RECIPROCAL REGULATION OF NEU TYROSINE KINASE-ACTIVITY AND CAVEOLIN-1 PROTEIN EXPRESSION IN-VITRO AND IN-VIVO - IMPLICATIONS FOR HUMAN BREAST-CANCER

Neu (c-erbB2) is a proto-oncogene product that encodes an epidermal gr owth factor-like receptor tyrosine kinase, Amplification of wild-type c-Neu and mutational activation of Neu (Neu T) have been implicated in oncogenic transformation of cultured fibroblasts and mammary tumorige nesis in vivo. Here, we examine the relationship between Neu tyrosine kinase activity and caveolin-1 protein expression in vitro and in vivo . Recent studies have suggested that caveolins may function as negativ e regulators of signal transduction, Our current results show that mut ational activation of c-Neu down-regulates caveolin-1 protein expressi on, but not caveolin-2, in cultured NIH 3T3 and Rat 1 cells. Conversel y, recombinant overexpression of caveolin-1 blocks Neu-mediated signal transduction in vice. These results suggest a reciprocal relationship between c-Neu tyrosine kinase activity and caveolin-1 protein express ion. We next analyzed a variety of caveolin-1 deletion mutants to map this caveolin-1-dependent inhibitory activity to a given region of the caveolin-1 molecule. Results from this mutational analysis show that this functional in vivo inhibitory activity is contained within caveol in-1 residues 82-95, in accordance with these in vivo studies, a 20-am ino acid peptide derived from this region (the caveolin-1 scaffolding domain) was sufficient to inhibit Neu autophosphorylation in an in vit ro kinase assay. To further confirm or refute the relevance of our fin dings in vivo, we next examined the expression levels of caveolin-1 in mammary tumors derived from c-Neu transgenic mice. Our results indica te that dramatic reduction of caveolin-1 expression occurs in mammary tumors derived from c-Neu-expressing transgenic mice and other transge nic mice expressing downstream effecters of Neu-mediated signal transd uction, such as Src and Ras, Taken together, our data suggest that a n ovel form of reciprocal negative regulation exists between c-Neu and c aveolin-1.