Analysis of phosphorylation of pRB and its regulatory proteins in breast cancer.

AIM

In order to study the role of retinoblastoma protein (pRB) in breast cancer, the phosphorylation of pRB and the expression of its related proteins-such as cyclin E, cyclin dependent kinase 2 (Cdk2), and p21/Cdk interacting protein 1 (Cip1)-were examined in 30 breast cancers in which pRB overexpression was confirmed immunohistochemically.

METHODS

The phosphorylation of pRB for 30 tumours was investigated with 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 pRB showed that 25 of 30 representative cancers (83%) were underphosphorylated, while only five tumours showed the hyperphosphorylated form of pRB. However, cyclin E and Cdk2-which promote phosphorylation of pRB-were expressed in all tumours. On the other hand p21/Cip1, a Cdk2 inhibitor, was expressed in 18 of 25 tumours with underphosphorylated pRB, while four of the five tumours with hyperphosphorylated pRB showed no expression of p21/Cip1. Examination of the relation between pRB phosphorylation and clinicopathological variables showed that the underphosphorylated group was characterised by low risk of lymph node metastasis (p < 0.01).

CONCLUSIONS

The phosphorylation of pRB appears to be regulated mainly by p21/Cip1 through the suppression of cyclin E and Cdk2 in breast cancer. The underphosphorylated form of pRB may be useful as a prognostic factor.