ANALYSIS OF PHOSPHORYLATION OF PRB AND ITS REGULATORY PROTEINS IN BREAST-CANCER

Aim - In order to study the role of retinoblastoma protein (pRB) in br east cancer, the phosphorylation of pRB and the expression of its rela ted proteins - such as cyclin E, cyclin dependent kinase 2 (Cdk2), and p21/Cdk interacting protein 1 (Cip1) - were examined in 30 breast can cers in which pRB overexpression was confirmed immunohistochemically. Methods - The phosphorylation of pRB for 30 tumours was investigated w ith western blotting. The expression of pRB, Cdk2/Cdc2, cyclin E, and p21/Cip1 was identified by immunohistochemistry and western blotting. Results - The expression of pRB was confirmed in 52 of 70 tumours (74% ) by immunostaining. Western blotting for PRE showed that 25 of 30 rep resentative cancers (83%) were underphosphorylated, while only five tu mours showed the hyperphosphorylated form of pRB. However, cyclin E an d Cdk2 - which promote phosphorylation of pRB - were expressed in all tumours. On the other hand p21/Cip1, a Cdk2 inhibitor, was expressed i n 18 of 25 tumours with underphosphorylated pRB, while four of the fiv e tumours with hyperphosphorylated pRB showed no expression of p21/Cip 1. Examination of the relation between pRB phosphorylation and clinico pathological variables showed that the underphosphorylated group was c haracterised by low risk of lymph node metastasis (p < 0.01). Conclusi ons - The phosphorylation of PRE appears to be regulated mainly by p21 /Cip1 through the suppression of cyclin E and Cdk2 in breast cancer. T he underphosphorylated form of pRB may be useful as a prognostic facto r.