No abstract

Human Monoglyceride Lipase (MGL) is a recently identified lipase and very little is known about its regulation and function in cellular regulatory processes, particularly in context to human malignancy. In this study, we investigated the regulation and function of Monoglyceride Lipase in human cancer(s) and report that MGL expression was either absent or reduced in the majority of primary colorectal cancers. Immunohistochemical studies showed that reduction of MGL expression in the colorectal tumor tissues predominantly occurred in the cancerous epithelial cells. MGL was found to reside in the core surface of a cellular organelle named “lipid body”. Furthermore, it was found to selectively interact with a number of phospholipids including phosphotidic acid and phosphoinositol(3,4,5)P3, phosphoinositol(3,5)P2, phosphoinositol(3,4)P2 and several other phosphoinositides, and among all phosphoinositides analyzed, its interaction with PI(3,4,5)P3 was found to be the strongest. In addition, overexpression of MGL suppressed colony formation in tumor cell lines and knockdown of MGL resulted in increased Akt phosphorylation. Together, our results suggest that MGL plays a negative regulatory role in PI3-K/Akt signaling and tumor cell growth.

WT p53 is critical for tumor suppression, whereas mutant p53 promotes tumor progression. Nerve injury-induced protein 1 (Ninj1) is a target of p53 and forms a feedback loop with p53 by repressing p53 mRNA translation. Here, we show that loss of increased mutant p53 expression and, subsequently, enhanced cell growth and migration in cells carrying a mutant p53. In contrast, loss of inhibited cell growth and migration in cells carrying a WT p53. To explore the biological significance of Ninj1, we generated a cohort of -deficient mice and found that mice were prone to systemic inflammation and insulitis, but not to spontaneous tumors. We also found that loss of altered the tumor susceptibility in both mutant and -null background. Specifically, in a mutant p53(R270H) background, deficiency shortened the lifespan, altered the tumor spectrum, and increased tumor burden, likely via enhanced expression of mutant p53. In a -null background, deficiency significantly increased the incidence of T-lymphoblastic lymphoma. Taken together, our data suggest that depending on p53 genetic status, Ninj1 has two opposing functions in tumorigenesis and that the Ninj1-p53 loop may be targeted to manage inflammatory diseases and cancer.

Rbm38 is a p53 target and an RNA-binding protein known to suppress p53 translation by preventing eukaryotic translation initiation factor 4E (eIF4E) from binding to p53 mRNA. In this study, we show that synthetic peptides corresponding to the binding interface between Rbm38 and eIF4E, including an 8 amino acid peptide (Pep8) derived from Rbm38, are effective in relieving Rbm38-mediated repression of p53. Molecular simulations showed that Ser-6 in Pep8 forms a hydrogen bond with Asp-202 in eIF4E. Substitution of Ser-6 with Lys, but not with Asp, enhanced the ability of Pep8 to inhibit the Rbm38-eIF4E complex. Importantly, Pep8 alone or together with a low dose of doxorubicin potently induced p53 expression and suppressed colony and tumor sphere formation and xenograft tumors in Rbm38-and p53-dependent manners. Together, we conclude that modulating the Rbm38-eIF4E complex may be explored as a therapeutic strategy for cancers that carry wild-type p53. Significance: Disruption of the Rbm38-eIF4E complex via synthetic peptides induces wild-type p53 expression, suppresses tumor growth and progression, and may serve as a novel cancer therapeutic strategy.

Esophageal cancer-related gene 2 (ECRG2) is a newer tumor suppressor whose function in the regulation of cell growth and apoptosis remains to be elucidated. Here we show that ECRG2 expression was upregulated in response to DNA damage, and increased ECRG2 expression induced growth suppression in cancer cells but not in non-cancerous epithelial cells. ECRG2-mediated growth suppression was associated with activation of caspases and marked reduction in the levels of apoptosis inhibitor, X chromosome-linked inhibitor of apoptosis protein (XIAP). ECRG2, via RNA-binding protein human antigen R (HuR), regulated XIAP mRNA stability and expression. Furthermore, ECRG2 increased HuR ubiquitination and degradation but was unable to modulate the non-ubiquitinable mutant form of HuR. We also identified missense and frame-shift ECRG2 mutations in various human malignancies and noted that, unlike wild-type ECRG2, one cancer-derived ECRG2 mutant harboring glutamic acid instead of valine at position 30 (V30E) failed to induce cell death and activation of caspases. This naturally occurring V30E mutant also did not suppress XIAP and HuR. Importantly, the V30E mutant overexpressing cancer cells acquired resistance against multiple anticancer drugs, thus suggesting that ECRG2 mutations appear to have an important role in the acquisition of anticancer drug resistance in a subset of human malignancies.