TRANSCRIPTION FACTOR SP1 IS NECESSARY FOR BASAL CALMODULIN GENE-TRANSCRIPTION AND FOR ITS SELECTIVE STIMULATION BY INSULIN

Insulin positively regulates transcription of rat calmodulin (CaM) I g ene and activates the low Km cyclic AMP (cAMP) phosphodiesterase (PDE) . To elucidate the mechanism of transcriptional regulation, rat hepato ma (H-411E) cells were transfected with DNA constructs containing the putative CaM promoters coupled to a luciferase reporter and challenged with insulin. Activation of the full length 1835 bp rat CaM I promote r containing all three Sp1 sites or truncated promoters with combinati ons of one to three of tile Sp1 sites was studied in Sp1 deficient Dro sophilia SL2 cells and in SL2 cells co-transfected with an Sp1 express ion vector and re-challenged with insulin. Our results demonstrate tha t Sp1 is obligatory for basal activation of the CaM promoter. The maxi mal insulin stimulation of CaM promoter is elicited only if it contain s at least two Sp1 sites.