Expression of the proenkephalin gene in cultured astroglial cells: analysis of cell cycle dependence

Abstract

Glial progenitors strongly express the proenkephalin (PEnk) gene during their proliferation in the subventricular zone of the neocortex. Also in primary culture, astroglial cells from rat neocortex produce PEnk mRNA. Since the basal expression sharply declines after the cultured cells reach confluence, it seems to be related to cell proliferation. In contrast, activation of protein kinases A and C strongly enhances the levels of PEnk mRNA in confluent cultures. Therefore, it was investigated in cultured neocortical astroglial cells, whether the basal and stimulated expression of the PEnk gene occurred during different phases of the cell cycle. Activation of protein kinases A and C with 8Br.cAMP and tetradecanoylphorbolacetate, respectively, enhanced the PEnk gene expression only during the G 1 phase. Unstimulated astroglial cells contained PEnk mRNA during the G 1 as well as the S phase of the cell cycle. Since confluent astroglial cells are arrested in the G 1 phase, they should have a basal expression of the gene which is comparable to that in preconfluent cells. Thus, the reduced basal expression after confluence indicates that other mechanisms may play a role, such as humoral factors or contact inhibition.