Maximizing the Efficacy of MAPK-Targeted Treatment in PTENLOF/BRAFMUT Melanoma through PI3K and IGF1R Inhibition

The role of PI3'-lipid signalling in melanoma initiation, progression and maintenance

Phosphatidylinositol-3'-kinases (PI3Ks) are a family of lipid kinases that phosphorylate the 3' hydroxyl (OH) of the inositol ring of phosphatidylinositides (PI). Through their downstream effectors, PI3K generated lipids (PI3K-lipids hereafter) such as PI(3,4,5)P3 and PI(3,4)P2 regulate myriad biochemical and biological processes in both normal and cancer cells including responses to growth hormones and cytokines; the cell division cycle; cell death; cellular growth; angiogenesis; membrane dynamics; and autophagy and many aspects of cellular metabolism. Engagement of receptor tyrosine kinase by their cognate ligands leads to activation of members of the Class I family of PI3'-kinases (PI3Kα, β, δ & γ) leading to accumulation of PI3K-lipids. Importantly, PI3K-lipid accumulation is antagonized by the hydrolytic action of a number of PI3K-lipid phosphatases, most notably the melanoma suppressor PTEN (lipid phosphatase and tensin homologue). Downstream of PI3K-lipid production, the protein kinases AKT1-3 are believed to be key effectors of PI3'-kinase signalling in cells. Indeed, in preclinical models, activation of the PI3K→AKT signalling axis cooperates with alterations such as expression of the BRAFV600E oncoprotein kinase to promote melanoma progression and metastasis. In this review, we describe the different classes of PI3K-lipid effectors, and how they may promote melanomagenesis, influence the tumour microenvironment, melanoma maintenance and progression to metastatic disease. We also provide an update on both FDA-approved or experimental inhibitors of the PI3K→AKT pathway that are currently being evaluated for the treatment of melanoma either in preclinical models or in clinical trials.

Resistance to Molecularly Targeted Therapies in Melanoma

Malignant melanoma is the most aggressive type of skin cancer with invasive growth patterns. In 2021, 106,110 patients are projected to be diagnosed with melanoma, out of which 7180 are expected to die. Traditional methods like surgery, radiation therapy, and chemotherapy are not effective in the treatment of metastatic and advanced melanoma. Recent approaches to treat melanoma have focused on biomarkers that play significant roles in cell growth, proliferation, migration, and survival. Several FDA-approved molecular targeted therapies such as tyrosine kinase inhibitors (TKIs) have been developed against genetic biomarkers whose overexpression is implicated in tumorigenesis. The use of targeted therapies as an alternative or supplement to immunotherapy has revolutionized the management of metastatic melanoma. Although this treatment strategy is more efficacious and less toxic in comparison to traditional therapies, targeted therapies are less effective after prolonged treatment due to acquired resistance caused by mutations and activation of alternative mechanisms in melanoma tumors. Recent studies focus on understanding the mechanisms of acquired resistance to these current therapies. Further research is needed for the development of better approaches to improve prognosis in melanoma patients. In this article, various melanoma biomarkers including BRAF, MEK, RAS, c-KIT, VEGFR, c-MET and PI3K are described, and their potential mechanisms for drug resistance are discussed.

Insulin Reduces the Efficacy of Vemurafenib and Trametinib in Melanoma Cells

Background

Despite the progress made in the clinical management of metastatic melanoma, a patient’s response to treatment cannot be fully predicted, and intrinsic or acquired resistance that is developed in most melanoma patients warrants further research efforts. In addition to genetic factors, microenvironmental input should be considered to explain the diversity of response to treatment among melanoma patients. In this study, we evaluated the impact of insulin on patient-derived BRAF^V600E melanoma cells, either untreated or treated with vemurafenib or trametinib, inhibitors of BRAF^V600 and MEK1/2, respectively.

Methods

Cells were cultured in serum-free conditions, either with or without insulin. The activity of the MAPK/ERK and PI3K/AKT pathways was assessed by Western blotting, cell viability, and percentages of Ki-67- and NGFR-positive cells by flow cytometry. Transcript levels were analyzed using qRT-PCR, and γ-H2AX levels by immunoblotting and confocal microscopy. A luminescence-based assay was used to measure glutathione content.

Results

While insulin did not influence the MAPK/ERK pathway activity, it had a strong influence on melanoma cells, in which this pathway was suppressed by either vemurafenib or trametinib. In the presence of insulin, both drugs were much less efficient in 1) inhibiting proliferation and reducing the percentage of Ki-67-positive cells, and 2) inducing apoptosis and phosphorylation of histone H2AX in melanoma cells. Changes induced by vemurafenib and trametinib in glutathione homeostasis and DNA repair gene expression were also attenuated by insulin. Moreover, insulin impaired the combined effects of targeted drugs and doxorubicin in melanoma cells. In addition to insulin-induced PI3K/AKT activity, which was either transient or sustainable depending on the cell line, an insulin-triggered increase in the percentage of cells expressing NGFR, a marker of neural crest stem-like cells, may contribute to the reduced drug efficacy.

