Abstract 1138: Novel SMARCA2 degrading bifunctional molecules as therapeutics in SMARCA4 mutant lung cancer

Lung cancer is the top cause of cancer mortality. Despite recent advances, the majority of patients with lung cancer still lack effective therapeutic options, underscoring the dire need for additional treatment approaches. Genomic studies have identified frequent mutations in subunits of the SWI/SNF chromatin remodeling complex including SMARCA4 and ARID1A in non-small cell lung cancer with a frequency of up to 33% in advanced stage disease, making it the most frequently mutated complex in lung cancer. Recent reports, and our own data, have identified the paralogue SMARCA2 to be synthetic lethal to SMARCA4 suggesting SMARCA2 is a valuable therapeutic target. However, the discovery of selective inhibitors of SMARCA2 has been challenging. To overcome this hurdle, we have utilized iterative structure-activity relationship (SAR) studies to develop novel, potent and selective SMARCA2 degrading small molecules based on proteolysis targeting chimera (PROTAC) technology. We demonstrated that YD23, our lead SMARCA2 PROTAC, potently and selectively induces degradation of SMARCA2. Using global proteomic analysis and quantification of more than 8000 proteins, we showed that our SMARCA2 PROTAC is highly selective for SMARCA2. Importantly, we showed that YD23 selectively inhibits growth of SMARCA4 mutant lung cancer cells in vitro. Mechanistically, we demonstrated that YD23 reduces chromatin accessibility only in SMARCA4 deficient cells. In particular, YD23 profoundly decreased chromatin accessibility at enhancers of a number of genes including cell cycle and cell growth regulatory genes. Gene expression profiling and pathway analysis indicated that various cell cycle genes were downregulated by YD23 consistent with the reduced chromatin accessibility at their regulatory regions. In conclusion, our study provides a potent chemical probe for studying the synthetic lethal interaction between SMARCA2 and SMARCA4, dissect the chromatin and epigenetic landscape alterations and lay the foundation for future preclinical and clinical development of SMARCA2 degraders as therapeutics.

Citation Format: Sasi Kotagiri, Nicholas Blazanin, Yuanxin Xi, Jing Wang, Yonathan Lissanu. Novel SMARCA2 degrading bifunctional molecules as therapeutics in SMARCA4 mutant lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1138.

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.

Twist1 is required for the development of UVB-induced squamous cell carcinoma

The transcription factor Twist1 has been reported to be essential for the formation and invasiveness of chemically induced tumors in mouse skin. However, the impact of keratinocyte-specific Twist1 deletion on skin carcinogenesis caused by UVB radiation has not been reported. Deletion of Twist1 in basal keratinocytes of mouse epidermis using K5.Cre × Twist1^flox/flox mice led to significantly reduced UVB-induced epidermal hyperproliferation. In addition, keratinocyte-specific deletion of Twist1 significantly suppressed UVB-induced skin carcinogenesis. Further analyses revealed that deletion of Twist1 in cultured keratinocytes or mouse epidermis in vivo led to keratinocyte differentiation. In this regard, deletion of Twist1 in epidermal keratinocytes showed significant induction of early and late differentiation markers, including TG1, K1, OVOL1, loricrin, and filaggrin. Similar results were obtained with topical application of harmine, a Harmala alkaloid that leads to degradation of Twist1. In contrast, overexpression of Twist1 in cultured keratinocytes suppressed calcium-induced differentiation. Further analyses using both K5.Cre × Twist1^flox/flox mice and an inducible system where Twist1 was deleted in bulge region keratinocytes showed loss of expression of hair follicle stem/progenitor markers, including CD34, Lrig1, Lgr5, and Lgr6. These data support the conclusion that Twist1 has a direct role in maintaining the balance between proliferation and differentiation of keratinocytes and keratinocyte stem/progenitor populations. Collectively, these results demonstrate a critical role for Twist1 early in the process of UVB skin carcinogenesis, and that Twist1 may be a novel target for the prevention of cutaneous squamous cell carcinoma.

