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 translocated from the cytosol to the cell membrane upon cell stimulation. We used confocal microscopy to study the spatial distribution of protein kinase C isoforms after stimulation of cultured vascular smooth muscle cells with different agonists. First, we analysed the effects of angiotensin II and platelet-derived growth factor (PDGF). Confocal microscopy showed a rapid assembly of PKC alpha along cytosolic fibres followed by a translocation towards the nucleus with angiotensin II. PDGF engendered a similar, but much slower response; however, a cytoskeletal distribution was not observed. We then investigated 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 contrast, thrombin had a smaller effect on nuclear translocation of PKC alpha and did not influence PKC epsilon, but instead induced a rapid nuclear translocation of PKC zeta. Thus, tyrosine kinase receptor activation via bFGF induces a rapid association of PKC alpha and epsilon within nuclear structures. Our results show that agonists cause, not only a translocation of protein kinase C isoforms into the cell membrane but also into the cell nucleus. Lastly, we analyzed the nuclear immunoreactivity of the PKC isoforms, alpha, delta, epsilon and zeta in vascular smooth muscle cells during the cell cycle. Resting cells were stimulated with foetal calf serum (FCS, 10%), which translocated PKC alpha and epsilon to the perinuclear region and into the nucleus, while PKC delta and zeta showed no increase in nuclear immunoreactivity. After 4 h of FCS, the nuclear immunoreactivity for PKC alpha and epsilon was reduced to or below control values. At 8 h, increased nuclear expression of isoforms alpha, epsilon and zeta was observed, while isoform delta was not affected. Our results demonstrate a complex spatial and temporal regulation of PKC isoforms in response to vasoactive hormones and growth factors. We suggest that protein kinase C may be important for nuclear signaling and demonstrate that nuclear translocation of PKC isoforms is differentially regulated during the cell cycle.