REDUCED O-GLYCOSYLATION OF SP1 IS ASSOCIATED WITH INCREASED PROTEASOME SUSCEPTIBILITY

Sp1 is a ubiquitously expressed transcription factor that is particula rly important for the regulation of TATA-less genes that encode housek eeping proteins. Most growth factors and receptors are also encoded by such genes. Sp1 is multiply O glycosylated by covalent linkage of the monosaccharide N-acetylglucosamine (O-GlcNAc) to serine and threonine residues, Based on an earlier observation that growth factor gene tra nscription can be regulated by glucose and glucosamine in vascular smo oth muscle cells, we determined whether Sp1 glycosylation could be reg ulated and if this modification altered Sp1 function, We found that Sp 1 becomes hyperglycosylated when cells are exposed to 5 mM glucosamine , whereas under glucose starvation, stimulation with cyclic AMP (cAMP) results in nearly complete deglycosylation of this protein, Correlati ng with this hypoglycosylated state, Sp1 is rapidly proteolytically de graded by an enzyme(s) that can be inhibited by specific proteasome in hibitors, lactacystin and LLnL, Treatment of cells with glucose or glu cosamine protects Sp1 from cAMP-mediated degradation, whereas blockade of glucosamine synthesis abrogates glucose but not glucosamine protec tion, This effect on Sp1 is specific, in that the Stat-3 and E2F trans cription factors did not undergo degradation under these conditions. T he O-GlcNAc modification of Sp1 may play a role as a nutritional check point, In the absence of adequate nutrition, Sp1 becomes hypoglycosyla ted and thereby subject to proteasome degradation. This process could potentially result in reduced general transcription, thereby conservin g nutrients.