1. ¹Department of Pharmacology & Experimental Therapeutics, University of Maryland School of Medicine, Baltimore, MD, USA
2. ²Department of Epidemiology & Preventive Medicine, University of Maryland School of Medicine, Baltimore, MD, USA

Correspondence: Dr Y Qiu, Department of Pharmacology & Experimental Therapeutics, University of Maryland School of Medicine, 655 W Baltimore St, Baltimore, MD 21201, USA. E-mail: yqiu@som.umaryland.edu

Received 29 March 2005; Revised 26 July 2005; Accepted 26 July 2005; Published online 26 September 2005.

Abstract

Protein kinase Pim-1 has been implicated in the development of hematopoietic and prostatic malignancies. Here, we present the evidence that two isoforms, the 44 and 33 kDa Pim-1, are expressed in all human prostate cancer cell lines examined. The subcellular localization of human 44 kDa Pim-1 is primarily on the plasma membrane, while the 33 kDa isoform is present in both the cytosol and nucleus in PCA cells. The 44 kDa Pim-1 contains the proline-rich motif at the N-terminus and directly binds to the SH3 domain of tyrosine kinase Etk. Such interaction leads to the activation of Etk kinase activity possibly by competing with the tumor suppressor p53. This is corroborated by the fact that overexpression of the 44 kDa Pim-1 in prostate cancer cells confers the resistance to chemotherapeutic drugs. Our results suggest that these two isoforms of Pim-1 kinase may regulate distinct substrates and the 44 kDa Pim-1 may play a more prominent role in drug resistance in prostate cancer cells.