THE SPECIFICITY OF TYROSINE KINASE INHIBITORS - THEIR EFFECT ON INSULIN RELEASE (SHORT-TERM EFFECT) AND INSULIN MESSENGER-RNA (LONG-TERM EFFECT) IN AN INSULIN-SECRETING CELL-LINE (INS-1)

Tyrosine kinases are involved in various intracellular signalling casc ades of different cells: Genistein has been shown to inhibit tyrosine kinase in INS-1 cells, an insulin-secreting cell line (Verspohl et al. , 1995). It is, however, not established how specific and selective th e tyrosine kinase inhibitors and their controls are. The tyrosine kina se inhibitors genistein and tyrphostin 25 increased insulin release, b ut not their negative controls with isoflavonoid structure (daidzein a nd genistin). In addition to this short-term effect a long-term effect was investigated. Genistein (100 mu M) time-dependently increased ins ulin mRNA levels in INS-1 cells. On the other hand the tyrosine kinase inhibitors tyrphostin 25 and lavendustin A (both at 100 mu M), which are structurally different from genistein, failed to increase the insu lin mRNA whereas daidzein and genistin, normally used as negative cont rols, increased insulin mRNA as potently as genistein did. However, an examination of the incubation medium revealed that genistin was degra ded to genistein by about 50% probably by nonspecific glucosidases fir st seen after 2 hours of incubation; genistin, therefore, does not app ear to be a proper control though often used in this way In conclusion , the suitability of the compounds used in recent studies is doubtful since other effects than the inhibition of tyrosine kinases are possib le. Whereas the involvement of tyrosine kinase in a shortterm effect ( insulin release) is obvious and clearly substantiated by using the est ablished pharmacological tools (negative controls), the involvement of tyrosine kinases in long-term effects is not that clear; only compoun ds with isoflavonoid structure are effective independent whether they normally are thought to be inhibitors or negative controls. One has to be cautious in using the above-mentioned compounds in an uncritical w ay.