Is prostate-specific membrane antigen a multifunctional protein?

Aptamer:toxin conjugates that specifically target prostate tumor cells

We have used RNA aptamer:gelonin conjugates to target and specifically destroy cells overexpressing the known cancer biomarker prostate-specific membrane antigen (PSMA). Aptamer:toxin conjugates have an IC50 of 27 nmol/L and display an increased potency of at least 600-fold relative to cells that do not express PSMA. The aptamer not only promotes uptake into target cells but also decreases the toxicity of gelonin in non-target cells. These results validate the notion that "escort aptamers" may be useful for the treatment of specific tumors expressing unique antigen targets.

Association of prostate-specific membrane antigen with caveolin-1 and its caveolae-dependent internalization in microvascular endothelial cells: Implications for targeting to tumor vasculature

Prostate-specific membrane antigen (PSMA) is a transmembrane protein with a highly restricted profile of expression. Expression is primarily limited to secretory cells of the prostatic epithelium, with elevated levels observed in prostate cancer. As an integral membrane protein correlated with prostate cancer, PSMA offers a potentially valuable target for immunotherapy. PSMA is also detected in the neovasculature of a variety of solid tumors but not in the endothelial cells of preexisting blood vessels. Although the significance of PSMA expression in these cells remains elusive, this pattern of expression implies that PSMA may perform a functional role in angiogenesis and may offer a therapeutic target for the treatment of a broad spectrum of solid tumors. In this study, we have expressed PSMA in human microvascular endothelial cells and demonstrate that PSMA binds to caveolin-1 and undergoes internalization via a caveolae-dependent mechanism. The association between PSMA and caveolae in endothelial cells may provide important insight into PSMA function and ways to best exploit this protein for therapeutic benefit.

Comparative study of PSMA expression in the prostate of mouse, dog, monkey, and human

BACKGROUND

Intraprostatic PSMA targeted prodrugs/protoxins are under development in our laboratory. Future toxicologic studies of these therapies require identification of animal models that express PSMA within the prostate.

METHOD

PSMA enzymatic activity and protein expression was determined. PSMA expression in the prostates of mouse, dog, and monkey were compared to humans by real-time PCR analysis.

RESULTS

No substrate hydrolysis was observed in dog or monkey prostate homogenates. Monkey prostate was negative for PSMA protein expression. No significant PSMA mRNA levels were detected by real time PCR in mouse, dog, or monkey prostate tissue compared to PSMA negative tissues.

CONCLUSIONS

PSMA is not expressed in any significant amount in the prostates of mouse, beagle dog, or macaque monkeys in this study but is expressed in high levels by human prostate. These non-human species, therefore, are not suitable toxicologic models to assess prostate damage from PSMA-activated intraprostatic prodrug/protoxin therapies.

Purification of prostate-specific membrane antigen using conformational epitope-specific antibody-affinity chromatography

Prostate-specific membrane antigen (PSMA) is a type II membrane protein that has attracted significant attention as a target for immunioscintigraphic and radioimmunotherapeutic applications for prostate cancer. However, definitive studies on its substrate and inhibitor specificity as well as protein-protein interactions have been somewhat limited by difficulties in the purification of native PSMA. In this study, we optimized the purification of native PSMA from LNCaP cells using conformational epitope-specific antibody-affinity chromatography. Western blot analysis and an HPLC-based enzymatic activity assay were used to compare the yield and activity of PSMA purified by different methods. The ratio of purified PSMA in a native and active conformation was determined by quantifying the amount of non-native PSMA not retained in a second antibody-affinity isolation. The addition of both a neutralization step and the inclusion of Zn(2+) to the equilibration buffer in desalting step provides considerable enhancement in the yield of active PSMA from LNCaP cells.

Proteomics profiling of epidermal mucus secretion of a cichlid (Symphysodon aequifasciata) demonstrating parental care behavior

The discus fish (Symphysodon aequifasciata) is a cichlid demonstrating advanced mode of parental care towards fry. Both male and female fish utilized epidermal mucus secreted from specialized epidermal cells to feed developing fry. We utilized proteomics to compare protein profile from parental and nonparental fish. Gel analysis revealed a total of 35 spots that were up-regulated in parental mucus. In tandem, another 18 spots were uniquely expressed in parental mucus. MS analysis of these spots identified proteins such as fructose biphosphate aldolase, nucleoside diphosphate kinase, and heat shock proteins, which are essential to support energy provision, cell repair and proliferation, stress mediation, and defense mechanism in parental fish during parental-care period. Concurrently, the detection of several antioxidant-related proteins such as thioredoxin peroxidase and hemopexin suggests a need to overcome oxidative stress during hypermucosal production in parental-care behavior. A C-type lectin was also found to be uniquely expressed in parental mucus and could have important role in providing antimicrobial defense to both parental fish and fry. In summary, our study shows that discus mucus proteome undergoes changes in protein expression during parental-care period.