Cyclin E dependent kinase activity in human breast cancer in relation to cyclin E, p27 and p21 expression and retinoblastoma protein phosphorylation

Publisher Website
Google Scholar
Abstract
The cell cycle machinery is regulated by cyclin dependent kinases and sets of activating and inhibitory proteins. The G1-S control mechanism is often deregulated in tumours supposedly leading to increased kinase activity, phosphorylation of substrates and subsequent S phase entrance. Increased kinase activity has been proposed to be essential in cell cycle aberrations, but few studies have actually shown enhanced kinase activity related to specific cell cycle defects in primary tumours. In the present study we have determined the cyclin E dependent kinase activity (cyclin Ekinase) in 59 primary breast cancers, using an H1-kinase assay, and related the activity to the expression of cyclin E, p27 and p21. In a subgroup of 48 tumours, we further characterized the association between cyclin Ekinase, in vivo phosphorylation of the retinoblastoma protein (pRb) and proliferation. The cyclin Ekinase correlated significantly with cyclin E content and inversely with p27 and p21 expression. P27, but not p21, was associated with low cyclin Ekinase in specimens with normal/low levels of cyclin E. At elevated cyclin E levels, suppression of cyclin Ekinase seemed to require high levels of both p21 and p27. The cyclin Ekinase correlated with the phosphorylation status of pRb as well as with proliferation. Surprisingly, pRb phosphorylation did not correlate with proliferation. Our results support that pRb is a substrate for cyclin Ekinase in primary breast cancer and that deregulation of cyclin E and p27 act through increased CDK-kinase activity, but cyclin E associated events beside pRb phosphorylation might be rate-limiting for entrance into S phase.