Abstract 5678: Novel glucocorticoid receptor degrading bifunctional molecules as therapeutics in castration-resistant prostate cancer

While the majority of men with prostate cancer will experience modest morbidity, advanced castration-resistant prostate cancer (CRPC) remains a devastating disease with significant mortality and limited treatment options. Recently approved anti-androgen therapies such as enzalutamide have shown promising clinical responses. Unfortunately, patients frequently develop resistance to these therapies. Signaling through the glucocorticoid receptor (GR) has emerged as a primary mechanism of resistance to enzalutamide in CRPC suggesting that GR is an attractive therapeutic target. However, existing small molecule antagonists of GR suffer from partial agonist activity leading to activation of a subset of GR target genes which limits their anti-tumor efficacy. To overcome this problem, we have developed bifunctional small molecules or proteolysis targeting chimeras (PROTACs) that induce potent and specific degradation of GR. Using iterative structure-activity relationship (SAR) studies, we have developed a novel chemical linker that imparted superior properties to PROTACs. These novel GR PROTACs induce profound degradation of GR. Using proteomic analysis and quantification of more than 8000 proteins, we show that our GR PROTAC is highly selective for GR. GR PROTACs inhibited proliferation of enzalutamide-resistant prostate cancer cells, have favorable pharmacokinetic properties and are active in vivo. Importantly, GR PROTACs work synergistically with enzalutamide to inhibit growth of prostate cancer xenografts in a castration-resistant mouse model of prostate cancer. Mechanistically, we identified improved suppression of AR and GR target genes in the combination of GR PROTAC and enzalutamide. More importantly, the combination was able to suppress crucial developmental pathways implicated in prostate cancer resistance to therapy and tumor dormancy. Our novel GR PROTACs highlight the power of targeted protein degradation to deliver excellent chemical probes and potentially impactful cancer therapeutics.

Citation Format: James T. Link, Nicholas Blazanin, Yonathan Lissanu Deribe. Novel glucocorticoid receptor degrading bifunctional molecules as therapeutics in castration-resistant prostate cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5678.

Selective Ah receptor ligands mediate enhanced SREBP1 proteolysis to restrict lipogenesis in sebocytes

The aryl hydrocarbon receptor (AHR) mediates 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) induced toxicity that can lead to chloracne in humans. A characteristic of chloracne, in contrast to acne vulgaris, is shrinkage or loss of sebaceous glands. Acne vulgaris, on the other hand, is often accompanied by excessive sebum production. Here, we examined the role of AHR in lipid synthesis in human sebocytes using distinct classes of AHR ligands. Modulation of AHR activity attenuated the expression of lipogenic genes and key pro-inflammatory markers in the absence of canonical DRE-driven transcription of the AHR target gene CYP1A1. Furthermore, topical treatment with TCDD, which mediates DRE-dependent activity, and SGA360, which fails to induce DRE-mediated responses, both exhibited a decrease in the size of sebaceous glands and the number of sebocytes within each gland in the skin. To elucidate the mechanism of AHR-mediated repression of lipid synthesis, we demonstrated that selective AHR modulators, SGA360 and SGA315 increased the protein turnover of the mature sterol regulatory element-binding protein (mSREBP-1), the principal transcriptional regulator of the fatty acid synthesis pathway. Interestingly, selective AHR ligand treatment significantly activated the AMPK-dependent kinase (AMPK) in sebocytes. Moreover, we demonstrated an inverse correlation between the active AMPK and the mSREBP-1 protein, which is consistent with the previously reported role of AMPK in inhibiting cleavage of SREBP-1. Overall, our findings indicate a DRE-independent function of selective AHR ligands in modulating lipid synthesis in human sebocytes, which might raise the possibility of using AHR as a therapeutic target for treatment of acne.

The multiple roles of the unfolded protein response regulator IRE1α in cancer

Cancer is associated with a number of conditions such as hypoxia, nutrient deprivation, cellular redox, and pH changes that result in accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) and trigger a stress response known as the unfolded protein response (UPR). The UPR is a conserved cellular survival mechanism mediated by the ER transmembrane proteins activating transcription factor 6, protein kinase-like endoplasmic reticulum kinase, and inositol-requiring enzyme 1α (IRE1α) that act to resolve ER stress and promote cell survival. IRE1α is a kinase/endoribonuclease (RNase) with multiple activities including unconventional splicing of the messenger RNA (mRNA) for the transcription factor X-Box Binding Protein 1 (XBP1), degradation of other mRNAs in a process called regulated IRE1α-dependent decay (RIDD) and activation of a pathway leading to c-Jun N-terminal kinase phosphorylation. Each of these outputs plays a role in the adaptive and cell death responses to ER stress. Many studies indicate an important role for XBP1 and RIDD functions in cancer and new studies suggest that these two functions of the IRE1α RNase can have opposing functions in the early and later stages of cancer pathogenesis. Finally, as more is learned about the context-dependent role of IRE1α in cancer development, specific small molecule inhibitors and activators of IRE1α could play an important role in counteracting the protective shield provided by ER stress signaling in cancer cells.

