T-Cadherin-Mediated Cell Growth Regulation Involves G₂ Phase Arrest and Requires p21^CIP1/WAF1 Expression

Abstract

Members of the cadherin family have been implicated as growth regulators in multiple tumor types. Based on recent studies from our laboratory implicating T-cadherin expression in mouse brain tumorigenesis, we examined the role of T-cadherin in astrocytoma growth regulation. In this report, we show that T-cadherin expression increased during primary astrocyte physiologic growth arrest in response to contact inhibition and serum starvation in vitro, suggesting a function for T-cadherin in astrocyte growth regulation. We further demonstrate that transient and stable reexpression of T-cadherin in deficient C6 glioma cell lines results in growth suppression. In addition, T-cadherin-expressing C6 cell lines demonstrated increased homophilic cell aggregation, increased cell attachment to fibronectin, and decreased cell motility. Cell cycle flow cytometry demonstrated that T-cadherin reexpression resulted in G 2 phase arrest, which was confirmed by mitotic index analysis. This growth arrest was p53 independent, as T-cadherin could still mediate growth suppression in p53 ؊/؊ mouse embryonic fibroblasts. T-cadherin-expressing C6 cell lines exhibited increased p21 CIP1/WAF1 , but not p27 Kip1 , expression. Lastly, T-cadherin-mediated growth arrest was dependent on p21 CIP1/WAF1 expression and was eliminated in p21 CIP1/WAF1 -deficient fibroblasts. Collectively, these observations suggest a novel mechanism of growth regulation for T-cadherin involving p21 CIP1/WAF1 expression and G 2 arrest.Astrocytomas are the most common primary malignant cancer affecting the nervous system, and despite aggressive therapy, the median survival of patients diagnosed with a highgrade astrocytoma (glioma) is only 9 to 12 months (35). These malignant glial tumors are hypothesized to develop as the result of the stepwise accumulation of specific genetic changes in astrocytes or astroglial precursors that promote astrocyte transformation and malignant progression (8). Genetic events important for astrocytoma formation and progression involve alterations in pathways involved in mitogenic signaling, cell cycle growth regulation, and environmental sensing. Previous studies have demonstrated that astrocytomas harbor changes in the epidermal and platelet-derived growth factor signaling pathways, involving amplification, mutation, or overexpression of these receptor tyrosine kinase molecules to result in increased mitogenic signaling. Similarly, astrocytomas harbor alterations in the p53 and retinoblastoma (Rb) cell cycle regulatory pathways. Inactivating mutations in the p53, p16, and Rb genes have been reported as well as overexpression of regulators of these pathways, including cyclin-dependent kinase 4 (cdk4) and MDM2.In contrast, alterations in molecules involved in environmental sensing have not been explored in great detail in astrocytomas. Gene expression profiling experiments from our laboratory on a mouse astrocytoma model have implicated the novel cadherin molecule, T-or H-cadherin, in astrocytoma progression. Based on these initial observ...

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