Samisubbu R. Naidu

PIASy-mediated Tip60 sumoylation regulates p53-induced autophagy

Posttranslational modifications of p53 integrate diverse stress signals and regulate its activity, but their combinatorial contribution to overall p53 function is not clear. We investigated the roles of lysine (K) acetylation and sumoylation on p53 and their relation to apoptosis and autophagy. Here we describe the collaborative role of the SUMO E3 ligase PIASy and the lysine acetyltransferase Tip60 in p53-mediated autophagy. PIASy binding to p53 and PIASy-activated Tip60 lead to K386 sumoylation and K120 acetylation of p53, respectively. Even though these two modifications are not dependent on each other, together they act as a “binary death signal” to promote cytoplasmic accumulation of p53 and execution of PUMA-independent autophagy. PIASy-induced Tip60 sumoylation augments p53 K120 acetylation and apoptosis. In addition to p14ARF inactivation, impairment in this intricate signaling may explain why p53 mutations are not found in nearly 50% of malignancies.

Chloroquine Promotes Apoptosis in Melanoma Cells by Inhibiting BH3 domain Mediated PUMA Degradation

The BH3-only protein PUMA counters Bcl-2 family anti-apoptotic proteins and promotes apoptosis. Although PUMA is a key regulator of apoptosis, the post-transcriptional mechanisms that control PUMA protein stability are not understood. We show that a lysosome-independent activity of chloroquine prevents degradation of PUMA protein, promotes apoptosis and reduces the growth of melanoma xenografts in mice. Compared to wild–type PUMA, a BH3 domain deleted PUMA protein showed impaired decay in melanoma cells. Fusion of the BH3 domain to a heterologous protein led to its rapid turnover that was inhibited by chloroquine. While both chloroquine and inhibitors of lysosomal proteases stalled autophagy, only choroquine stabilized PUMA protein and promoted apoptosis. Our results reveal a lysosomal protease independent activity of chloroquine that selectively promotes apoptosis in melanoma cells.

Glucose-independent Acetate Metabolism Promotes Melanoma Cell Survival and Tumor Growth

Tumors rely on multiple nutrients to meet cellular bioenergetics and macromolecular synthesis demands of rapidly dividing cells. Although the role of glucose and glutamine in cancer metabolism is well understood, the relative contribution of acetate metabolism remains to be clarified. We show that glutamine supplementation is not sufficient to prevent loss of cell viability in a subset of glucose-deprived melanoma cells, but synergizes with acetate to support cell survival. Glucose-deprived melanoma cells depend on both oxidative phosphorylation and acetate metabolism for cell survival. Acetate supplementation significantly contributed to maintenance of ATP levels in glucose-starved cells. Unlike acetate, short chain fatty acids such as butyrate and propionate failed to prevent loss of cell viability from glucose deprivation. In vivo studies revealed that in addition to nucleo-cytoplasmic acetate assimilating enzyme ACSS2, mitochondrial ACSS1 was critical for melanoma tumor growth in mice. Our data indicate that acetate metabolism may be a potential therapeutic target for BRAF mutant melanoma.

Impaired PIASy-Tip60 signaling weakens activation of p53 in melanoma.

The tumor suppressor p53 plays a central role in preventing tumor development by promoting transcription of genes that stall cell cycle and induce cell death. Although the majority of melanomas express wild-type p53, the molecular mechanisms that impede its activation remain unclear. We previously reported that the SUMO E3 ligase PIASy and the histone acetyltransferase Tip60 signaling cascade promote p53-dependent autophagy and apoptosis. We hypothesized that impairment in this signaling attenuates p53, thus disabling its apoptotic function in melanoma. Here, we show that human melanoma patient samples and cell lines maintain p53 expression but PIASy and/or Tip60 are frequently lost. We observed dysregulation of Tip60-mediated p53 transcription program in melanoma cell lines. Reconstitution of PIASy and Tip60 in melanoma cells increased genotoxic stress-induced apoptosis. Our study provides a clinical link of how sumoylation signaling may activate p53-mediated cell death in melanoma.

Lapachol inhibits glycolysis in cancer cells by targeting pyruvate kinase M2

Reliance on aerobic glycolysis is one of the hallmarks of cancer. Although pyruvate kinase M2 (PKM2) is a key mediator of glycolysis in cancer cells, lack of selective agents that target PKM2 remains a challenge in exploiting metabolic pathways for cancer therapy. We report that unlike its structural analog shikonin, a known inhibitor of PKM2, lapachol failed to induce non-apoptotic cell death ferroxitosis in hypoxia. However, melanoma cells treated with lapachol showed a dose-dependent inhibition of glycolysis and a corresponding increase in oxygen consumption. Accordingly, in silico studies revealed a high affinity-binding pocket for lapachol on PKM2 structure. Lapachol inhibited PKM2 activity of purified enzyme as well as in melanoma cell extracts. Blockade of glycolysis by lapachol in melanoma cells led to decreased ATP levels and inhibition of cell proliferation. Furthermore, perturbation of glycolysis in melanoma cells with lapachol sensitized cells to mitochondrial protonophore and promoted apoptosis. These results present lapachol as an inhibitor of PKM2 to interrogate metabolic plasticity in tumor cells.

Cyclic Amp-epac signaling pathway contributes to repression of Puma transcription in melanoma cells

The universal second messenger cAMP regulates numerous cellular processes. Although the cAMP-signaling pathway leads to induction of gene transcription, it remains unknown whether this pathway contributes toward suppression of transcription. Here, we show that blockade of cAMP signaling using MDL12330A led to an increase in PUMA transcript levels, but not p21 in melanoma cells. cAMP downstream component Epac activation was essential for suppression of PUMA transcription as an Epac agonist reversed the effects of MDL12330A. These results suggest that transcriptional repression is one of the functions of the cAMP-Epac signaling pathway.