Abstract 2257: Divergent IRE1α endonuclease outputs dictate the senescence response of mouse keratinocytes to oncogenic HRAS

Oncogenic RAS plays a pivotal role in neoplastic transformation, however in primary cells it initially triggers a proliferative response followed by irreversible growth arrest and premature senescence, a potent barrier to tumor development. How premature senescence is bypassed to enable full transformation and tumor progression is not clear. Here we show that in primary mouse keratinocytes IRE1α, an endoplasmic reticulum (ER) transmembrane kinase/endoribonuclease (RNase) important in the unfolded protein response, regulates cell fate in response to oncogenic v-Ha-RAS (HRAS) through its two distinct RNase outputs. During the initial proliferation response, oncogenic HRAS activates IRE1α-mediated cleavage of XBP1 mRNA to produce the active transcription factor that is MEK-ERK dependent. Microarray analysis of keratinocytes with IRE1α knockdown using shRNA also showed that HRAS activates the regulated IRE1α-dependent mRNA decay (RIDD) function of IRE1α and this is responsible for downregulation of a subset of mRNAs by HRAS. Interestingly, IRE1α and XBP1 knockdown had distinct effects on premature senescence in HRAS keratinocytes. XBP1 knockdown induced premature senescence that was associated with extensive vacuolization whereas IRE1α knockdown did not despite similar inhibition of proliferation. Pharmacologic inhibition of IRE1α RNase activity inhibited premature senescence in XBP1-depleted HRAS keratinocytes and enhanced proliferation demonstrating a role for RIDD in this response. In addition, the potent cytostatic growth factor TGFβ1 which we previously showed induces rapid senescence in HRAS keratinocytes, inhibited IRE1α-mediated XBP1 splicing and enhanced RIDD possibly through alteration of IRE1α phosphorylation. Similarly, shRNA knockdown or pharmacologic inhibition of IRE1α bypassed TGFβ1-induced senescence, while XBP1 knockdown was susceptible to it. Using the RIDD gene signature obtained from microarray analysis of HRAS keratinocytes, we performed gene set enrichment analysis (GSEA) on previously identified gene expression profiles of benign and malignant mouse epidermal squamous tumors. Our results, validated by qPCR, show that genes in the RIDD signature are downregulated in benign tumors, paralleling the in vitro studies, but are upregulated in malignant tumors. In contrast, XBP1 splicing is similar in both tumor types. These results suggest that RIDD is linked senescence in vitro and benign tumor phenotype in vivo and that suppression of RIDD, but not XBP1 splicing, is a prerequisite for malignant progression. We propose that the balance between XBP1 splicing and RIDD determines whether a premalignant tumor cell undergoes proliferation and tumor progression or senescence and tumor suppression. Selective pharmacological agents that inhibit XBP1 splicing or enhance RIDD could be an important therapeutic modality to the eradication of cancer.

Citation Format: Nicholas Blazanin, Christian John, Alayna Craig-Lucas, Adam Glick. Divergent IRE1α endonuclease outputs dictate the senescence response of mouse keratinocytes to oncogenic HRAS. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2257. doi:10.1158/1538-7445.AM2014-2257

The nuclear receptor peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) promotes oncogene-induced cellular senescence through repression of endoplasmic reticulum stress

Endoplasmic reticulum (ER) stress and ER stress-associated unfolded protein response (UPR) can promote cancer cell survival, but it remains unclear whether they can influence oncogene-induced senescence. The present study examined the role of ER stress in senescence using oncogene-dependent models. Increased ER stress attenuated senescence in part by up-regulating phosphorylated protein kinase B (p-AKT) and decreasing phosphorylated extracellular signal-regulated kinase (p-ERK). A positive feed forward loop between p-AKT, ER stress, and UPR was discovered whereby a transient increase of ER stress caused reduced senescence and promotion of tumorigenesis. Decreased ER stress was further correlated with increased senescence in both mouse and human tumors. Interestingly, H-RAS-expressing Pparβ/δ null cells and tumors having increased cell proliferation exhibited enhanced ER stress, decreased cellular senescence, and/or enhanced tumorigenicity. Collectively, these results demonstrate a new role for ER stress and UPR that attenuates H-RAS-induced senescence and suggest that PPARβ/δ can repress this oncogene-induced ER stress to promote senescence in accordance with its role as a tumor modifier that suppresses carcinogenesis.

