The siRNA-mediated downregulation of N-Ras sensitizes human melanoma cells to apoptosis induced by selective BRAF inhibitors

New Approaches to Targeted Therapy in Melanoma

It was just slightly more than a decade ago when metastatic melanoma carried a dismal prognosis with few, if any, effective therapies. Since then, the evolution of cancer immunotherapy has led to new and effective treatment approaches for melanoma. However, despite these advances, a sizable portion of patients with advanced melanoma have de novo or acquired resistance to immune checkpoint inhibitors. At the same time, therapies (BRAF plus MEK inhibitors) targeting the BRAF^V600 mutations found in 40-50% of cutaneous melanomas have also been critical for optimizing management and improving patient outcomes. Even though immunotherapy has been established as the initial therapy in most patients with cutaneous melanoma, subsequent effective therapy is limited to BRAF^V600 melanoma. For all other melanoma patients, driver mutations have not been effectively targeted. Numerous efforts are underway to target melanomas with NRAS mutations, NF-1 LOF mutations, and other genetic alterations leading to activation of the MAP kinase pathway. In this era of personalized medicine, we will review the current genetic landscape, molecular classifications, emerging drug targets, and the potential for combination therapies for non-BRAF^V600 melanoma.

Mechanisms of Acquired BRAF Inhibitor Resistance in Melanoma: A Systematic Review

This systematic review investigated the literature on acquired v-raf murine sarcoma viral oncogene homolog B1 (BRAF) inhibitor resistance in patients with melanoma. We searched MEDLINE for articles on BRAF inhibitor resistance in patients with melanoma published since January 2010 in the following areas: (1) genetic basis of resistance; (2) epigenetic and transcriptomic mechanisms; (3) influence of the immune system on resistance development; and (4) combination therapy to overcome resistance. Common resistance mutations in melanoma are BRAF splice variants, BRAF amplification, neuroblastoma RAS viral oncogene homolog (NRAS) mutations and mitogen-activated protein kinase kinase 1/2 (MEK1/2) mutations. Genetic and epigenetic changes reactivate previously blocked mitogen-activated protein kinase (MAPK) pathways, activate alternative signaling pathways, and cause epithelial-to-mesenchymal transition. Once BRAF inhibitor resistance develops, the tumor microenvironment reverts to a low immunogenic state secondary to the induction of programmed cell death ligand-1. Combining a BRAF inhibitor with a MEK inhibitor delays resistance development and increases duration of response. Multiple other combinations based on known mechanisms of resistance are being investigated. BRAF inhibitor-resistant cells develop a range of 'escape routes', so multiple different treatment targets will probably be required to overcome resistance. In the future, it may be possible to personalize combination therapy towards the specific resistance pathway in individual patients.

Upregulation of S100A9 contributes to the acquired resistance to BRAF inhibitors

BACKGROUNDS

Acquired resistance is a significant clinical challenge in targeted therapy of melanomas using BRAF inhibitors. We previously identified that downregulation of miR-92a-1-5p confers acquired resistance to BRAF inhibition using an miRNA array platform.

OBJECTIVE

In this study, we investigated the target genes of miR-92a-1-5p and their functional significance in BRAF inhibitor resistance.

METHODS

The miRNA target prediction data were combined with RNA-Seq data to identify possible target genes for miR-92a-1-5p. Cellular effects of target genes were further examined using siRNA knockdown, WST-1 assay, and immunoblotting analysis.

RESULTS

We selected S100 calcium-binding protein A9 (S100A9) as a possible target gene for functional validation. S100A9 knockdown abrogated resistance to PLX4720 in A375P/Mdr cells. This result was similar to those described earlier for miR-92a-1-5p, indicating that miR-92a-1-5p inhibits cell viability by targeting S100A9. S100A9 overexpression partially conferred PLX4720 resistance to A375P cells. We also demonstrated that MAPK re-activation does not contribute to the promotion of BRAF inhibitor resistance by S100A9.

CONCLUSION

Taken together, our results indicate that S100A9 might be functionally involved in development of resistance to BRAF inhibitors and might be a target for melanoma therapy in the future.

A Theoretical Study on Inhibition of Melanoma with Controlled and Targeted Delivery of siRNA via Skin Using SPACE-EGF

Melanoma is a potentially lethal skin cancer with high mortality rate. Recently, the peptide-mediated transdermal delivery of small interference RNA (siRNA) emerges as a promising strategy to treat melanoma by inducing the apoptosis of tumor cells, but the related theoretical model describing the delivery of siRNA under the effect of SPACE-EGF, the growth inhibition of melanoma and the dynamic expanding of the bump on the skin due to the growth of melanoma has not been reported yet. In this article, a theoretical model is developed to describe the percutaneous siRNA delivery mediated by SPACE-EGF to melanoma and the growth inhibition of melanoma. The results present the spatial-temporal distribution of siRNA and the growth of melanoma under the inhibition of siRNA, which shows a good consistency with the experimental results. In addition, this model represents the uplift process of tumors on the skin surface. The model presented here is a useful tool to understand the whole process of the SPACE-EGF-mediated delivery of the siRNA to melanoma through skin, to predict the therapeutic effect, and to optimize the therapeutic strategy, providing valuable references for the treatment of melanoma.