Conclusion

Our results demonstrate the role of insulin in reducing the efficacy of vemurafenib and trametinib. This needs clinical assessment.

Targeting effector pathways in RAC1P29S-driven malignant melanoma

Malignant melanoma is characterized by mutations in a number of driver genes, most notably BRAF and NRAS. Recent genomic analyses revealed that 4-9% of sun-exposed melanomas bear activating mutations in RAC1, which encodes a small GTPase that is known to play key roles in cell proliferation, survival, and migration. The RAC1 protein activates several effector pathways, including Group A p21-activated kinases (PAKs), phosphoinositol-3-kinases (PI3Ks), in particular the beta isoform, and the serum-response factor/myocardin-related transcription factor (SRF/MRTF). Having previously shown that inhibition of Group A PAKs impedes oncogenic signalling from RAC1^P29S, we here extend this analysis to examine the roles of PI3Ks and SRF/MRTF in melanocytes and/or in a zebrafish model. We demonstrate that a selective Group A PAK inhibitor (Frax-1036), a pan-PI3K (BKM120), and two PI3Kβ inhibitors (TGX221, GSK2636771) impede the growth of melanoma cells driven by mutant RAC1 but not by mutant BRAF, while other PI3K selective inhibitors, including PI3Kα, δ and γ, are less effective. Using these compounds as well as an SRF/MRTF inhibitor (CCG-203,971), we observed similar results in vivo, using embryonic zebrafish development as a readout. These results suggest that targeting Group A PAKs, PI3Kβ, and/or SRF/MRTF represent a promising approach to suppress RAC1 signalling in malignant melanoma.

Activation of the IGF Axis in Thyroid Cancer: Implications for Tumorigenesis and Treatment

The Insulin-like growth factor (IGF) axis is one of the best-established drivers of thyroid transformation, as thyroid cancer cells overexpress both IGF ligands and their receptors. Thyroid neoplasms encompass distinct clinical and biological entities as differentiated thyroid carcinomas (DTC)-comprising papillary (PTC) and follicular (FTC) tumors-respond to radioiodine therapy, while undifferentiated tumors-including poorly-differentiated (PDTC) or anaplastic thyroid carcinomas (ATCs)-are refractory to radioactive iodine and exhibit limited responses to chemotherapy. Thus, safe and effective treatments for the latter aggressive thyroid tumors are urgently needed. Despite a strong preclinical rationale for targeting the IGF axis in thyroid cancer, the results of the available clinical studies have been disappointing, possibly because of the crosstalk between IGF signaling and other pathways that may result in resistance to targeted agents aimed against individual components of these complex signaling networks. Based on these observations, the combinations between IGF-signaling inhibitors and other anti-tumor drugs, such as DNA damaging agents or kinase inhibitors, may represent a promising therapeutic strategy for undifferentiated thyroid carcinomas. In this review, we discuss the role of the IGF axis in thyroid tumorigenesis and also provide an update on the current knowledge of IGF-targeted combination therapies for thyroid cancer.

Uveal Versus Cutaneous Melanoma; Same Origin, Very Distinct Tumor Types

Here, we critically evaluated the knowledge on cutaneous melanoma (CM) and uveal melanoma (UM). Both cancer types derive from melanocytes that share the same embryonic origin and display the same cellular function. Despite their common origin, both CM and UM display extreme differences in their genetic alterations and biological behavior. We discuss the differences in genetic alterations, metastatic routes, tumor biology, and tumor-host interactions in the context of their clinical responses to targeted- and immunotherapy.

RNAi/CRISPR Screens: from a Pool to a Valid Hit

High-throughput genetic screens interfering with gene expression are invaluable tools to identify gene function and phenotype-to-genotype interactions. Implementing such screens in the laboratory is challenging, and the choice between currently available technologies based on RNAi and CRISPR/Cas9 (CRISPR-associated protein 9) is not trivial. Identifying reliable candidate hits requires a streamlined experimental setup adjusted to the specific biological question. Here, we provide a critical assessment of the various RNAi/CRISPR approaches to pooled screens and discuss their advantages and pitfalls. We specify a set of best practices for key parameters enabling a reproducible screen and provide a detailed overview of analysis methods and repositories for identifying the best candidate gene hits.