PPARβ/δ promotes HRAS-induced senescence and tumor suppression by potentiating p-ERK and repressing p-AKT signaling

Peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) inhibits skin tumorigenesis through mechanisms that may be dependent on HRAS signaling. The present study examined the hypothesis that PPARβ/δ promotes HRAS-induced senescence resulting in suppression of tumorigenesis. PPARβ/δ expression increased p-ERK and decreased p-AKT activity. Increased p-ERK activity results from the dampened HRAS-induced negative feedback response mediated in part through transcriptional upregulation of RAS guanyl-releasing protein 1 (RASGRP1) by PPARβ/δ. Decreased p-AKT activity results from repression of integrin-linked kinase (ILK) and phosphoinositide-dependent protein kinase-1 (PDPK1) expression. Decreased p-AKT activity in turn promotes cellular senescence through upregulation of p53 and p27 expression. Both over-expression of RASGRP1 and shRNA-mediated knockdown of ILK partially restore cellular senescence in Pparβ/δ-null cells. Higher PPARβ/δ expression is also correlated with increased senescence observed in human benign neurofibromas and colon adenoma lesions in vivo. These results demonstrate that PPARβ/δ promotes senescence to inhibit tumorigenesis and provide new mechanistic insights into HRAS-induced cellular senescence.

Abstract 2172: Anti-oncogenic role of peroxisome proliferator-activated receptor-α/β (PPARδ/α) involves inhibition of mitosis and regulation of Hras1-induced senescence

Ligand activation of PPARδ/α inhibits chemically-induced skin tumorigenesis and Pparα/α-null mice exhibit enhanced chemically-induced skin tumorigenesis compared to wild-type mice. The mechanism that mediates inhibition of skin carcinogenesis by PPARδ/α is still being unraveled and was examined in this study. Ligand activation of PPARδ/α caused a negative selection against cells expressing higher levels of HRAS by inducing a mitotic block. Mitosis-related genes that are predominantly regulated by E2F were induced to a higher level in HRAS-expressing Pparα/α-null keratinocytes as compared to HRAS-expressing wild-type keratinocytes. Ligand activated PPARδ/α repressed expression of these genes by direct binding with p130/p107, facilitating nuclear translocation, and increasing promoter recruitment of p130/p107. In addition, co-treatment with a PPARδ/α ligand and various mitosis inhibitors increases the efficacy of increasing G2/M arrest. PPARδ/α is also required for HRAS-induced senescence and suppression of malignant conversion of benign papillomas. HRAS-expressing Pparα/α-null keratinocytes evade senescence by utilizing the PI3K-AKT rather than the MEK-ERK pathway. PPARδ/α positively regulates MEK-ERK pathway by maintaining RASGRP1 levels and negatively regulating the PI3K-AKT pathway by repressing ILK and PDPK1 expression. Heightened AKT activity increases endoplasmic reticulum (ER) stress in Pparα/α-null keratinocytes, which in turn helps maintain higher AKT activity, leading to evasion of senescence. In addition, enhanced HRAS activity increases the synthesis of endogenous PPARδ/α ligands, thus providing a PPARδ/α-dependent cell-autonomous mechanism to suppress HRAS-induced carcinogenesis. (Supported by CA124533).

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2172. doi:1538-7445.AM2012-2172

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

TGFβ has both tumor suppressive and oncogenic roles in cancer development. We previously showed that SB431542 (SB), a small molecule inhibitor of the TGFβ type I receptor (ALK5) kinase, suppressed benign epidermal tumor formation but enhanced malignant conversion. Here, we show that SB treatment of primary K5rTA/tetORASV12G bitransgenic keratinocytes did not alter HRASV12G-induced keratinocyte hyperproliferation. However, continuous SB treatment significantly enhanced HRASV12G-induced cornified envelope formation and cell death linked to increased expression of enzymes transglutaminase (TGM) 1 and TGM3 and constituents of the cornified envelope small proline-rich protein (SPR) 1A and SPR2H. In contrast, TGFβ1 suppressed cornified envelope formation in HRASV12G keratinocytes. Similar results were obtained in HRASV12G transgenic mice treated topically with SB or by coexpressing TGFβ1 and HRASV12G in the epidermis. Despite significant cell death, SB-resistant HRASV12G keratinocytes repopulated the primary culture that had overcome HRas-induced senescence. These cells expressed reduced levels of p16(ink4a) and were growth stimulated by SB but remained sensitive to a calcium-induced growth arrest. Together these results suggest that differential responsiveness to cornification may represent a mechanism by which pharmacologic blockade of TGFβ signaling can inhibit the outgrowth of preneoplastic lesions but may cause a more progressed phenotype in a separate keratinocyte population.

