We have carried out an investigation into the processing of the enkephalin-
like immunoreactivity reported in breast tissue using two human breast tumo
ur cell lines and a mouse tumour cell line. A 46 kDa form of proenkephalin
(PE) has been observed in the cell lysates of two human breast tumour cell
Lines (MCF-7, ZR-75-1) and the mouse androgen-responsive Shionogi breast ca
rcinoma cell line (SC115). PE processing in the cell lysates of these cells
was assessed by a specific met-enkephalin RIA. The basal levels of process
ed PE in the MCF-7, ZR-75-1 and SC115 cell lysates were 30, 30 and 76% resp
ectively. The processing enzymes PC1 and PC2, which have been implicated in
the differential processing of PE, were detected by immunoblot analysis in
these cells. PC1 was found within the cell extracts of all three cell line
s. PC2 was only observed in the SC115 cell line, which may account for the
higher percentage of processed PE measured, The cDNA of PC2 has been transf
ected into ZR-75-1 cells and this was accompanied by an increase in the lev
el of processed PE from 30 to 76%. These breast tumour cell lines may provi
de a useful insight into the function of enkephalin-containing peptides in
breast cancer.