Autocrine signalling through erbB receptors promotes constitutive activation of protein kinase B/Akt in breast cancer cell lines.
The protein kinase PKB/Akt plays a pivotal role in promoting cell survival and proliferation. This study investigated the regulation of PKB/Akt activity in breast cancer cells. In primary invasive breast cancers PKB/Akt exhibited elevated phosphorylation at regulatory site Ser473 in 80% of cases, using immunohistochemistry. The degree of phospho-PKB/Akt immunoreactivity was positively correlated with the extent of its nuclear accumulation. Moderate/strong staining was seen in 31% of the samples but was absent in tumour-associated normal breast epithelia. To examine the mechanisms of PKB/Akt activation, we studied its phosphorylation in a panel of breast cancer cell lines. PKB/Akt was constitutively phosphorylated on both regulatory sites (Thr308 and Ser473) in the absence of serum growth factors in 7 of 8 lines but not in two cell lines derived from normal breast epithelia. Further analysis revealed that constitutive PKB/Akt phosphorylation was associated with loss of PTEN phosphatase expression (CAL51, MDA-MB-468, BT549 cells) and constitutive activation of erbB2 (SKBR3, BT474 cells). In two further breast cancer lines (T47D and HS578T) PKB/Akt phosphorylation was dependent upon autocrine factors acting primary through the epidermal growth factor receptor (EGFR) and erbB2. Conditioned medium from HS578T cells stimulated EGFR-dependent PKB/Akt phosphorylation in normal breast cells. These results demonstrate that PKB/Akt is frequently activated in breast cancer through diverse mechanisms, including autocrine signalling via erbB receptors.
Division of Cancer Studies, School of Medicine, University of Manchester, Manchester, UK.
K M
KM
Nicholson
C H
CH
Streuli
N G
NG
Anderson
D001943:Breast Neoplasms / Q000201:enzymology* / Q000378:metabolism / Q000473:pathology; D045744:Cell Line, Tumor; D016895:Culture Media, Serum-Free; D004789:Enzyme Activation; D005260:Female; D006801:Humans; D015151:Immunoblotting; D007150:Immunohistochemistry; D010766:Phosphorylation; D011233:Precipitin Tests; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt; D020893:Receptor, ErbB-3 / Q000378:metabolism*
D016895:Culture Media, Serum-Free; D011518:Proto-Oncogene Proteins; D020893:Receptor, ErbB-3; C494918:AKT1 protein, human; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt