Transcriptional and epigenetic regulation of gene expression by arsenic in cancer and normal cells

Inorganic arsenic is a toxic metalloid that occurs naturally on the earth’s crust. Millions of people worldwide are exposed to inorganic arsenic via drinking water, and more recently also via contaminated food, in particular rice. Many studies have focused on exploring the adverse health effects of arsenic as well as its mode of action once entering the body. Still, little is known about the intracellular processes that drive arsenic toxicity. In the present thesis we investigated the effects of arsenic on transcriptional and epigenetic processes both in normal and cancer cell lines.

In order to understand how exogenous compounds affect transcriptional regulators such as MAML1, which is involved in many different signaling pathways and has been related to developmental processes and human diseases (e.g. cancer), it’s important to understand which cellular processes regulate the protein levels. Consequently, we studied the transcriptional co-activator MAML1 and the ubiquitination and degradation process. We show that MAML1 protein levels are regulated via ubiquitination and that this process is enhanced by p300 and repressed by N1ICD. On top of that, we also investigated MAML1 involvement in cell proliferation and epigenetic regulation as well as arsenic effect on MAML1 expression and kidney cell proliferation. We show that MAML1 interacts with DNMT1 and PCNA, both members of the DNMT1-PCNA-HDAC2 repressive complex, which is involved in epigenetic regulation. We further report that arsenic decreases kidney cell proliferation and we suggest this occurs via MAML1 downregulation.

In order to further explore the effects of arsenic on the epigenome, we studied arsenic exposure in relation to the post-translational histone modifications (PTHMs) H3K9me3 and H3K9Ac in lymphocytes isolated from exposed individuals as well as in vitro in cell culture system. We report arsenic-related changes in H3K9me3 epigenetic mark in CD4+ lymphocytes isolated from the arsenic exposed individuals and changes in H3K9Ac in in vitro cultured T lymphoblasts exposed to arsenic.

In conclusion, our data suggest that MAML1 protein levels are regulated via ubiquitination, a process that could also be targeted by arsenic and in this way influence gene expression. Moreover, we suggest that arsenic regulates MAML1 protein levels and could thereby also influence the cell signaling pathways depending on MAML1 transcriptional activity. Arsenic also targets epigenetic processes by altering the global levels of H3K9me3 and H3K9Ac in lymphocytes, which could lead to adverse health effects in the human population.

List of scientific papers

I. M. Farshbaf, M. J. Lindberg, A. Truong, Z. Bevens, E. Chambers, A. Pournara, A. E. Wallberg and J. B. White (2015). Mastermind-Like 1 Is Ubiquitinated: Functional Consequences for Notch Signaling. PLoS One, 10, e0134013.
https://doi.org/10.1371/journal.pone.0134013

II. A. Pournara, T. Holmlund, Y. Lu, R. Ceder, M. Putnik, R. Grafstrom, M. Vahter and A. E. Wallberg (2014). Arsenic-induced suppression of kidney cell proliferation and the transcriptional coregulator MAML1. Metallomics, 6, 498-504.
https://doi.org/10.1039/c3mt00333g

III. A. Pournara, M. Kippler, T. Holmlund, R.Ceder, R. Grafström, M. Vahter, K. Broberg and A. E. Wallberg (2016). Arsenic alters global histone modifications in lymphocytes in vitro and in vivo. [Manuscript]