Regulatory module involving FGF13, miR-504, and p53 regulates ribosomal biogenesis and supports cancer cell survival.

The microRNA miR-504 targets TP53 mRNA encoding the p53 tumor suppressor. miR-504 resides within the fibroblast growth factor 13 (FGF13) gene, which is overexpressed in various cancers. We report that the FGF13 locus, comprising FGF13 and miR-504, is transcriptionally repressed by p53, defining an additional negative feedback loop in the p53 network. Furthermore, we show that FGF13 1A is a nucleolar protein that represses ribosomal RNA transcription and attenuates protein synthesis. Importantly, in cancer cells expressing high levels of FGF13, the depletion of FGF13 elicits increased proteostasis stress, associated with the accumulation of reactive oxygen species and apoptosis. Notably, stepwise neoplastic transformation is accompanied by a gradual increase in FGF13 expression and increased dependence on FGF13 for survival ("nononcogene addiction"). Moreover, FGF13 overexpression enables cells to cope more effectively with the stress elicited by oncogenic Ras protein. We propose that, in cells in which activated oncogenes drive excessive protein synthesis, FGF13 may favor survival by maintaining translation rates at a level compatible with the protein quality-control capacity of the cell. Thus, FGF13 may serve as an enabler, allowing cancer cells to evade proteostasis stress triggered by oncogene activation.

MAGE-A tumor antigens target p53 transactivation function through histone deacetylase recruitment and confer resistance to chemotherapeutic agents.

The MAGE gene family is characterized by a conserved domain (MAGE Homology Domain). A subset of highly homologous MAGE genes (group A; MAGE-A) belong to the chromosome X-clustered cancer/testis antigens. MAGE-A genes are normally expressed in the human germ line and overexpressed in various tumor types; however, their biological function is largely unknown. Here we present evidence indicating that MageA2 protein, belonging to the MAGE-A subfamily, confers wild-type-p53-sensitive resistance to etoposide (ET) by inducing a novel p53 inhibitory loop involving recruitment of histone deacetylase 3 (HDAC3) to MageA2/p53 complex, thus strongly down-regulating p53 transactivation function. In fact, enhanced MageA2 protein levels, in addition to ET resistance, correlate with impaired acetylation of both p53 and histones surrounding p53-binding sites. Association between MAGE-A expression levels and resistance to ET treatment is clearly shown in short-term cell lines obtained from melanoma biopsies harboring wild-type-p53, whereas cells naturally, or siRNA-mediated expressing low MAGE-A levels, correlate with enhanced p53-dependent sensitivity to ET. In addition, combined trichostatin A/ET treatment in melanoma cells expressing high MAGE-A levels reestablishes p53 response and reverts the chemoresistance.

Lovastatin alters cytoskeleton organization and inhibits experimental metastasis of mammary carcinoma cells.

Lovastatin is a competitive inhibitor of 3-hydroxy 3-methylglutaryl coenzyme A reductase, the key regulatory enzyme of cholesterol biosynthesis. This enzyme catalyzes the formation of mevalonate, which is also the precursor of isoprenoid moieties, such as farnesol and geraniol, that are incorporated into several molecules essential for tumor cell signaling. Here, we describe that pretreatment with a non-cytotoxic concentration of lovastatin (10 microM) dramatically inhibited the metastatic ability of F311 mammary carcinoma cells in syngeneic BALB/c mice. Similarly, daily i.p. treatment of animals with a well-tolerated dose of lovastatin (10 mg/kg/day) significantly reduced the number of experimental lung metastases. In vitro, incubation of F3II monolayers in the presence of lovastatin caused a rounded-cell morphology. Immunofluorescence analysis revealed a lack of cortical actin organization, micrutubule disruption and inhibition of integrin-mediated focal contacts in lovastatin-treated cells. Exposure of F3II cells to lovastatin significantly inhibited tumor cell adhesion and migration, and coincubation with the cholesterol precursor mevalonate prevented these effects. Lovastatin reduced membrane localization of Rho protein, a signaling molecule involved in the regulation of actin-based cell motility that needs geranylation for membrane association and activation. In addition, lovastatin induced a dose-dependent inhibition in the secretion of urokinase, a key proteolytic enzyme during tumor invasion and metastasis, and a significant increase of tissue-type plasminogen activator, a marker of good prognosis in mammary cancer. These data suggest that antimetastatic properties of lovastatin are strongly associated with alterations in cytoskeleton organization and the consequent modulation of adhesion, motility and proteolysis.