AN IMMUNOCHEMICAL ANALYSIS OF MDM2 EXPRESSION IN HUMAN BREAST-CANCER AND THE IDENTIFICATION OF A GROWTH-REGULATED CROSS-REACTING SPECIES P170

mdm2 is a 491 amino acid nuclear protein which is involved in complex interactions with important cell-cycle and stress-response regulators including p53, Rb and E2F, Recent data implicate mdm2 in the regulatio n of both p53 activity and level, and burgeoning data suggest that mdm 2 may be invoiced in human epithelial tumourigenesis, including breast cancer. In this study the expression of mdm2 protein has been investi gated in a series of 54 human breast carcinomas using immunoblotting m ethods. Overexpression of the predominant p90 mdm2 isoform is common i n breast cancer (54 per cent) and this is not frequently a consequence of gene amplification. There is no relationship between p90 expressio n and either p53 protein expression or p53 mutational status. Addition al mdm2 immunoreactive species of differing mobilities are identifiabl e, greatly complicating the analysis, For example, a p170 form is seen in many breast cancer samples (44 per cent) using 2A10 but is not ide ntified by 3G5. This 2A10 immunoreactive species, which is almost cert ainly not an mdm2 isoform, is a growth-regulated protein, being undete ctable in resting peripheral blood lymphocytes and rising to high leve ls after PHA stimulation. In contrast to mdm2 (p90), p170 is not induc ed by DNA damage caused by UV light. p170 is identifiable in mdm2 null cells by immunoblotting and is detected as a nuclear protein. While m dm2 immunostaining studies are increasing, this report highlights the complexity of mdm2 analysis in vivo and emphasizes the need to correla te immunohistological and biochemical assays since, in some mdm2 (p90) negative tumours, nuclear immunoreactivity may be identified as a con sequence of cross-reacting species such as p170, (C) 1998 John Wiley & Sons, Ltd.