Targeting Insulin-Like Growth Factor-I and Extracellular Matrix Interactions in Melanoma Progression

Insulin-like growth factor (IGF)-I binds to the ECM protein vitronectin (VN) through IGF binding proteins (IGFBPs) to enhance proliferation and migration of skin keratinocytes and fibroblasts. Although evidence exists for the role of individual components of the complex (IGF-I, IGFBP-3 and VN), the cellular functions stimulated by these proteins together as a complex remains un-investigated in melanoma cells. We report here that the IGF-I:IGFBP-3:VN trimeric complex stimulates a dose-dependent increase in the proliferation and migration of WM35 and Sk-MEL28 melanoma cells. In 3D Matrigel^™ and hydrogel cultures, both cell lines formed primary tumor-like spheroids, which increased in size in a dose-dependent manner in response to the trimeric complex. Furthermore, we reveal IGFBP-3:VN protein complexes in malignant melanoma and squamous cell carcinoma patient tissues, where the IGFBP-3:VN complex was seen to be predominantly tumor cell-associated. Peptide antagonists designed to target the binding of IGF-I:IGFBP-3 to VN were demonstrated to inhibit IGF-I:IGFBP-3:VN-stimulated cell migration, invasion and 3D tumor cell growth of melanoma cells. Overall, this study provides new data on IGF:ECM interactions in skin malignancies and demonstrates the potential usefulness of a growth factor:ECM-disrupting strategy for abrogating tumor progression.

Aberrant SIRT6 expression contributes to melanoma growth: Role of the autophagy paradox and IGF-AKT signaling

Melanoma is among the most life-threatening cancers. The pathogenesis of melanoma has not been fully elucidated. Recently, dysregulated macroautophagy/autophagy has been found to play a critical but inconsistent role in modulating melanoma growth at different stages, with the regulatory mechanism unclear. The histone deacetylase SIRT6 (sirtuin 6) is a known autophagy regulator, and its involvement in cancer development has been reported. Therefore, we sought to determine the role of SIRT6 in melanoma growth and detect its possible link with autophagy in the current study. We initially observed that the expression of SIRT6 decreased in primary melanoma but increased in metastatic melanoma compared with melanocytic nevus. Notably, the expression of SIRT6 was significantly correlated with the expression of autophagy biomarkers including MAP1LC3/LC3 and SQSTM1/p62. Furthermore, SIRT6 suppressed the growth of primary melanoma but promoted metastatic melanoma development in an autophagy-dependent way in vitro. Moreover, SIRT6 exerted its regulation on melanoma growth via the IGF-AKT signaling pathway, and the intervention of AKT could partly reverse the effects of SIRT6 on melanoma growth by regulating autophagy. At last, we determined the effects of SIRT6 on melanoma development in vivo. Taken together, our findings demonstrate that the bimodal expression of SIRT6 at different melanoma stages plays a critical role in regulating melanoma growth through an autophagy-dependent manner, which indicates the potential of SIRT6 to be a biomarker and a therapeutic target in melanoma.

Overcoming resistance to BRAF inhibitors

The discovery of activating mutations in the serine/threonine (S/T) kinase BRAF followed by a wave of follow-up research manifested that the MAPK-pathway plays a critical role in melanoma initiation and progression. BRAF and MEK inhibitors produce an unparalleled response rate in melanoma, but it is now clear that most responses are transient, and while some patients show long lasting responses the majority progress within 1 year. In accordance with the key role played by the MAPK-pathway in BRAF mutant melanomas, disease progression is mostly due to the appearance of drug-resistance mechanisms leading to restoration of MAPK-pathway activity. In the present article we will review the development, application and clinical effects of BRAF and MEK inhibitors both, as single agent and in combination in the context of targeted therapy in melanoma. We will then describe the most prominent mechanisms of resistance found in patients progressed on these targeted therapies. Finally we will discuss strategies for further optimizing the use of MAPK inhibitors and will describe the potential of alternative combination therapies to either delay the onset of resistance to MAPK inhibitors or directly target specific mechanisms of resistance to BRAF/MEK inhibitors.

Insulin-Like Growth Factor (IGF) Pathway Targeting in Cancer: Role of the IGF Axis and Opportunities for Future Combination Studies

