Prostate-specific Antigen: Its Usefulness in Clinical Medicine

Different proportions of various prostate-specific antigen (PSA) and human kallikrein 2 (hK2) forms are present in noninduced and androgen-induced LNCaP cells

BACKGROUND

Human prostate-specific antigen (PSA) and human kallikrein 2 (hK2) are expressed primarily by prostate epithelial cells. PSA and hK2 both exist as free protein and complexed with protease inhibitors (e.g., alpha1-antichymotrypsin, ACT) in serum. The expression of PSA and hK2 in LNCaP cells is upregulated by androgen.

METHODS

LNCaP, a prostate cancer cell line that secretes both PSA and hK2, was used as a model to study the biosynthesis and processing of PSA and hK2 upon androgen induction.

RESULTS

Precursor (zymogen or pro) forms of both PSA and hK2 were detected in spent media of induced and noninduced LNCaP cells, indicating that PSA and hK2 are secreted as proPSA (pPSA) and prohK2 (phK2), respectively, and are converted to the mature forms extracellularly. A 3-fold higher ratio of mature to pPSA was detected in the spent media of mibolerone-induced LNCaP cells compared to noninduced cells. In addition to the inactive proform of PSA, more than half of the mature unclipped PSA present in the spent media did not complex with exogenously added ACT. Spent media of mibolerone-induced LNCaP cells contained nearly 100% mature hK2, whereas the spent media of noninduced cells contained mostly phK2.

CONCLUSIONS

These results indicate that androgens not only upregulate the expression of these kallikreins, but also have a significant effect on the processing of PSA and hK2. These results also show that LNCaP cells express a heterogeneous mixture of inactive PSA and hK2 forms that may serve as a model for the genesis of these forms in physiological fluids. These findings may also provide insights into the forms and ratios of PSA and hK2 in normal and malignant breast tissues.

Prostate-specific Antigen: A Cancer Fighter and a Valuable Messenger?

Background: Prostate-specific antigen (PSA) is a valuable prostatic cancer biomarker that is now widely used for population screening, diagnosis, and monitoring of patients with prostate cancer. Despite the voluminous literature on this biomarker, relatively few reports have addressed the issue of its physiological function and its connection to the pathogenesis and progression of prostate and other cancers.

Approach: I here review literature dealing with PSA physiology and pathobiology and discuss reports that either suggest that PSA is a beneficial molecule with tumor suppressor activity or that PSA has deleterious effects in prostate, breast, and possibly other cancers.

Content: The present scientific literature on PSA physiology and pathobiology is confusing. A group of reports have suggested that PSA may act as a tumor suppressor, a negative regulator of cell growth, and an apoptotic molecule, whereas others suggest that PSA may, through its chymotrypsin-like activity, promote tumor progression and metastasis.

Summary: The physiological function of PSA is still not well understood. Because PSA is just one member of the human kallikrein gene family, it is possible that its biological functions are related to the activity of other related kallikreins. Only when the physiological functions of PSA and other kallikreins are elucidated will we be able to explain the currently apparently conflicting experimental data.

Identification and characterization of KLK-L4, a new kallikrein-like gene that appears to be down-regulated in breast cancer tissues

Kallikreins are a subgroup of serine proteases and these proteolytic enzymes have diverse physiological functions in many tissues. Growing evidence suggests that many kallikreins are implicated in carcinogenesis. In rodents, kallikreins constitute a large multigene family, but in humans, only three genes were identified. By using the positional candidate gene approach, we were able to identify a new kallikrein-like gene, tentatively named KLK-L4 (for kallikrein-like gene 4). This new gene maps to chromosome 19q13. 3-q13.4, is formed of five coding exons and four introns, and shows structural similarity to other kallikreins and kallikrein-like genes. KLK-L4 is expressed in a variety of tissues including prostate, salivary gland, breast, and testis. Our preliminary results show that KLK-L4 is down-regulated, at the mRNA level, in breast cancer tissues and breast cancer cell lines. Its expression is regulated by steroid hormones in the breast cancer cell line BT-474. This gene may be involved in the pathogenesis and/or progression of breast cancer and may find applicability as a novel cancer biomarker.

The expanded human kallikrein gene family: locus characterization and molecular cloning of a new member, KLK-L3 (KLK9)

In rodents, kallikreins are encoded by a large multigene family but in humans, only three kallikrein genes were thought to exist. Based on the homology between the human and the rodent kallikrein loci, we defined a 300-kb human kallikrein gene region on chromosome 19q13. 3-q13.4. By using linear sequence information, restriction analysis, PCR, and blotting techniques, we were able to construct the first detailed map of the human kallikrein gene locus. Comparative analysis of genes located in this area enabled us to expand the human kallikrein multigene family with some recently identified serine proteases and establish common structural features. We further identified a new kallikrein-like gene, named kallikrein-like gene 3 (KLK-L3; HGMW-approved symbol KLK9). We describe the structural characterization of the KLK-L3 gene, together with its precise chromosomal localization in relation to other kallikreins and its tissue expression pattern and hormonal regulation.

