PROMOTION-RESISTANT JB6 MOUSE EPIDERMAL-CELLS EXHIBIT DEFECTS IN PHOSPHATIDYLETHANOLAMINE SYNTHESIS AND PHORBOL ESTER-INDUCED PHOSPHATIDYLCHOLINE HYDROLYSIS

The tumour-promotion-sensitive (P+) and -resistant (P-) variants of mo use JB6 epidermis-derived cells have often been used to study the requ irements for the tumour-promoting effect of PMA. As part of an effort to identify the defect(s) in JB6 P- cells that might prevent the promo ting effect of PMA, stimulation of phospholipase D (PLD)-mediated hydr olysis of phosphatidylcholine (PtdCho) and phosphatidylethanolamine (P tdEtn) by PMA as well as the rate of phospholipid synthesis were compa red in three P+ variants, two P- variants and a transformed variant of the JB6 cell line. PMA (5-100 nM) had significantly less stimulatory effect on PtdCho hydrolysis in P- cells than in P+ or transformed JB6 cells. The effects of PMA on PtdEtn hydrolysis in the P+ and P- cell l ines were similar, whereas in transformed cells PMA had slightly less effect. Each JB6 cell line was found to contain similar amounts of Ptd Cho. In contrast, P- cells contained significantly less PtdEtn and a c orrespondingly higher level of ethanolamine phosphate compared with P and transformed cells. P- cells also secreted ethanolamine phosphate into the medium; this process was greatly enhanced by PMA. In the two P- variants the synthesis of PtdEtn from [C-14]ethanolamine was reduce d to various extents, whereas the rate of PtdCho synthesis was compara ble in each JB6 cell line. The synthesis of PtdCho, but not PtdEtn, wa s greatly stimulated by PMA in both the P+ and P- clones. The results indicate that decreased synthesis/level of PtdEtn and suboptimal funct ioning of a PtdCho-specific PLD are common characteristics of the P- J B6 cells examined so far. The observed alterations in phospholipid met abolism may play a role in the resistance of P- cells to the tumour-pr omoting action of PMA.