AMACR (P504S protein, 48 kDa) plays a role in the beta-oxidation of branched-chain fatty acids and their derivatives. AMACR is found in mitochondria and peroxisomes of numerous tissues such as prostate, liver, biliary tract, kidney and lung. Using immunohistochemistry, non-neoplastic prostate show a weak or focal staining reaction in about 20% of the cases. The reactivity tends to decrease with age but the correlation coefficient is low.
A diffuse staining pattern for AMACR has been found in more than 90% of prostate carcinomas irrespective of Gleason score. Particularly foamy, pseudohyperplastic and atrophic variants of carcinoma may be negative. In prostatic intraepithelial neoplasia (PIN), the positive rate of AMACR ranges from 13% to 72%.
AMACR is mainly used in prostate lesions. Since negative staining for high molecular weight cytokeratin (HMW-CK) in atypical prostate glands may not be sufficient for a definitive diagnosis of malignancy, p63 may enhance the ability to diagnose limited prostate cancer. However, AMACR should always be used in conjunction with (HMW-CK) and/or p63. A cocktail staining is applicable. It is also important to note that AMACR positive glands in a prostate biopsy may represent seminal vesicle or periurethral glands.
Kidney is recommended as positive tissue control for AMACR: Virtually all epithelial cells of the proximal tubules must show a strong and distinct granular cytoplasmic staining, whereas epithelial cells of the distal tubules must display a weak granular cytoplasmic staining reaction. Normal prostate is recommended as negative tissue control for AMACR: The epithelial cells must be negative or only show a focal staining reaction.
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