Mdm2 overexpression and p14ARF inactivation are two mutually exclusive events in primary human lung tumors

Pathways involving p53 and pRb tumor suppressor genes are frequently deregulated during lung carcinogenesis. Through its location at the interface of these pathways, Mdm2 can modulate the function of both p53 and pRb genes. We have examined here the pattern of expression of Mdm2 in a series of 192 human lung carcinomas of all histological types using both immunohistochemical and Western blot analyses and four distinct antibodies mapping different epitopes onto the Mdm2 protein. Using Immunohistochemistry (MC), Mdm2 was overexpressed as compared to normal lung in 31% (60 out of 192) of all tumors analysed, whatever their histological types. Western blotting was performed on 28 out of the 192 tumoral samples. Overexpression of p85/90, p74/76 and p57 Mdm2 isoforms was detected in 18% (5 out of 28), 25% (7 out of 28) and 39% (11 out of 28) of the cases respectively. Overall, overexpression of at least one isoform was observed in 14 out of 28 (50%) lung tumors and concomittant overexpression of at least two isoforms in 7 out of 28 (25%) cases. A good concordance (82%) was observed between immunohistochemical and Western blot data. Interestingly, a highly significant inverse relationship was detected between p14(ARF) loss and Mdm2 overexpression either in NSCLC (P = 0.0089) or in NE lung tumors (P < 0.0001). Furthermore, a Mdm2/P14(ARF) > 1 ratio was correlated with a high grade phenotype among NE tumors overexpressing Mdm2 (P = 0.0021). Taken together, these data strongly suggest that p14(ARF) and Mdm2 act on common pathway(s) to regulate p53 and/or pRb-dependent or independent functions and that the Mdm2:p14(ARF) ratio might act as a rheostat in modulating the activity of both proteins.