Role of Oncogene-Induced Senescence in Thyroid Carcinogenesis

Vizioli, Maria Grazia (2012). Role of Oncogene-Induced Senescence in Thyroid Carcinogenesis. PhD thesis The Open University.



Oncogene-induced senescence (OIS) is an irreversible G1 cell cycle arrest triggered by aberrant expression of oncogenes. The arrest observed during senescence is implemented mainly through activation of p53 and the upregulation of the cyclin-dependent kinase (CDK) inhibitors, p16INK4a and p21CIP1. In vivo OIS may act as a barrier to neoplastic transformation, and bypass of OIS is a prerequisite for tumorigenesis.

In this work the role of OIS in thyroid carcinogenesis was investigated by in combination of in vitroand in vivo approaches.

We found that the expression of different thyroid tumour-associated oncogenes (BRAF, RAS, RET and TRK) in human primary thyrocytes triggers senescence, as demonstrated by the presence of OIS hallmarks. For instance, cells appeared flatted and enlarged; they displayed senescence-associated B-galactosidase activity (SA-β-Gal), senescence associated heterochromatic foci (SAHF), and high expression levels of the CDK inhibitors p16INK4a and p21CIP1 and p53. Additionally, using RNA-interference strategy we directly assessed the individual contribution of p16INK4a in BRAFV600E-induced senescence. In our experimental setting the inactivation of p16INK4a did not have any impact on oncogene-induced senescence suggesting the possibility that p16INK4a may cooperate with other factors in triggering senescence.

A major consequence of oncogene activation is the secretion of a plethora of proteins including growth factors, cytokines, and chemokines. Some of these secreted proteins are known to be regulators of OIS. For example, IGFBP7 is recently proposed as a mediator of BRAFV600E-induced senescence in melanocytes.

We showed that IGFBP7 gene is frequently downregulated in Papillary thyroid tumour (PTC); restoration of its expression in a PTC-derived, IGFBP7-negative cell line affects growth properties, migration and invasion, and this is associated to the induction of apoptosis. Thus, our results implicate IGFBP7 as a tumour suppressor protein in thyroid tumours. Additionally, we investigated the possible involvement of IGFBP7 in our cellular senescence model. Our data indicate that IGFBP7 expression is not changed during OIS.

We also established and characterized OIS inducible system in our cellular model; we transduced human primary thyrocytes with a retroviral vector carrying an activated form of RAS oncogene coupled to the ligand binding domain of the estrogen receptor. Upon addition of 4-hydroxytamoxifen (4-OHT), we observed a robust induction of senescence, including significantly reduced incorporation of BrdU and high levels of p16INK4a p21CIP1, and p53. Moreover, we assessed the relative importance of tumour suppressor proteins, p16INK4a and p53 in mediating OIS in H-RASG12V transfected thyrocytes. Surprisingly, silencing of p53 expression did not alter the growth arrest induced by H-RASG12V, whereas p16INK4a seems to be the dominant effector of senescence in this experimental setting, as its specific inactivation delays the onset of senescence.

We also found that senescence induced by H-RASG12V is associated with activation of inflammatory transcriptome including IL8 and its receptor CXCR2. We reported that knocking down the chemokine receptor CXCR2 either IL8 prevents H-RASG12V cells from undergoing OIS. To translate information from in vitroexperiments to a clinic setting, senescence markers identified in cultured cells were used to detect OIS in a panel of thyroid tumours characterized by different aggressiveness. Our immunohistochemical analysis showed that the expression of OIS markers is upregulated at early stages of the thyroid tumour and lost during tumour progression. These evidences support the notion that OIS may counteract oncogenic activity in thyroid tumours, and its escape allows thyroid tumour progression.

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