SELECTIVE-INHIBITION OF THE EPIDERMAL GROWTH-FACTOR AND HER2 NEU RECEPTORS BY TYRPHOSTINS/

The HER2 (neu/erb-B2) proto-oncogene codes for a transmembrane recepto r with tyrosine kinase activity and with high homology to the EGF rece ptor (HER1). The high incidence of HER2 overexpression in breast and o vary carcinomas prompted us to synthesize protein tyrosine kinase inhi bitors (tyrphostins) which selectively inhibit the HER2 kinase activit y. Two groups of tyrphostins were developed: one highly selective in i nhibiting HER1 as opposed to HER2, the other highly selective in inhib iting HER2. Both the HER1 and the HER2 selective blockers were competi tive with ATP binding. This suggests that even though the kinase domai ns of the respective receptors show an 80% degree of homology it is po ssible to design small molecules capable of discriminating between the m. These results also show that the two kinases differ in their ATP bi nding sites. Mitogenic signaling induced by EGF in NIH3T3 cells overex pressing either HER1 or HER1-2 (possessing the HER2 kinase domain) was blocked identically by the agents that discriminate between the two i n vitro. This paradox was further explored and elucidated. We propose that high intracellular ATP levels prevent inhibitor binding to the re ceptor. The antiproliferative action of the two distinct selective tyr phostins observed may result from the inhibition of a downstream eleme nt, presumably a tyrosine kinase, which mediates mitogenic signaling.