Abstract 1227: Anti-oncogenic role of Peroxisome proliferator-activated receptor- β/Δ (Pparβ/Δ) involves regulation of viral Hras1 (v-Hras1)-induced senescence and endoplasmic reticulum (ER) stress in mouse primary keratinocytes

Pparβ/Δ-null mice exhibit enhanced chemically-induced skin tumorigenesis compared to wild-type counterparts. Since an oncogenic mutation of Harvey sarcoma ras virus gene (v-Hras1) is a critical event in chemical carcinogenesis, the role of PPARβ/Δ in mouse primary keratinocytes infected with a retrovirus encoding viral Hras1 (v-Hras1) gene was examined. While v-Hras1 induced senescence in wild-type mouse primary keratinocytes, the level of v-Hras1-induced senescence was significantly reduced in Pparβ/Δ-null keratinocytes, which was due in part to decreased phospho-MEK and phospho-ERK during the early stage of v-Hras1 expression. In addition, while total v-Hras1 protein level was comparable between wild-type and Pparβ/Δ-null keratinocytes, the level of the GTP-bound form of v-Hras1 was significantly decreased in Pparβ/Δ-null keratinocytes from 4 day to 7 day post v-Hras1 infection. Surprisingly, the decreased activity of v-Hras1 did not lead to a decreased level of phospho-AKT in Pparβ/Δ-null keratinocytes. However, Pparβ/Δ-null keratinocytes were more dependent upon PI3K-AKT pathway for maintenance of proliferation and evasion of senescence. Differentially expressed genes between wild-type and Pparβ/Δ-null keratinocytes that are involved in the regulation of senescence were determined by cDNA microarray analysis. Differential expressions of both positive and negative regulators of RAS pathway were observed between wild-type and Pparβ/Δ-null keratinocytes following v-Hras1 infection. Notably, the mRNA encoding CDK inhibitor p27Kip1 was lower in Pparβ/Δ-null keratinocytes, In addition, Pparβ/Δ-null keratinocytes showed enhanced endoplasmic reticulum (ER) stress and unfolded protein response (UPR) post v-Hras1 infection, characterized by enhanced vacuolation and higher levels of major ER stress markers. However, the enhanced ER stress did not lead to apoptosis in Pparβ/Δ-null keratinocytes. These results suggest an anti-oncogenic role of PPARβ/Δ in v-Hras1-transformed mouse primary keratinocytes that involves both the promotion of anti-tumorigenic senescence and attenuation of pro-tumorigenic ER stress and UPR. (Supported by CA124533)

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1227. doi:10.1158/1538-7445.AM2011-1227

TGFbeta1-induced inflammation in premalignant epidermal squamous lesions requires IL-17

Overexpression of transforming growth factor-beta1 (TGFbeta1) in the normal epidermis can provoke an inflammatory response, but whether this occurs within a developing tumor is not clear. To test this, we used an inducible transgenic mouse to overexpress TGFbeta1 in premalignant squamous lesions. Within 48 hours of TGFbeta1 induction, there was an increase in IL-17 production by both CD4(+) and gammadelta(+) T cells, together with increased expression of T-helper-17 (Th17)-polarizing cytokines. Induction of TGFbeta1 in premalignant primary keratinocytes elevated the expression of proinflammatory and Th17-polarizing cytokines, and the keratinocyte-conditioned media caused IL-17 production by naive T cells that was dependent on T-cell TGFbeta1 signaling. Microarray analysis showed significant upregulation of proinflammatory genes 2 days after TGFbeta1 induction, and this was followed by increased MPO(+), F4/80(+), and CD8(+) cells in tumors, increased CD8(+) effectors and IFNgamma(+) cells in skin-draining LNs, and tumor regression. In parallel, the percentage of tumor CD11b(+)Ly6G(+) neutrophils was reduced. Neutralization of IL-17 blocked TGFbeta1-induced CD11b(+) Ly6G(-) tumor infiltration but did not alter the reduction of neutrophils or tumor regression. Thus, TGFbeta1 overexpression causes IL-17-dependent and IL-17-independent changes in the premalignant tumor inflammatory microenvironment.