Relapse and cure rates of prostate cancer patients after radical prostatectomy and 5 years of follow-up

OBJECTIVES

We have compared the ability of an ultrasensitive prostate specific antigen assay and a regular PSA assay to identify relapse and cure rates of prostate cancer patients after radical prostatectomy, during a 5-year follow-up period.

DESIGN AND METHODS

We measured PSA by an ultrasensitive assay (detection limit 0.001 ng/mL) and a conventional PSA assay (detection limit 0.1 ng/mL) in serial serum samples obtained from 197 patients who have undergone radical prostatectomy.

RESULTS

Based on ultrasensitive PSA analysis, we have identified three groups of patients: 62% of patients did not show any significant changes in serum PSA; 15% of patients demonstrated slow PSA increases over time but none of the measurements exceeded 0.1 ng/mL within 4 years; and 23% of the patients had relatively significant increases of serum PSA and were classified as having 'fast relapse'. The vast majority of these patients were subsequently identified to have relapse by the regular PSA assay. The ultrasensitive PSA assay detected relapse by an average of eighteen months earlier than the conventional PSA method. Fast relapsing patients were associated with other prognostic indicators of the disease including pre-operative PSA, tumor volume, Gleason score, clinical stage, surgical margin positivity, periprostatic tissue involvement, capsular invasion and seminal vesicle invasion. The group with slowly rising PSA had prognosis which was between the patients in remission and fast relapsing patients.

CONCLUSIONS

The use of ultrasensitive PSA analysis for monitoring patients after radical prostatectomy provides earlier detection of relapse (by 18 months) and identifies three distinct groups of patients. Fast relapsing patients should be good candidates for early therapeutic interventions.

The new human kallikrein gene family: implications in carcinogenesis

The traditional human kallikrein gene family consists of three genes, namely KLK1 [encoding human kallikrein 1 (hK1) or pancreatic/renal kallikrein], KLK2 (encoding hK2, previously known as human glandular kallikrein 1) and KLK3 [encoding hK3 or prostate-specific antigen (PSA)]. KLK2 and KLK3 have important applications in prostate cancer diagnostics and, more recently, in breast cancer diagnostics. During the past two to three years, new putative members of the human kallikrein gene family have been identified, including the PRSSL1 gene [encoding normal epithelial cell-specific 1 gene (NES1)], the gene encoding zyme/protease M/neurosin, the gene encoding prostase/KLK-L1, and the genes encoding neuropsin, stratum corneum chymotryptic enzyme and trypsin-like serine protease. Another five putative kallikrein genes, provisionally named KLK-L2, KLK-L3, KLK-L4, KLK-L5 and KLK-L6, have also been identified. Many of the newly identified kallikrein-like genes are regulated by steroid hormones, and a few kallikreins (NES1, protease M, PSA) are known to be downregulated in breast and possibly other cancers. NES1 appears to be a novel breast cancer tumor suppressor protein and PSA a potent inhibitor of angiogenesis. This brief review summarizes recent developments and possible applications of the newly defined and expanded human kallikrein gene locus.

Comparison of the percent free prostate-specific antigen levels in the serum of healthy men and in men with recurrent prostate cancer after radical prostatectomy

The percentage of free PSA in serum is currently used to better discriminate between patients with prostate cancer and patients with benign prostatic hyperplasia, in prostate cancer screening programs. We measured using non-competitive immunological techniques, the total PSA and free PSA in post-surgical serum of prostate cancer patients who underwent radical prostatectomy and then relapsed. We compared these data with those of a group of 40 age-matched men with no evidence of prostatic disease. Although in general, patients with prostate cancer had lower percentage of free PSA in serum in comparison to the controls, a subset of these patients (approximately 20%) had percent free PSA significantly higher than the levels considered as exclusive of prostate cancer in screening programs. We also found that percent free PSA does not correlate significantly with most of the standard clinical or pathological indicators of prostate cancer aggressiveness. Only a weak negative association with Gleason Score was observed. The percent free PSA in serum of relapsing prostate cancer patients varies within a relatively wide range and does not correlate significantly with indicators of cancer aggressiveness. The use of percent free PSA for excluding prostate cancer in screening programs must be approached with caution until the mechanism of low percent free PSA in the majority but not all prostate cancer patients is elucidated.

