Hexamethylene bisacetamide (HMBA)-induced growth inhibition and differentiation of the rat C6 glioma cell line were found to be accompanied by down-regulation of the constitutively expressed fra-1 gene. In order to check if the fra-1 gene down-regulation was essential for HMBA's growth inhibitory effect, C6 cells were stably transfected with vector expressing fra-1 cDNA under CMV promoter in either sense or antisense orientation. Contrary to the expectations, fra-1 overexpression was found to inhibit proliferation and induce morphological differentiation of C6 cells. Furthermore, all three differentiation inducers studied viz. dibutyryl cyclic AMP (dbcAMP), staurosporine, and HMBA have greater growth inhibitory effect on fra-1 overexpressing clones as compared to the parental C6 cells. dbcAMP and staurosporine not only inhibit proliferation but bring about complete apoptosis of fra-1 overexpressing clones. Spontaneous apoptosis is seen in fra-1 overexpressing clones especially in confluent cultures. fra-1 overexpression also results in substantial reduction in anchorage-independent growth and tumourigenicity of C6 cells. Overexpression of fra-1 leading to proliferation inhibition of C6 glioma cells is consistent with the concept that fra-1 functions as a negative regulator of AP-1 activity.
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