INSULIN-LIKE GROWTH-FACTORS STIMULATE GROWTH AND L-CYSTEINE UPTAKE BYTHE INTESTINAL PARASITE GIARDIA-LAMBLIA

Giardia lamblia, a parasitic protozoan responsible for diarrhea and ma labsorption in humans, grows axeni- cally only in media that contain s erum and a high concentration of L-cysteine. During our attempts to gr ow Giardia in the absence of serum, we found that: (a) human insulin-l ike growth factors (especially IGF-II), but not insulin, promote the g rowth and L-cysteine uptake by G. lamblia trophozoites; (b) the growth stimulation was inhibited by alpha IR3, an anti-type 1 IGF receptor m onoclonal antibody, but an anti-type 2 IGF receptor antibody had no ef fect; and (c) IGFs act on Giardia through a type 1 IGF receptor-like p rotein, which can bind IGF-II with higher affinity than IGF-I, and mos t likely possesses intrinsic phosphotyrosine kinase activity.