Genomic organization, mapping, tissue expression, and hormonal regulation of trypsin-like serine protease (TLSP PRSS20), a new member of the human kallikrein gene family

The cDNA for the trypsin-like serine protease gene (TLSP, HGMW-approved symbol PRSS20) has been recently identified. TLSP is expressed in brain and skin tissues but little else is known about this new serine protease gene. In this paper, we describe the complete genomic organization and precise mapping of the TLSP gene. This gene spans 5.3 kb of genomic sequence on chromosome 19q13.3-q13. 4. The gene consists of six exons, the first of which is untranslated. All splice junctions follow the GT/AG rule, and the intron phases are identical to those of other kallikrein-like genes, including zyme (PRSS9), NES1 (PRSSL1), and neuropsin (PRSS19). Fine-mapping of the area indicates that TLSP lies downstream from the PSA, zyme, neuropsin, and NES1 genes. Significant sequence homologies were found between TLSP and other human kallikreins. Furthermore, there is conservation of the catalytic triad (histidine, aspartic acid, serine) and of the number of coding exons (five; the same in all members of the kallikrein gene family). We thus suggest that TLSP is a new member of the human kallikrein gene family. TLSP is expressed in many tissues including cerebellum, prostate, salivary glands, stomach, lung, thymus, small intestine, spleen, liver, and uterus. TLSP expression appears to be regulated by steroid hormones in the breast carcinoma cell line BT-474.

The new kallikrein-like gene, KLK-L2. Molecular characterization, mapping, tissue expression, and hormonal regulation

Since in rodents the kallikreins are represented by a large multi-gene family, the restriction of this family in humans to three genes is somewhat surprising. In an effort to identify new human kallikrein genes, we examined a genomic area of about 300 kilobases on chromosome 19q13.3-q13.4, a region that contains most of the currently known kallikreins. By using the positional candidate approach, we were able to identify a new gene named KLK-L2 (for kallikrein- like gene 2). Screening of human EST libraries allowed us to delineate the full genomic and cDNA structure of the new gene. KLK-L2 consists of 5 coding exons and 4 introns and has significant similarities to other members of the kallikrein multi-gene family. Homology studies suggest that the protein is likely secreted. KLK-L2 is expressed mainly in breast, brain, and testis and to a lesser extent in many other tissues. KLK-L2 is up-regulated by estrogens and progestins in the breast cancer cell line BT-474.

Prostate-specific antigen in acute hepatitis and hepatocellular carcinoma

BACKGROUND

Prostate-specific antigen (PSA) is the most important tumor marker in prostate cancer diagnosis and follow-up. Its catabolism by the liver has not influenced its use as a prostate marker until the recent report of a significant increase in a man and a woman with acute hepatitis. In addition, PSA was detected in liver tumor extracts, which warranted its evaluation in liver cytolysis and hepatocellular carcinoma. In this study, PSA was evaluated in a cohort of both sexes presenting either acute hepatitis or hepatocellular carcinoima.

METHODS

Forty-two patients with acute hepatitis (21 male patients, 21 female patients) and 54 patients with hepatocellular carcinoma (31 male patients, 23 female patients) were tested for PSA by equimolar immunoassay (Abbott AxSYM Total PSA, Abbott Diagnostics, Rungis, France) and for relevant liver biological parameters (alpha-fetoprotein, alanine aminotransferase, aspartate aminotransferase, total bilirubin, and prothrombin rate).

RESULTS

PSA was undetectable in all the female patients and was consistent with age in the males (PSA median and range in acute hepatitis, 0.36 microg/l (range, 0.05-1.3); in hepatocellular carcinoma, 0.36 microg/l (range, 0.02-3.9)). It did not correlate with alpha-fetoprotein and aminotransferases.

CONCLUSIONS

Our results confirm the well-established reliability of PSA, and show that PSA remains a valid prostate marker in patients with acute hepatitis and hepatocellular carcinoma.

The Combination of Human Glandular Kallikrein and Free Prostate-specific Antigen (PSA) Enhances Discrimination Between Prostate Cancer and Benign Prostatic Hyperplasia in Patients with Moderately Increased Total PSA

Background: Prostate-specific antigen (PSA) is the most reliable tumor marker available and is widely used for the diagnosis and management of prostate cancer. Unfortunately, PSA cannot distinguish efficiently between benign and malignant disease of the prostate, especially within the range of 4–10 μg/L. Among the refinements developed to enhance PSA specificity is the free/total PSA ratio, which is useful in discriminating between the two diseases within the diagnostic “gray zone”. Recent data indicate that human glandular kallikrein (hK2), a protein with high homology to PSA, may be an additional serum marker for the diagnosis and monitoring of prostate cancer.

