PSA, PAP, NKX3.1, prostein, androgen receptors, ERG, human glandular kallikrein-2 (hK2), AMACR (p504S)

High molecular weight cytokeratins (CK5, CK6, CK14, CK34βE12), p40, p63

Prostate specific antigen (PSA) also known as kallikrin-3 is a single chain glycoprotein and a serine protease synthesized by the epithelium of prostatic gland and secreted into prostatic ducts. Normally, protease inhibitors rapidly inactivate PSA that enter the blood circulation. PSA is one of the most specific markers for prostatic parenchyma and prostatic carcinoma. Metastatic carcinoma positive for pan-cytokeratin but negative for cytokeratins 5/7/14/20 suggests a primary prostatic carcinoma, and the expression of PSA and/or NKX3.1 will confirm the prostatic origin.

About 10% of high-grade prostatic carcinomas are negative for PSA. In such cases, other prostate-specific markers such as NKX3.1, prostate-specific membrane antigen, prostatic acid phosphatase, and androgen receptors are useful to confirm the diagnosis. Low levels of PSA expression are reported in tumors other than prostatic carcinoma. Weak expression level of PSA is found in a subset of salivary duct carcinoma. Weak expression of PSA is also reported in small cell carcinoma and breast carcinoma in addition to endometrioid carcinoma.

Prostein (solute carrier family 45, type 4 (SLC45A4)) is a transmembrane transporter protein found in the Golgi apparatus of prostatic secretory epithelia. Prostein is more specific to determine a prostatic origin than PSA and slightly more sensitive. Prostein can thus be successfully used in a panel with NKX3.1 and PSA to classify metastases of unknown primary tumor or to discriminate between prostatic, urothelial, or colorectal carcinomas [1]. The loss of prostein expression is associated with unfavorable clinical course [2].

Negativity for prostein does not rule out prostatic origin.

Prostatic acid phosphatase (PAP) is an enzyme secreted by prostatic epithelium and a major component of prostatic fluid. PAP is more sensitive but less specific than PSA for prostatic glands and prostatic carcinoma. PAP can be successfully used in a panel with PSA to classify metastases of unknown primary tumor.

Similar to PSA, PAP can be also expressed in neuroendocrine carcinomas of different origin. This feature is important for the differentiation between poorly differentiated prostatic carcinoma, prostatic carcinoma with neuroendocrine differentiation, and neuroendocrine tumors.

Androgen receptor (AR) is a member of the steroid family of ligand-dependent transcription factors. Androgen receptor is expressed in different tissue types including prostatic glands and skin adnexa. Neoplastic prostatic glands are usually positive for AR, but studies show no direct correlation between the intensity of AR expression and the response to hormonal therapy [3]. The nuclear expression pattern of AR makes it useful for the immunohistochemical double stain with other antibodies with cytoplasmic or membranous expression pattern.

The expression of AR is not restricted to prostatic carcinoma and can be found in other carcinoma types with similar morphology such as salivary duct carcinoma, breast carcinoma, and apocrine carcinoma.

Alpha-methylacyl-CoA racemase (also known as p504S) is a member of the isomerases enzyme family involved in the metabolism of branched-chain fatty acids and synthesis of bile acids. It is expressed in the mitochondria and peroxisomes of various normal and neoplastic cells. p504S is overexpressed in prostatic carcinoma compared to benign prostatic glands (Fig. 13.1) [4, 5]. In combination with p63, alpha-methylacyl-CoA racemase (AMACR) is now widely used for the diagnosis of prostatic carcinoma (so-called PIN cocktail). p63 is a myoepithelial marker exhibiting a nuclear stain [6]. The immunohistochemical double stain with the PIN cocktail can show one of the following three results:

Low molecular weight cytokeratins such as CK5/6/14 can be used as alternatives to p63 in a separate reaction (Fig. 13.2).

The expression of AMACR is found in many neoplasms types and cannot be considered as a specific marker of prostatic tumors [7].

Is an androgen-regulated tumor suppressor gene located on chromosome 8p221.1 and strongly expressed in the nuclei of normal prostatic epithelium and persist in prostatic acinar carcinomas. NKX3.1 is a specific marker for primary prostatic carcinoma, and the intensity of the nuclear expression correlates with the differentiation grade of prostatic carcinoma, which can be very weak in poorly differentiated carcinoma (Fig. 13.3) [8].

NKX3.1 is also expressed in testicular germ cells and seminoma in situ (GCNIS) but lost in invasive seminoma and embryonal carcinoma. Different expression intensity of NKX3.1 is also found in estrogen- and androgen-positive breast carcinomas, i.e., invasive lobular carcinoma [9]. Mucinous units of salivary and bronchial glands also reveal a nuclear expression of NKX3.1 which to consider in the interpretation of small biopsies [10]. Furthermore, the TAL-1 genetic aberration associated with a subset of T-ALL causes the activation of NKX3.1 expression in neoplastic lymphocytes [11].

V-ETS avian erythroblastosis virus E26 oncogene homolog (ERG ) encoded by the gene located on chromosome 21q22.3. ERG is a member of the ETS family transcription factors, which also include Fli-1 and EST-1. ERG is normally expressed in endothelial cells and tumors derived from these cells [12].

The immunohistochemical results using this marker must be carefully interpreted as positive staining is observed in about 29% of high-grade PIN and occasionally benign glands; thus, the gold standard remains the labeling of the myoepithelial basal cells [17]. Both antibodies to ERG and p63 can be used as a cocktail for the diagnosis of prostatic carcinoma but has less sensitivity than the above disrobed PIN cocktail.

PTEN is a tumor suppressor mentioned in a previous chapter. The loss of PTEN is found in a fraction of high Gleason prostatic carcinoma, which is usually resistant to the antiandrogen therapy. Furthermore, the loss PTEN expression is a useful marker to distinguish intraductal carcinoma from PIN usually positive for PTEN.