Compartmentalized expression of kallikrein 4 (KLK4/hK4) isoforms in prostate cancer: nuclear, cytoplasmic and secreted forms

The prostate-specific antigen-related serine protease gene, kallikrein 4 (KLK4), is expressed inthe prostate and, more importantly, overexpressed in prostate cancer. Several KLK4 mRNAsplice variants have been reported, but it is still not clear which of these is most relevant toprostate cancer. Here we report that, in addition to the full-length KLK4 (KLK4-254) transcript,the exon 1 deleted KLK4 transcripts, in particular, the 50-truncated KLK4-205 transcript, isexpressed in prostate cancer. Using V5/His6 and green fluorescent protein (GFP) carboxy terminaltagged expression constructs and immunocytochemical approaches, we found that hK4-254 iscytoplasmically localized, while the N-terminal truncated hK4-205 is in the nucleus of transfectedPC-3 prostate cancer cells. At the protein level, using anti-hK4 peptide antibodies specific todifferent regions of hK4-254 (N-terminal and C-terminal), we also demonstrated that endogenoushK4-254 (detected with the N-terminal antibody) is more intensely stained in malignant cells than inbenign prostate cells, and is secreted into seminal fluid. In contrast, for the endogenous nuclearlocalizedN-terminal truncated hK4-205 form, there was less difference in staining intensity betweenbenign and cancer glands. Thus, KLK4-254/hK4-254 may have utility as an immunohistochemicalmarker for prostate cancer. Our studies also indicate that the expression levels of the truncatedKLK4 transcripts, but not KLK4-254, are regulated by androgens in LNCaP cells. Thus, these datademonstrate that there are two major isoforms of hK4 (KLK4-254/hK4-254 and KLK4-205/hK4-205)expressed in prostate cancer with different regulatory and expression profiles that imply bothsecreted and novel nuclear roles