling feedback sequelae of mTORC1 loss of function in epithelial tissue. In mice with conditional epidermal loss of the mTORC1 components Rheb or Rptor, mTORC1 loss of function unexpectedly resulted in a profound skin barrier defect with epidermal abrasions, blistering, and early postnatal lethality, due to a thinned epidermis with decreased desmosomal protein expression and incomplete biochemical differentiation. In mice with mTORC1 loss of function, we found that Rho kinase (ROCK) signaling was constitutively activated, resulting in increased cytoskeletal tension and impaired cell-cell adhesion. Inhibition or silencing of ROCK1 was sufficient to rescue keratinocyte adhesion and biochemical differentiation in these mice. mTORC1 loss of function also resulted in marked feedback upregulation of upstream TGF-β signaling, triggering ROCK activity and its downstream effects on desmosomal gene expression. These findings elucidate a role for mTORC1 in the regulation of epithelial barrier formation, cytoskeletal tension, and cell adhesion, underscoring the complexity of signaling feedback following mTORC1 inhibition.

iPSC-derived cardiomyocytes reveal abnormal TGF-β signalling in left ventricular non-compaction cardiomyopathy

Left ventricular non-compaction (LVNC) is the third most prevalent cardiomyopathy in children and its pathogenesis has been associated with the developmental defect of the embryonic myocardium. We show that patient-specific induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) generated from LVNC patients carrying a mutation in the cardiac transcription factor TBX20 recapitulate a key aspect of the pathological phenotype at the single-cell level and this was associated with perturbed transforming growth factor beta (TGF-β) signalling. LVNC iPSC-CMs have decreased proliferative capacity due to abnormal activation of TGF-β signalling. TBX20 regulates the expression of TGF-β signalling modifiers including one known to be a genetic cause of LVNC, PRDM16, and genome editing of PRDM16 caused proliferation defects in iPSC-CMs. Inhibition of TGF-β signalling and genome correction of the TBX20 mutation were sufficient to reverse the disease phenotype. Our study demonstrates that iPSC-CMs are a useful tool for the exploration of pathological mechanisms underlying poorly understood cardiomyopathies including LVNC.

Genetic and pharmacological analysis identifies a physiological role for the AHR in epidermal differentiation

Stimulation of the aryl hydrocarbon receptor (AHR) by xenobiotics is known to affect epidermal differentiation and skin barrier formation. The physiological role of endogenous AHR signaling in keratinocyte differentiation is not known. We used murine and human skin models to address the hypothesis that AHR activation is required for normal keratinocyte differentiation. Using transcriptome analysis of Ahr(-/-) and Ahr(+/+) murine keratinocytes, we found significant enrichment of differentially expressed genes linked to epidermal differentiation. Primary Ahr(-/-) keratinocytes showed a significant reduction in terminal differentiation gene and protein expression, similar to Ahr(+/+) keratinocytes treated with AHR antagonists GNF351 and CH223191, or the selective AHR modulator (SAhRM) SGA360. In vitro keratinocyte differentiation led to increased AHR levels and subsequent nuclear translocation, followed by induced CYP1A1 gene expression. Monolayer cultured primary human keratinocytes treated with AHR antagonists also showed an impaired terminal differentiation program. Inactivation of AHR activity during human skin equivalent development severely impaired epidermal stratification, terminal differentiation protein expression, and stratum corneum formation. As disturbed epidermal differentiation is a main feature of many skin diseases, pharmacological agents targeting AHR signaling or future identification of endogenous keratinocyte-derived AHR ligands should be considered as potential new drugs in dermatology.

The TGFβ1 pathway is required for NFκB dependent gene expression in mouse keratinocytes

