Traditionally, natural products have played a major role in drug discovery by serving as the source of earliest medicine. In spite of the scientific advancements and the promise of alternative drug discovery strategies in the recent decades, there is still a shortage of drugs leading to clinical trials for various diseases. Calothrixin B and its N-oxide derivative, calothrixin A are two bioactive metabolites isolated from the cyanobacteria Calothrix. Calothrixins possess a wide array of biological activities and they are effective in inhibiting the growth of various human cancer cells at nanomolar concentrations. The present study was undertaken to elucidate whether calothrixin A has the ability to block the growth of melanoma cells, as melanoma is the leading cause of death due to skin diseases in the United States. Here, we report that treatment of human melanoma cells (A375 and Hs294t) with calothrixin A significantly inhibited the activity of histone deacetylases (HDACs) while enhanced the activity of histone acetylation (HAT) in melanoma cells. Calothrixin A inhibited HDAC activity by 10-60% in A375 cells and 20-50% in Hs294t cells in a dose-dependent manner when cells were treated for 24 h. Whereas, HAT activity was increased significantly in melanoma cells in a dose-dependent (0, 0.4, 0.8, 1.6 μM) manner. Class I HDACs have been considered as potential therapeutic targets for the treatment of cancers. Western blot analysis revealed that the treatment of melanoma cells with calothrixin A resulted in a dose-dependent decrease in the levels of class I HDAC proteins (HDAC 1, 2, 3 and 8). These epigenetic changes in melanoma cells by calothrixin A were associated with significant reduction in cell viability of melanoma cells (4-64% in A375; 4-80% in Hs294t cells) in a dose- and time-dependent manner. Further, treatment of cells with calothrixin A resulted in reactivation of tumor suppressor gene, as evident by the enhanced expression of Cip1/WAF1/p21 protein. HDACs also regulate many biologic processes, including cell cycle regulation and cell differentiations. Western blot analysis revealed that treatment of melanoma cells with calothrixin A resulted in reduction in the levels of regulatory proteins of G0/G1 phase of cell cycle, such as cyclins (cyclin D1 and D2) and cyclin dependent kinases (CDKs). Flow cytometry analysis revealed that calothrixin A enhanced apoptotic cell death of melanoma cells, which was associated with the increased expression of proapoptotic protein Bax and decreased expression of antiapoptotic proteins (Bcl-2 and Bcl-xL). In conclusion, our study demonstrates for the first time that calothrixin A has the ability to block the growth of melanoma cells and it is mediated through targeting class I HDACs in micromolar concentrations.

Keywords: Calothrixin A, Cyanobacteria, Calothrix, HDAC, Melanoma

Citation Format: Tripti Singh, Su Xu, Sadanandan Velu, Santosh K. Katiyar. Calothrixin A, a metabolite from Calothrix cyanobacteria, inhibits class I histone deacetylases leading to suppression of cell growth and induction of apoptosis in human melanoma cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5386. doi:10.1158/1538-7445.AM2015-5386