Role of Oncogenic Protein Kinase C-iota in Melanoma Progression; A Study Based on Atypical Protein Kinase-C Inhibitors

Irrespective of plentiful efforts to enhance primary prevention and early detection, the number of melanoma cases in the United States has increased steadily over the past 30 years, thus greatly affecting public health and the economy. We have investigated the effects of five novel aPKC inhibitors; 2-acetyl-1,3-cyclopentanedione (ACPD), 3,4-Diaminonaphthalene-2,7-disulfonic acid (DNDA), [4-(5-amino-4-carbamoylimidazol-1-yl)-2,3-dihydroxycyclopentyl] methyl dihydrogen phosphate (ICA-1T) along with its nucleoside analog 5-amino-1-((1R,2S,3S,4R)-2,3-dihydroxy-4-methylcyclopentyl)-1H-imidazole-4-carboxamide (ICA-1S) and 8-hydroxy-1,3,6-naphthalenetrisulfonic acid (ζ-Stat) on cell proliferation, apoptosis, migration and invasion of two malignant melanoma cell lines compared to normal melanocyte cell lines. Molecular docking data suggested that both ACPD and DNDA specifically bind to protein kinase C-zeta (PKC-ζ) and PKC-iota (PKC-ι) while both ICA-1 compounds specifically bind to PKC-ι, and ζ-Stat showed a high affinity towards PKC-ζ. Kinase activity assays were carried out to confirm these observations. Results suggest that PKC-ι is involved in melanoma malignancy than PKC-ζ. Both isoforms promote the activation of nuclear factor (NF)-κB and protein kinase B (AKT) thereby supporting survival and progression. In addition, we demonstrated that PKC-ι induced the metastasis of melanoma cells by activating Vimentin, and PKC-ι inhibition downregulated epithilial-mesencymal transition (EMT), while inducing apoptosis. Of note, PKC-ἱ specific inhibitors downregulated the expression of both PKC-ι and phosphorylated PKC-ι, suggesting that PKC-ι plays a role in regulating its own expression in melanoma. We also report the underlaying mechanisms of the transcriptional regulation of PKC-ι (PRKCI gene) expression in melanoma. c-Jun, interferon-stimulated gene factor 3 (ISGF3), paired box gene 3 (PAX3), early growth response protein 1 (EGR1) and forkhead box protein O1 (FOXO1), which bind on or near the promoter sequence of the PRKCI gene, were analyzed for their role in PKC-ι regulation in SK-MEL-2 and MeWo cell lines. We silenced selected transcription factors using siRNA, and the results revealed that the silencing of c-Jun and FOXO1 significantly altered the expression of PRKCI. The levels of both phosphorylated and total PKC-ι increased upon FOXO1 silencing and decreased upon c-Jun silencing, suggesting that c-Jun acts as an upregulator, while FOXO1 acts as a downregulator of PRKCI expression. We also used a multiplex ELISA to analyze multiple pathways other than NF-κB that were affected by treatment with PKC-ι inhibitor. The silencing of NF-κB p65 and PKC-ι by siRNA suggested that the regulation of PKC-ι expression was strongly associated with FOXO1. In addition, we observed a significant decrease in the mRNA levels of both interleukin (IL)-6 and IL-8, with a significant increase in the levels of IL-17E and intercellular adhesion molecule 1 (ICAM-1) upon the knockdown of expression of PKC-ι in both cell lines. This suggested that PKC-ι expression was affected by these cytokines in an autocrine manner. Overall, the findings of this study suggest that PKC-ι inhibition suppresses its own expression, diminishing oncogenic signaling, while upregulating anti-tumor signaling, thus rendering it an effective novel biomarker for use in the design of novel targeted therapeutics for melanoma