TGFβ1 Modulation of Inflammation in Premalignant Squamous Lesions is IL-17-dependent and Independent (95.5)

The proinflammatory functions of TGFβ1 are dependent on the local cytokine milieu. Overexpression of TGFβ1 in epidermis provokes an inflammatory response in the skin while its role in skin tumors is still not clear. To evaluate the effects of TGFβ1 in modulating inflammatory cytokine production, we overexpressed TGFβ1 in premalignant primary keratinocytes derived from an inducible TGFβ1 transgenic mouse. TGFβ1 induced expression of TNFα, IL-1β and IL-23a in vitro and conditioned media from these cells polarized naïve T cells to a Th17 phenotype that was dependent on TGFβ1 signaling. Overexpression of TGFβ1 in premalignant squamous lesions resulted in an increase in IL-17 production by CD4+ and γδ+ T cells as well as rapid regression of the tumors. Microarray analysis of gene expression following induction of TGFβ1 in premalignant skin tumors indicated a significant upregulation of proinflammatory genes at day 2. This was followed by an increase in MPO+, F4/80+ and CD8+ cells in the tumors and increase in CD8+ effectors and IFNγ+ cells in the skin draining lymph nodes at day 5. Concomitantly, the percentage of tumor CD11b+Ly6G+ cells was reduced in TGFβ1 overexpressing tumors. Neutralization of IL-17 blocked TGFβ1 induced CD11b+Ly6g- macrophage infiltration in the tumor but did not alter the reduction of CD11b+Ly6g+ cells or tumor regression. Thus, TGFβ1 overexpression causes IL-17 dependent and independent changes in the premalignant tumor inflammatory microenvironment.

Tumor suppressor and oncogene actions of TGFbeta1 occur early in skin carcinogenesis and are mediated by Smad3

Interactions between TGFbeta1 and ras signaling pathways play an important role in cancer development. Here we show that in primary mouse keratinocytes, v-ras(Ha) does not block the early biochemical events of TGFbeta1 signal transduction but does alter global TGFbeta1 mediated gene expression in a gene specific manner. Expression of Smad3 dependent TGFbeta1 early response genes and the TGFbeta1 cytostatic gene expression response were not altered by v-ras(Ha) consistent with an intact TGFbeta1 growth arrest. However, TGFbeta1 and v-ras(Ha) cause significant alteration in genes regulating matrix remodeling as the TGFbeta1 induction of extracellular matrix genes was blocked by v-ras(Ha) but specific matrix proteases associated with cancer progression were elevated. Smad3 deletion in keratinocytes repressed normal differentiation maker expression and caused expression of Keratin 8 a simple epithelial keratin and marker of malignant conversion. Smad3 was required for the TGFbeta1 cytostatic response in v-ras(Ha) keratinocytes, but also for protease induction, keratinocyte attachment and migration. These results show that pro-oncogenic activities of TGFbeta1 can occur early in carcinogenesis before loss of its tumor suppressive function and that selective regulation rather than complete inactivation of Smad3 function may be crucial for tumor progression.

Ligand activation of peroxisome proliferator-activated receptor beta/delta (PPAR beta/delta) inhibits chemically induced skin tumorigenesis

Peroxisome proliferator-activated receptor (PPAR)beta/delta-null mice exhibit enhanced tumorigenesis in a two-stage chemical carcinogenesis model as compared with wild-type mice. Previous work showed that ligand activation of PPARbeta/delta induces terminal differentiation and inhibits proliferation of primary keratinocytes, and this effect does not occur in the absence of PPARbeta/delta expression. In the present studies, the effect of ligand activation of PPARbeta/delta on skin tumorigenesis was examined using both in vivo and ex vivo skin carcinogenesis models. Inhibition of chemically induced skin tumorigenesis was observed in wild-type mice administered GW0742, and this effect was likely the result of ligand-induced terminal differentiation and inhibition of replicative DNA synthesis. These effects were not found in similarly treated PPARbeta/delta-null mice. Ligand activation of PPARbeta/delta also inhibited cell proliferation and induced terminal differentiation in initiated/neoplastic keratinocyte cell lines representing different stages of skin carcinogenesis. These studies suggest that topical administration of PPARbeta/delta ligands may be useful as both a chemopreventive and/or a chemotherapeutic approach to inhibit skin cancer.