The transforming growth factor-beta 1 (TGFβ1) and NFκB pathways are important regulators of epidermal homeostasis, inflammatory responses and carcinogenesis. Previous studies have shown extensive crosstalk between these pathways that is cell type and context dependent, but this has not been well-characterized in epidermal keratinocytes. Here we show that in primary mouse keratinocytes, TGFβ1 induces NFκB-luciferase reporter activity that is dependent on both NFκB and Smad3. TGFβ1-induced NFκB-luciferase activity was blocked by the IκB inhibitor parthenolide, the IκB super-repressor, a dominant negative TGFβ1-activated kinase 1 (TAK1) and genetic deletion of NFκB1. Coexpression of NFκB p50 or p65 subunits enhanced NFκB-luciferase activity. Similarly, inhibition of the TGFβ1 type I receptor with SB431542 or genetic deletion of Smad3 blocked TGFβ1 induction of NFκB-luciferase. TGFβ1 rapidly induced IKK phosphorylation but did not cause a detectable decrease in cytoplasmic IκB levels or nuclear translocation of NFκB subunits, although EMSA showed rapid NFκB nuclear binding activity that could be blocked by SB431542 treatment. TNFα, a well characterized NFκB target gene was also induced by TGFβ1 and this was blocked in NFκB+/- and -/- keratinocytes and by the IκB super-repressor. To test the effects of the TGFβ1 pathway on a biologically relevant activator of NFκB, we exposed mice and primary keratinocytes in culture to UVB irradiation. In primary keratinocytes UVB caused a detectable increase in levels of Smad2 phosphorylation that was dependent on ALK5, but no significant increase in SBE-dependent gene expression. Inhibition of TGFβ1 signaling in primary keratinocytes with SB431542 or genetic deletion of Tgfb1 or Smad3 suppressed UVB induction of TNFα message. Similarly, UVB induction of TNFα mRNA was blocked in skin of Tgfb1+/- mice. These studies demonstrate that intact TGFβ1 signaling is required for NFκB-dependent gene expression in mouse keratinocytes and skin and suggest that a convergence of these pathways in the nucleus rather than the cytoplasm may be critical for regulation of inflammatory pathways in skin by TGFβ1.

The Role of TGFβ Signaling in Squamous Cell Cancer: Lessons from Mouse Models

TGFβ1 is a member of a large growth factor family including activins/inhibins and bone morphogenic proteins (BMPs) that have a potent growth regulatory and immunomodulatory functions in normal skin homeostasis, regulation of epidermal stem cells, extracellular matrix production, angiogenesis, and inflammation. TGFβ signaling is tightly regulated in normal tissues and becomes deregulated during cancer development in cutaneous SCC and many other solid tumors. Because of these diverse biological processes regulated by TGFβ1, this cytokine and its signaling pathway appear to function at multiple points during carcinogenesis with distinct effects. The mouse skin carcinogenesis model has been a useful tool to dissect the function of this pathway in cancer pathogenesis, with transgenic and null mice as well as small molecule inhibitors to alter the function of the TGFβ1 pathway and assess the effects on cancer development. This paper will review data on changes in TGFβ1 signaling in human SCC primarily HNSCC and cutaneous SCC and different mouse models that have been generated to investigate the relevance of these changes to cancer. A better understanding of the mechanisms underlying the duality of TGFβ1 action in carcinogenesis will inform potential use of this signaling pathway for targeted therapies.

Efficient tumorigenesis by mutation-induced failure to terminate microRNA-mediated adaptive hyperplasia

Seven current contending cancer theories consider different sets of critical events as sufficient for tumorigenesis. These theories, most recently the microRNA dysregulation (MRD) theory, have overlapping attributes and extensive empirical support, but also some discrepancies, and some do not address both benign and malignant tumorigenesis. By definition, the most efficient tumorigenic pathways will dominate under conditions that selectively activate those pathways. The MRD theory provides a mechanistic basis to combine elements of the current theories into a new hypothesis that: (i) tumors arise most efficiently under stress that induces and sustains either protective or regenerative states of adaptive hyperplasia (AH) that normally are epigenetically maintained unless terminated; and (ii) if dysregulated by a somatic mutation that prevents normal termination, these two AH states can generate benign and malignant tumors, respectively. This hypothesis, but not multistage cancer theory, predicts that key participating AH-stem-cell populations expand markedly when triggered by stress, particularly chronic metabolic or oxidative stress, mechanical irritation, toxic exposure, wounding, inflammation, and/or infection. This hypothesis predicts that microRNA expression patterns in benign vs. malignant tumor tissue will correlate best with those governing protective vs. regenerative AH in that tissue, and that tumors arise most efficiently inmutagen-exposed stem cells that either happen to be in, or incidentally later become recruited into, an AH state.