Intracellular targeting and protein kinase C in vascular smooth muscle cells: specific effects of different membrane-bound receptors

Protein kinase C is an important second messenger system, which is transloc ated from the cytosol to the cell membrane upon cell stimulation. We used c onfocal microscopy to study the spatial distribution of protein kinase C is oforms after stimulation of cultured vascular smooth muscle cells with diff erent agonists. First, we analysed the effects of angiotensin II and platel et-derived growth factor (PDGF). Confocal microscopy showed a rapid assembl y of PKC alpha along cytosolic fibres followed by a translocation towards t he nucleus with angiotensin II. PDGF engendered a similar, but much slower response; however, a cytoskeletal distribution was not observed. We then in vestigated the effects of thrombin and bFGF on nuclear translocation. bFGF induced a rapid translocation of the isoform towards the perinuclear region and into the nucleus. bFGF had a similar effect on PKC epsilon. In contras t, thrombin had a smaller effect on nuclear translocation of PKC alpha and did not influence PKC epsilon, but instead induced a rapid nuclear transloc ation of PKC zeta. Thus, tyrosine kinase receptor activation via bFGF induc es a rapid association of PKC alpha and epsilon within nuclear structures. Our results show that agonists cause, not only a translocation of protein k inase C isoforms into the cell membrane but also into the cell nucleus. Las tly, we analyzed the nuclear immunoreactivity of the PKC isoforms alpha, de lta, epsilon and zeta in vascular smooth muscle cells during the cell cycle . Resting cells were stimulated with foetal calf serum (FCS, 10%), which tr anslocated PKC alpha and epsilon to the perinuclear region and into the nuc leus, while PKC delta and zeta showed no increase in nuclear immunoreactivi ty. After 4 h of FCS, the nuclear immunoreactivity for PKC alpha and epsilo n was reduced to or below control values. At 8 h, increased nuclear express ion of isoforms alpha, epsilon and zeta was observed, while isoform delta w as not affected. Our results demonstrate a complex spatial and temporal reg ulation of PKC isoforms in response to vasoactive hormones and growth facto rs. We suggest that protein kinase C may be important for nuclear signaling and demonstrate that nuclear translocation of PKC isoforms is differential ly regulated during the cell cycle.