Grb2 downregulation leads to Akt inactivation in heregulin-stimulated and ErbB2-overexpressing breast cancer cells

Abstract

ErbB2 can be activated by its own overexpression or be transactivated by the heregulin polypeptide growth factor. Activation of ErbB2 leads to breast cancer cell proliferation, presumably by inducing the activation of extracellular signal-regulated kinases 1,2 (Erk1,2) and Akt. We have previously reported that the growth factor receptor bound protein-2 (Grb2) is required for the proliferation of ErbB2-overexpressing breast cancer cells. We investigated here whether Grb2 protein plays a role in heregulin-stimulated proliferation. Grb2 protein inhibition led to growth inhibition of heregulin-stimulated breast cancer cells, but not Erk1,2 inactivation. These ®ndings are similar to our earlier observations in ErbB2-overexpressing cells. Since Akt can also be activated by heregulin, the e ects of Grb2 inhibition on Akt were examined. Akt was inactivated following Grb2 downregulation in heregulin-stimulated breast cancer cells. We then examined the e ects of Grb2 downregulation on Akt in ErbB2-overexpressing cells in the absence of heregulin. Similar to heregulin-stimulated cells, Grb2 inhibition also led to Akt inactivation in ErbB2-overexpressing breast cancer cells. Our results indicate that the activation of ErbB2 by heregulin or by its overexpression requires Grb2 to stimulate the Akt pathway to propagate mitogenic signals. Oncogene (2000) 19, 6271 ± 6276.

Her2/neu induces all-transretinoic acid (ATRA) resistance in breast cancer cells

“…We found that serum growth factors and Grb2 predominantly regulate Akt activity in Her2/neu-overexpressing cells, while serum growth factors and Grb2 predominantly regulate Erk1,2 activities in EGFR-overexpressing cells Lim et al, 2000). Other groups illustrated that it was the expression/ activation of Her2/neu, not EGFR, which promotes the retention of Raf-1 in the plasma membrane (Zhang et al, 2002), reinitiates the proliferation of growth-arrested mammary epithelial acini, and induces the luminal repopulation of mammary epithelial acini (Muthuswamy et al, 2001).…”

“…We have shown that Grb2 regulates Akt activity in heregulin-stimulated and Her2/neu overexpressing breast cancer cells (Lim et al, 2000). Increased Akt activity has been found in many types of cancer.…”

“…We have earlier shown that Grb2 protein regulates Akt activity in breast cancer cells that have high levels of Her2/neu or heregulin (Lim et al, 2000). Since down-regulation of Grb2 protein sensitized Her2/neuoverexpressing breast cancer cells to ATRA, inhibition of Akt activity might also sensitize breast cancer cells to ATRA.…”

“…We have previously demonstrated that Grb2 protein is vital to the proliferation of breast cancer cells that have high expression levels of Her2/neu or heregulin (Tari et al, 1999;Lim et al, 2000), so we examined whether Grb2 protein may also be vital to ATRA sensitivity.…”

“…We have previously reported the sequences of the Grb2 antisense and the control oligos (Tari et al, 1999;Lim et al, 2000) as follows: Grb2 antisense, 5'-ATATTTGGCGATGGCTTC-3'; control oligo, 5'-GGGCTTTTGAACTCTGCT-3'. Grb2 antisense and control oligos were mixed with dioleoylphosphocholine (Avanti Polar Lipids, Alabaster, AL, USA) in the presence of tertiary butanol and prepared as described (Tari et al, 1999;Lim et al, 2000).…”