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target EGFR, may trigger the release of ligands that induce HER4 cleavage. Indeed we observed that AG 1478 and Iressa induced the cleavage PFI-1 from the precursor proheregulin 1 creating mature heregulin, whichmigrates amongst 35 and 50 kDa . Essentially the most substantial cleavage of proheregulin 1 was noticed with AG 1478 treatment despite the fact that there was also an increase on Iressa treatment. The treatment with either drug also elevated the production of betacellulin inMCF 7 cells . In contrast to heregulin release, the maximum increase of betacellulin was noticed with acute Iressa treatment as opposed to AG 1478 . MCF 7 cells are typically considered to be resistant to physiological doses of Iressa. Working with cell viability assays we confirmed that for the duration of acute treatment with 1 mMIressa, MCF 7 growth was not prevented and in addition there was an increase in cell proliferation in comparison to the manage .
Immediately after seven days of treatment, MCF 7 cell growth was only minimally inhibited by 1 mM of Iressa . SKBR3 cells are recognized to be PFI-1 sensitive to Iressa because of the inhibition of EGFR HER2 and EGFR HER3 and we've confirmed their sensitivity to Iressa working with cell viability assays . We have also shown that there was an increase in cleavage of pro heregulin 1 also as an increase in betacellulin production induced by two hours of Iressa treatment in sensitive SKBR3 cells . We have shown that the activation and proteolytic cleavage of HER4 occurred for the duration of acute treatment of EGFR tyrosine kinase inhibitors correlated with all the release of ligands which includes betacellulin and heregulin in both resistant MCF 7 cells and sensitive SKBR3 cells.
Prolonged Iressa treatment caused reactivation of HER3 activity in both resistant Clindamycin MCF 7 cells and sensitive SKBR3 Iressa has been shown to inhibit the PI3K PKB pathway through HER3 . We observed a fast decrease of phospho HER3 and phospho PKB upon acute treatment of AG1478 via inhibition of EGFR HER3 . On the other hand, acute treatment of Iressa induced the release of heregulin in both MCF 7 and SKBR3 causing dimerization of HER2 and HER4 . Given that heregulin could be the ligand for both HER3 and HER4, we considered that acute Iressa treatment may have induced dimerization of HER2 HER3 also as HER2 HER4, maintaining HER2 activation. Figure 3A shows that seven days of Iressa treatment was not able to abolish HER2 phosphorylation even in sensitive SKBR3 .
Immediately after seven days of Iressa treatment, the remaining surviving cells had an enhanced HER2 phosphorylation monitored by FRET in comparison to basal conditions . Moreover, not just was HER2 phosphorylation maintained in surviving SKBR3 cells , but phospho HER3 was reactivated with prolonged Iressa treatment NSCLC . The reactivation occurred right after the initial decrease in HER3 activation through inhibition of EGFR HER3 in both SKBR3 and MCF 7 cells. The reactivation was not because of the degradation from the drugs due to the fact the dose of Iressa was replenished right after a few days. We also observed the recovery of phospho PKB and phospho ERK1 2 within 48 hours , consistent with activation of alternative HER pathways which includes HER2 HER3 and HER2 HER4 through autocrine release of ligands.
The autocrine ligand release mediates resistance to Iressa in sensitive SKBR3 cells To test the hypothesis that activation of alternative HER receptors via the autocrine release of ligands mediates resistance to Iressa, we stimulated sensitive SKBR3 cells with TGF a, heregulin b, heregulin b 1 or betacellulin although the cells were Clindamycin treated PFI-1 with Iressa for 4 days. Figure 3C shows that all of the ligands rendered the sensitive SKBR3 resistant to Iressa. The greatest effect was noticed with Iressa treatment in combination with either heregulin b or heregulin b 1. The results are consistent with previous experiments where EGFR inhibition by tyrosine kinase inhibitors sensitises the cells to exogenous heregulin stimulation in terms of HER2 activation and hence induced enhanced proliferation. This experiment confirms the role of ligands in mediating resistance to Iressa.
To test if the resistance of SKBR3 cells was accounted by the autocrine ligand release, a neutralising antibody was employed. An anti betacellulin antibody in combination with Iressa was discovered to potentiate the inhibitory effect of Iressa in cell viability experiments . The results Clindamycin indicate a role of autocrine ligand release in mediating resistance to Iressa. Combined therapy with Herceptin and Iressa exerts a greater suppression in EGFR and HER2 activation We showed above that Iressa failed to abolish HER2 phosphorylation in surviving SKBR3 cells as a result of activation of alternative HER3 and HER4 receptors through the autocrine release of numerous ligands. Given that Herceptin targets the HER2 receptor, we proceeded to investigate no matter whether combined treatment of Hercep tin with Iressa would abolish HER2 phosphorylation in SKBR3 cells. It has been shown that the combined treatment with Herceptin and Iressa in SKBR3 was either additive or synergistic in exerting anti proliferative effects as well