Muscarinic M3 acetylcholine receptor immunostaining in paraffin-embedded normal and neoplastic prostatic gland tissue

Recent in vitro studies suggest that muscarinic receptors are important indicators of prostatic epithelial differentiation. Using choline in the mapping of prostatic lesions, radiographic studies have been interpreted as showing malignant transformation when high levels of receptors are found, but to our knowledge no correlation with the grade of prostatic cancer has been reported. To demonstrate the relationship of muscarinic receptors to differentiation, we immunostained normal and neoplastic prostatic tissue with antibodies that recognize receptors of acetylcholine, dividing the neoplastic tissue into two groups according to grade of differentiation. As far as we know, this is the first time that immunostaining of benign and neoplastic prostatic gland tissue with cholinergic receptors has been reported. Thirty-five cases of prostatic carcinoma were retrieved from our files and subdivided into two groups; a well to moderately differentiated group and a poorly differentiated group. Both groups were immunostained with M3 muscarinic receptor, along with a proliferation marker (Ki-67). The proliferating index measured with Ki-67 in the well to moderately differentiated group was low (<5% of the cells) and high in the poorly differentiated group (>20% of the cells). A strong correlation was observed between the strength of expression of the muscarinic M3 receptor and the differentiation of the lesion. The 18 cases of well to moderately differentiated carcinoma showed a mean staining intensity of approximately 1 with no case showing an intensity grade greater than 2. Normal controls along with normal peritumoral prostatic tissue also demonstrated a staining intensity of approximately 1. However, none of the 11 cases of poorly differentiated carcinoma showed staining. Our conclusion is that the presence of muscarinic acetylcholine receptor M3 by immunostaining distinguishes normal glandular epithelium and low-grade carcinomas from poorly differentiated tumors, which suggests that its demonstration in vivo could be a useful tool in distinguishing grade of tumor clinically.