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Enhanced Antitumor Therapy by Inhibition of p21^waf1 in Human Malignant Mesothelioma

Authors' Affiliations: ¹ Department of Molecular Pathology and Innovative Therapies, Marche University, Ancona, Italy; ² Center of Cytology, Research Department, Italian National Research Centers on Aging, Ancona, Italy; and ³ Pathology Unit, Azienda Ospedaliera, Alessandria, Italy

Requests for reprints: Alfonso Catalano, Dipartimento di Patologia Molecolare, Politecnica delle Marche, Via Tronto 10/A, 60100, Ancona, Italy. Phone: 39-0712206245; Fax: 39-0712206240; E-mail: a.catalano{at}univpm.it.

Purpose: The p21 cyclin-dependent kinase inhibitor was frequently expressed in human malignant pleural mesothelioma (MPM) tissues as well as cell lines. Recent data indicate that p21 keeps tumor cells alive after DNA damage, favoring a survival advantage. In this study, we assessed the possibility of p21 suppression as a therapeutic target for MPM.

Experimental Design: We established two different MPM-derived (from H28 and H2052 cells) subclones using vector-based short hairpin RNA (shRNA). Then, chemosensitivity against low doses of antineoplastic DNA-damaging agents was investigated by colony formation assays, and furthermore, the type of cell response induced by these drugs was analyzed. To examine the effect of p21 shRNA on chemosensitivity in vivo, tumor formation assays in nude mice were done.

Results: In colony formation assay, the IC₅₀ of doxorubicin was 33 ± 3.0 nmol/L in p21 shRNA-transfected cells with respect to 125 ± 10 nmol/L of control vector–transfected cells. This enhancement of growth inhibition was achieved by converting a senescence-like growth arrest to apoptosis in response to doxorubicin, etoposide, and CPT11. In the in vivo assays, CPT11 and loss-of-expression of p21 in combination led to considerable suppression of tumor growth associated with a substantially enhanced apoptotic response, whereas CPT11 alone was ineffective at inducing these responses.

Conclusions: These results indicated that p21 might play an important role in chemosensitivity to anticancer agents, and the suppression of its expression might be a potential therapeutic target for MPM.