The regulated expression of protein kinase C (PKC) isoforms was examined during the differentiation program of 3T3-L1 preadipocytes. In a parallel analysis, differentiation was blocked by treatment of the cells with tumor necrosis factor-alpha (TNF) to determine differentiation-specific changes in isoform expression from growth or treatment-induced effects. This analysis revealed that the expression of the conventional PKC-alpha isoform was reduced by 85% as cells attained the adipocyte phenotype. PKC-beta expression was measurable only during the early stages of the differentiation process and was not detectable in fully differentiated cells. An upregulation of PKC-theta, a novel PKC isoform, occurred during the latter stage of differentiation. Expression of PKC-zeta an atypical PKC isoform suggested to participate in TNF signal transduction, occurred throughout the time course with similar levels of expression in both preadipocytes and adipocytes. Nuclear run-on analysis demonstrated an approximately 85% reduction in the transcription of the PKC-alpha gene during differentiation. The reduced expression of this isoform corresponded with the decreased ability to activate nuclear factor kapppaB (NF-kappaB) in response to phorbol 12-myristate 13-acetate (PMA) treatment in the adipocytes. These data suggest that PMA responsiveness in 3T3-L1 adipocytes is markedly diminished.