Nicotinamide Promotes Apoptosis In Chronic Lymphocytic Leukemia through Activation of the p53/Mir-34a/SIRT1 Tumor Suppressor Network

Abstract 4627

Nicotinamide (Nam), is the main precursor of nicotinamide adenine dinucleotide (NAD⁺). It regulates intracellular levels of NAD⁺ and consequently activities of four classes of NAD⁺-consuming enzymes, including NADases, mono-ADP-ribosyl transferases (ARTs), poly-ADP-ribose polymerases (PARPs) and sirtuins.

Pharmacological doses of Nam inhibit the physiological activation and proliferation of mouse B lymphocytes, suggesting that this agent might affect also human B cell homeostasis. We approaches this issue by comparing the effects of Nam on normal vs. leukemic B lymphocytes. Chronic lymphocytic leukemia (CLL) was selected as disease model, for testing in vitro the therapeutic potential of Nam, due its intrinsic resistance to apoptosis, mediated by an imbalance in the mechanisms regulating cell death, mainly regulated through the activities of NAD⁺-dependent enzymes.

This study shows that pharmacological doses of Nam (5-10 mM) significantly inhibit proliferation and induce apoptosis of CLL cells. At earlier time points, Nam markedly reduces phosphorylation of multiple intracellular substrates, including ERK1/2. Normal B lymphocytes, used as control, were significantly less sensitive to the action of Nam. We hypothesized that these effects could be explained at least in part as a consequence of the inhibitory effects of Nam on NAD⁺-consuming enzymes. Attention was focused on SIRT1, a deacetylase that plays a critical role in cancer and that acts as a longevity factor. The results demonstrate that Nam exposure inhibits the activity, and also the expression of SIRT1. This effect is apparent only in leukemic cells, where SIRT1 protein levels are significantly higher than in normal B lymphocytes, obtained from spleen or tonsils, markedly less sensitive to Nam effects. The functional block of SIRT1 induced by Nam is followed by activation of p53, transcription of miR-34a and translational repression of SIRT1 mRNA (p53/miR-34a/SIRT1 functional loop). The endpoint is the activation of apoptosis. The same loop is the target of conventional DNA-damaging drugs, such as etoposide. Thus, addition of Nam to conventional DNA-damaging chemotherapeutics agents, leads to an inhibition of SIRT1 through two independent and synergic pathways, resulting in additive effects on apoptosis.

In conclusion this work suggests that Nam represents a potentially useful non-chemotherapeutic agent, characterized by a known and established safety profile, to be associated to conventional cytotoxic drugs in the treatment of selected forms of CLL.