Methods: We analyzed 206 serum samples (all before treatment was initiated) from men with histologically confirmed benign prostatic hyperplasia (n = 100) or prostatic carcinoma (n = 106) with total PSA in the range of 2.5–10 μg/L. Total and free PSA and hK2 were measured with noncompetitive immunological procedures. Statistical analysis was performed to investigate the potential utility of the various markers or their combinations in discriminating between benign prostatic hyperplasia and prostatic carcinoma.

Results: hK2 concentrations were not statistically different between the two groups of patients. There was a strong positive correlation between hK2 and free PSA in the whole patient population. hK2/free PSA ratio (area under the curve = 0.69) was stronger predictor of prostate cancer than the free/total PSA ratio (area under the curve = 0.64). At 95% specificity, the hK2/free PSA ratio identified 30% of patients with total PSA between 2.5–10 μg/L who had cancer. At 95% specificity, the hK2/free PSA ratio identified 25% of patients with total PSA between 2.5 and 4.5 μg/L who had cancer.

Conclusions: Our data suggest that hK2 in combination with free and total PSA can enhance the biochemical detection of prostate cancer in patients with moderately increased total PSA concentrations. More specifically, the hK2/free PSA ratio appears to be valuable in identifying a subset of patients with total PSA between 2.5 and 4.5 μg/L who have high probability of cancer and who should be considered for biopsy.

Human Glandular Kallikrein in Breast Milk, Amniotic Fluid, and Breast Cyst Fluid

Background: Human glandular kallikrein (hK2) belongs to the serine protease family of enzymes and has high sequence homology with prostate-specific antigen (PSA). The physiological role of hK2 has not as yet been determined, but there is evidence that it can regulate the proteolytic activity of PSA through processing and activating pro-PSA, an inactive precursor. Thus, it is conceivable that these two secreted proteins may coexist in biological fluids. Currently, hK2 is considered an androgen-regulated and prostate-specific protein. Recently, it has been demonstrated that hK2 is expressed in the breast cancer cell line T-47D after stimulation by steroid hormones, and we reported that hK2 can be detected in a subset of breast tumor extracts. These data suggest that hK2 may be expressed in tissues other than the prostate, such as those in which PSA has already been detected. Because hK2 is a secreted protein, it may be present in various biological fluids.

Methods: We analyzed milk samples from lactating women, amniotic fluid from pregnant women, and breast cyst fluid from patients with gross breast cystic disease, using a highly sensitive and specific immunoassay for hK2.

Results: hK2 was present in all three biological fluids. We suggest that the female breast may produce hK2 and provide evidence that hK2 may have value as an additional marker for the discrimination between type I and type II breast cysts.

Conclusions: The female breast produces hK2 in addition to PSA. More studies are necessary to establish the role of this kallikrein in nondiseased breast, gross breast cystic disease, and breast cancer.

Quantitative Reverse Transcription-PCR Assay with an Internal Standard for the Detection of Prostate-specific Antigen mRNA

Background: Circulating prostate cells can be detected with a reverse transcription-PCR (RT-PCR) assay for prostate-specific antigen (PSA) mRNA. We have developed a new quantitative RT-PCR method for measuring PSA mRNA.

Methods: The method uses a PSA-like internal standard (IS) mRNA that is added into the sample at the beginning of the RNA extraction and coamplified by RT-PCR with the PSA in the sample. After PCR amplification, the IS and PSA products are selectively detected by hybridization in a microtitration plate using probes labeled with fluorescent europium chelates.

Results: The method was validated with PSA and IS mRNAs and PSA-expressing cells to obtain a detection limit of 50 PSA mRNA copies (i.e., signal 2 times the mean of zero signal), linearity up to 106 copies, and detection of a single PSA-expressing cell. In preliminary evaluations, 60% (n = 10) of the prostate cancer patients with skeletal metastases gave results above the detection limit (500 PSA mRNA copies in 5 mL of blood). The total number of PSA copies ranged from 900 ± 200 to 44 100 ± 4900 (mean ± SD) in the samples, corresponding to ∼1–100 PSA-expressing cells in 5 mL of blood. In the controls (n = 34), none of the healthy females and 2 of 19 healthy males had detectable PSA mRNA [700 ± 100 and 2000 ± 900 (mean ± SD) PSA mRNA copies in 5 mL of blood for the 2 males].

Conclusions: The assay provides sensitive and quantitative detection of PSA mRNA expression from blood samples and can be used to establish the clinically significant number of PSA mRNA copies in prostate cancer.