Despite a strong preclinical rationale for targeting the insulin-like growth factor (IGF) axis in cancer, clinical studies of IGF-1 receptor (IGF-1R)-targeted monotherapies have been largely disappointing, and any potential success has been limited by the lack of validated predictive biomarkers for patient enrichment. A large body of preclinical evidence suggests that the key role of the IGF axis in cancer is in driving treatment resistance, via general proliferative/survival mechanisms, interactions with other mitogenic signaling networks, and class-specific mechanisms such as DNA damage repair. Consequently, combining IGF-targeted agents with standard cytotoxic agents, other targeted agents, endocrine therapies, or immunotherapies represents an attractive therapeutic approach. Anti-IGF-1R monoclonal antibodies (mAbs) do not inhibit IGF ligand 2 (IGF-2) activation of the insulin receptor isoform-A (INSR-A), which may limit their anti-proliferative activity. In addition, due to their lack of specificity, IGF-1R tyrosine kinase inhibitors are associated with hyperglycemia as a result of interference with signaling through the classical metabolic INSR-B isoform; this may preclude their use at clinically effective doses. Conversely, IGF-1/IGF-2 ligand-neutralizing mAbs inhibit proliferative/anti-apoptotic signaling via IGF-1R and INSR-A, without compromising the metabolic function of INSR-B. Therefore, combination regimens that include these agents may be more efficacious and tolerable versus IGF-1R-targeted combinations. Herein, we review the preclinical and clinical experience with IGF-targeted therapies to-date, and discuss the rationale for future combination approaches as a means to overcome treatment resistance.

Enhanced response of melanoma cells to MEK inhibitors following unbiased IGF-1R down-regulation

Due to its ability to compensate for signals lost following therapeutic MAPK-inhibition, insulin-like growth factor type 1 receptor (IGF-1R) co-targeting is a rational approach for melanoma treatment. However IGF-1R conformational changes associated with its inhibition can preferentially activate MAPK-pathway in a kinase-independent manner, through a process known as biased signaling. We explored the impact of biased IGF-1R signaling, on response to MAPK inhibition in a panel of skin melanoma cell lines with differing MAPK and p53 mutation statuses. Specific siRNA towards IGF-1R down-regulates the receptor and all its signaling in a balanced manner, whilst IGF-1R targeting by small molecule Nutlin-3 parallels receptor degradation with a transient biased pERK1/2 activity, with both strategies synergizing with MEK1/2 inhibition. On the other hand, IGF-1R down-regulation by a targeted antibody (Figitumumab) induces a biased receptor conformation, preserved even when the receptor is exposed to the balanced natural ligand IGF-1. This process sustains MAPK activity and competes with the MEK1/2 inhibition. Our results indicate that IGF-1R down-regulation offers an approach to increase the sensitivity of melanoma cells to MAPK inhibition, and highlights that controlling biased signaling could provide greater specificity and precision required for multi-hit therapy.

Bis-anthracycline WP760 abrogates melanoma cell growth by transcription inhibition, p53 activation and IGF1R downregulation

Anthracycline chemotherapeutics, e.g. doxorubicin and daunorubicin, are active against a broad spectrum of cancers. Their cytotoxicity is mainly attributed to DNA intercalation, interference with topoisomerase activity, and induction of double-stranded DNA breaks. Since modification of anthracyclines can profoundly affect their pharmacological properties we attempted to elucidate the mechanism of action, and identify possible molecular targets, of bis-anthracycline WP760 which previously demonstrated anti-melanoma activity at low nanomolar concentrations. We studied the effect of WP760 on several human melanoma cell lines derived from tumors in various development stages and having different genetic backgrounds. WP760 inhibited cell proliferation (IC₅₀ = 1-99 nM), impaired clonogenic cell survival (100 nM), and inhibited spheroid growth (≥300 nM). WP760 did not induce double-stranded DNA breaks but strongly inhibited global transcription. Moreover, WP760 caused nucleolar stress and led to activation of the p53 pathway. PCR array analysis showed that WP760 suppressed transcription of ten genes (ABCC1, MTOR, IGF1R, EGFR, GRB2, PRKCA, PRKCE, HDAC4, TXNRD1, AKT1) associated with, inter alia, cytoprotective mechanisms initiated in cancer cells during chemotherapy. Furthermore, WP760 downregulated IGF1R and upregulated PLK2 expression in most of the tested melanoma cell lines. These results suggest that WP760 exerts anti-melanoma activity by targeting global transcription and activation of the p53 pathway and could become suitable as an effective therapeutic agent.

Enhancing the evaluation of PI3K inhibitors through 3D melanoma models

Targeted therapies for mutant BRAF metastatic melanoma are effective but not curative due to acquisition of resistance. PI3K signaling is a common mediator of therapy resistance in melanoma; thus, the need for effective PI3K inhibitors is critical. However, testing PI3K inhibitors in adherent cultures is not always reflective of their potential in vivo. To emphasize this, we compared PI3K inhibitors of different specificity in two- and three-dimensional (2D, 3D) melanoma models and show that drug response predictions gain from evaluation using 3D models. Our results in 3D demonstrate the anti-invasive potential of PI3K inhibitors and that drugs such as PX-866 have beneficial activity in physiological models alone and when combined with BRAF inhibition. These assays finally help highlight pathway effectors that could be involved in drug response in different environments (e.g. p4E-BP1). Our findings show the advantages of 3D melanoma models to enhance our understanding of PI3K inhibitors.