THE SIGNALS FOR STARVATION RESPONSE ARE TRANSDUCED THROUGH ELEVATED [CA2+](I) IN DICTYOSTELIUM CELLS

The mechanism by which cells recognize starvation to allow subsequent cellular development was analyzed using Dictyostelium discoideum, with special emphasis on Ca2+ as a crucial signal transducer in intra- and intercellular communications. As was expected, the cytosolic Ca2+ con centration ([Ca2+](i)) in aequorin-expressing cells (RHI76 derived fro m D. discoideum Ax-3) was temporarily increased, when 3-5 mu M thapsig argin (Tg), a specific inhibitor of the Ca2+-ATPase, was added into th e cells incubated in semistarvation medium (SS-medium: 1 vol of growth medium plus 7 vol either of 20 mM: Na-2/K-phosphate buffer (pH 6.2) o r of Bonner's salt solution (BSS)). Essentially the same result was ob tained by the application of 5 mu M nigericin (Ng), an acid ionophore to cells under the semistarved condition. Here it is of interest to no te that in the SS-medium Tg and Ng are capable of enhancing cell diffe rentiation as exemplified well by the earlier acquisition of chemotact ic response to cAMP, possibly inducing the starvation response through the [Ca2+](i) increase. From Western blot analysis of phosphotyrosine (pTyr)-containing proteins using anti-pTyr antibody, it was found tha t the pTyr-phosphorylation levels of 97-, 80-, and 45-kDa proteins inc rease specifically in response to starvation. Interestingly, Tg and Ng induced such a change of the 80-kDa protein in the cells incubated in the SS-medium. Taken together these results strongly suggest that the temporal increase of [Ca2+](i) may be a matter of importance for sign al transduction coupled with starvation response. (C) 1998 Academic Pr ess.