Topical and Targeted Delivery of siRNAs to Melanoma Cells Using a Fusion Peptide Carrier

Topical application of siRNAs through the skin is a potentially effective strategy for the treatment of melanoma tumors. In this study, we designed a new and safe fusion peptide carrier SPACE-EGF to improve the skin and cell penetration function of the siRNAs and their targeting ability to B16 cells, such that the apoptosis of B16 cells can be induced. The results show that the carrier is stable and less toxic. The EGF motif does not affect the skin and cell penetration function of the SPACE. Because EGF can strongly bind EGFR, which is overexpressed in cancer cells, the targeting ability of the SPACE-EGF-siRNA complex is increased. In vitro experiments indicate that GAPDH siRNAs conjugated with SPACE-EGF can significantly reduce the GAPDH concentration in B16 cells, and c-Myc siRNAs can cause the gene silencing of c-Myc and thus the apoptosis of cells. In vivo experiments show that the topical application of c-Myc siRNAs delivered by SPACE-EGF through the skin can significantly inhibit the growth of melanoma tumors. This work may provide insight into the development of new transdermal drug carriers to treat a variety of skin disorders.

Induction of Resistance to BRAF Inhibitor Is Associated with the Inability of Spry2 to Inhibit BRAF-V600E Activity in BRAF Mutant Cells

The clinical benefits of oncogenic BRAF inhibitor therapies are limited by the emergence of drug resistance. In this study, we investigated the role of a negative regulator of the MAPK pathway, Spry2, in acquired resistance using BRAF inhibitor-resistant derivatives of the BRAF-V600E melanoma (A375P/Mdr). Real-time RT-PCR analysis indicated that the expression of Spry2 was higher in A375P cells harboring the BRAF V600E mutation compared with wild-type BRAF-bearing cells (SK-MEL-2) that are resistant to BRAF inhibitors. This result suggests the ability of BRAF V600E to evade feedback suppression in cell lines with BRAF V600E mutations despite high Spry2 expression. Most interestingly, Spry2 exhibited strongly reduced expression in A375P/Mdr cells with acquired resistance to BRAF inhibitors. Furthermore, the overexpression of Spry2 partially restored sensitivity to the BRAF inhibitor PLX4720 in two BRAF inhibitor-resistant cells, indicating a positive role for Spry2 in the growth inhibition induced by BRAF inhibitors. On the other hand, long-term treatment with PLX4720 induced pERK reactivation following BRAF inhibition in A375P cells, indicating that negative feedback including Spry2 may be bypassed in BRAF mutant melanoma cells. In addition, the siRNA-mediated knockdown of Raf-1 attenuated the rebound activation of ERK stimulated by PLX4720 in A375P cells, strongly suggesting the positive role of Raf-1 kinase in ERK activation in response to BRAF inhibition. Taken together, these data suggest that RAF signaling may be released from negative feedback inhibition through interacting with Spry2, leading to ERK rebound and, consequently, the induction of acquired resistance to BRAF inhibitors.

Low inducible expression of p21Cip1 confers resistance to paclitaxel in BRAF mutant melanoma cells with acquired resistance to BRAF inhibitor

The therapeutic efficacy of oncogenic BRAF inhibitor is limited by the onset of acquired resistance. In this study, we investigated the potential therapeutic effects of the mitotic inhibitor paclitaxel on three melanoma cell lines with differing sensitivity to the BRAF inhibitor. Of the two BRAF inhibitor-resistant cell lines, A375P/Mdr cells harboring the BRAF V600E mutant were resistant and the wild-type BRAF SK-MEL-2 cells were sensitive to paclitaxel. In particular, paclitaxel caused the growth inhibition of SK-MEL-2 cells to a much greater extent than it caused growth inhibition of A375P cells. Paclitaxel exhibited no significant effect on the phosphorylation of MEK-ERK in any cell lines tested, regardless of both the BRAF mutation and the drug resistance, implying that paclitaxel activity is independent of MEK-ERK inhibition. In A375P cells, paclitaxel treatment resulted in a marked emergence of apoptotic cells after mitotic arrest, concomitant with a remarkable induction of p21(Cip1). However, paclitaxel only moderately increased the levels of p21(Cip1) in A375P/Mdr cells, which exhibited a strong resistance to paclitaxel. The p21(Cip1) overexpression partially conferred paclitaxel sensitivity to A375P/Mdr cells. Interestingly, we found an extremely low background expression level of p21(Cip1) in SK-MEL-2 cells lacking normal p53 function, which caused much greater G2/M arrest than that seen in A375P cells. Taken together, these results suggest that paclitaxel may be an effective anticancer agent through regulating the expression of p21(Cip1) for the treatment of BRAF mutant melanoma cells resistant to BRAF inhibitors.