The specificity of tyrosine kinase inhibitors: Their effect on insulin release (short-term effect) and insulin mRNA (long-term effect) in an insulin-secreting cell line (INS-1)

Summary

Tyrosine kinases are involved in various intracellular signalling cascades of different cells: Genistein has been shown to inhibit tyrosine kinase in INS-I cells, an insulin-secreting cell line (Verspohl et al., 1995). lt is, however, not established how specific and selective the tyrosine kinase inhibitors and their controls are. The tyrosine kinase inhibitors genistein and tyrphostin 25 increased insulin release, but not their negative controls with isoflavonoid structure (daidzein and genistin). In addition to this short-term effect a long-term effect was investigated. Genistein (100 μM) timedependently increased insulin mRNA levels in INS-I cells. On the other hand the tyrosine kinase inhibitors tyrphostin 25 and lavendustin A (both at 100 μM), which are structurally different from genistein. failed to increase the insulin mRNA whereas daidzein and genistin, normally used as negative controls, increased insulin mRNA as potently as genistein did. However, an examination of the incubation medium revealed that genistin was degraded to genistein by about 50% probably by nonspecific glucosidases first seen after 2 hours of incubation; genistin, therefore, does not appear 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 possible. Whereas the involvement of tyrosine kinase in a short. term effect (insulin release) is obvious and clearly substantiated by using the established pharmacological tools (negative controls), the involvement of tyrosine kinases in long-term effects is not that clear: only compounds with isoflavonoid structure are effective independent whether they normally are thought to be inhibitors or regative controls. One has to be cautious in using the above-mentioned compounds in an uncritical way.

Key words

Tyrosine kinase - insulin release - insulin mRNA - INS-I