Abstract

Human prostate-specific antigen (hPSA, KLK3) and glandular kallikrein 2 (hK2, KLK2), two members of a large human tissue kallikrein enzyme family, were produced as recombinant mature proteins for the first time and characterized. Furthermore, their association with prostate cancer was studied. Both proteins were produced with baculovirus expression vector system in pilot-scale using bioreactors. Recombinant hPSA was either active with chymotrypsin-like activity or inactive with incorrect processing of N-terminus. The molecular weight of active recombinant hPSA was 31 kD and it formed stable complexes with serine protease inhibitors, α1-antichymotrypsin (ACT) and α2-macroglobulin (2αM). Two polymorphic forms of KLK2, Arg²²⁶hK2 and Trp²²⁶hK2, were found. The recombinant Arg²²⁶hK2 had trypsin-like activity, while recombinant Trp²²⁶hK2 was inactive. The Arg²²⁶hK2 was labile with low production yields. The molecular weights of hK2 polymorphic forms were 33 kD.

hPSA isoforms secreted by prostate cancer cells, LNCaP, were isolated and characterized. These proteins were N-terminally heterogeneous: 10-60% of LNCaP-PSAs were correctly processed. Molecular modeling suggested that the additions or deletions of two or four N-terminal amino acids could affect the three-dimensional structure and reduce the activity of LNCaP-PSA. Active isoforms had chymotrypsin-like activity and formed stable complexes with ACT and 2αM.

The expression of hPSA and hK2 was studied with in situ hybridization and immunohistochemistry techniques in benign and cancerous prostate tissue. hK2 mRNA was expressed at a significantly higher level in prostate cancer tissue than in benign prostate tissue (P < 0.0005). The hPSA mRNA expression levels were reversed (P = 0.06). In benign tissue, the mean level of hK2 mRNA was 82% of the respective value of hPSA (P < 0.003), whereas in tumor tissue the mean hK2 expression level was 21% higher than that of hPSA (P < 0.01). The results at protein level supported the mRNA findings. There was a correlation between hPSA and hK2 mRNA levels in both benign (r = 0.735; P < 0.01) and malignant (r = 0.767; P < 0.01) prostate tissue. It was shown with competitively differential PCR that the KLK2 gene was amplified in prostate tumor tissue, while the KLK3 gene was not. These results suggest that hK2 and hPSA have a diverse value in the diagnosis of prostate cancer.