Degradation of insulin-like growth factors in small intestine of suckling rats.

Insulin-like growth factors (IFGs), IGF-I and IGF-II, present in mammalian milk, play an important role during gastrointestinal tract development. In this study we identified and localized the activities of the common intestinal proteolytic enzymes and investigated their degradation effect on IGFs. Results indicated that the enzymatic activities of chymotrypsin, trypsin, and elastase progressed from the lowest in the duodenum, to the highest in the midjejunum, and declined in the ileum. Chymotrypsin exhibited the greatest IGFs degradation activities in neonatal intestinal lumen followed by elastase. These data furnish a potential strategic design to supplement IGFs into milk formulas.

Effects of geldanamycin, a heat-shock protein 90-binding agent, on T cell function and T cell nonreceptor protein tyrosine kinases.

The benzoquinoid ansamycins geldanamycin (GA), herbimycin, and their derivatives are emerging as novel therapeutic agents that act by inhibiting the 90-kDa heat-shock protein hsp90. We report that GA inhibits the proliferation of mitogen-activated T cells. GA is actively toxic to both resting and activated T cells; activated T cells appear to be especially vulnerable. The mechanism by which GA acts is reflected by its effects on an essential hsp90-dependent protein, the T cell-specific nonreceptor tyrosine kinase lck. GA treatment depletes lck levels in cultured T cells by a kinetically slow dose-dependent process. Pulse-chase analyses indicate that GA induces the very rapid degradation of newly synthesized lck molecules. GA also induces a slower degradation of mature lck populations. These results correlate with global losses in protein tyrosine kinase activity and an inability to respond to TCR stimuli, but the activity of mature lck is not immediately compromised. Although the specific proteasome inhibitor lactacystin provides marginal protection against GA-induced lck depletion, proteasome inhibition also induces changes in lck detergent solubility independent of GA application. There is no other evidence for the involvement of the proteosome. Lysosome inhibition provides quantitatively superior protection against degradation. These results indicate that pharmacologic inhibition of hsp90 chaperone function may represent a novel immunosuppressant strategy, and elaborate on the appropriate context in which to interpret losses of lck as a reporter for the pharmacology of GA in whole organisms.