RB1 PROTEIN IN NORMAL AND MALIGNANT HUMAN COLORECTAL TISSUE AND COLON-CANCER CELL-LINES

The pattern of human retinoblastoma (RB1) gene protein expression was directly examined in normal and malignant human colorectal tissues and in seven colorectal carcinoma cell lines by immunohistochemistry usin g the mouse monoclonal antibody (RB-MAb-1) directed against the retino blastoma protein (RB). This is the first demonstration of RB immunosta ining in adult human colonic epithelium and colorectal carcinomas. Spe cificity using RB-MAb-1 was confirmed by western blot analysis, which showed bands of 110-116 kDa corresponding to the sizes of unphosphoryl ated and phosphosrylated RB. RB staining of normal adult colonic epith elium was confined to the nucleus and was most intense in the transiti onal zone of the crypt, whereas lumenal cells (fully differentiated) w ere RB negative. Primary colorectal carcinomas and all the colon cance r cell lines stained positively for nuclear RB, but the expression was heterogeneous with varying fractions of RB negative cells present. Be cause we and others have previously shown that loss or inactivation of the RB1 gene is infrequent in colorectal carcinomas, reduced RB expre ssion in such cells is probably due to a cellular regulatory mechanism . For example, RB negative cells may be those in early-G1 phase (known to have reduced RB levels) or growth-arrested cells that have differe ntiated. The ability to directly detect RB in primary colorectal carci nomas will permit assessment of whether heterogeneous expression of th e RB1 gene product has prognostic significance for survival of patient s with this cancer.