Abstract 367: Nuclear FGF-2 Triggers Reciprocal Communication Between Nucleus and Mitochondria Resulting in ERK1/2 Pathway-Dependent Chromatin Compaction and Cell Death
Fibroblast growth factor 2 (FGF-2) is produced as CUG-initiated, 22–34 kDa or AUG-initiated 18 kDa isoforms (referred to by us as hi- and lo-FGF-2, respectively), with distinct intracrine/ nuclear activities. Previously we reported that ectopic expression of human or rat hi-FGF-2 in neonatal cardiomyocytes elicited chromatin compaction leading to cell death with apoptotic features. Hi-FGF-2 exerted similar effects on the HEK293 cell line. In addressing the mechanism of hi-FGF-2-induced cell death first we established that nuclear localization of the intact protein was essential, as expression of a non-nuclear mutant had no effect. Equally ineffective (despite nuclear localization) were overexpression of the amino-terminal extension of hi-FGF-2 or the 18 kDa core FGF-2 sequence. Chromatin compaction by hi-FGF-2 was accompanied by increased cytosolic cytochrome C, and was attenuated either by a peptide inhibitor of the pro-apoptotic protein Bax, or by overerexpression of Bcl-2, indicating mitochondrial involvement. Hi-FGF-2 expression elicited sustained activation of total and nuclear extracellular signal regulated kinase (ERK1/2) by an intracrine route, as it was not prevented by neutralizing anti-FGF-2 antibodies. The non-nuclear, non-apoptotic, hi FGF-2 was equally capable of activating ERK1/2. ERK1/2 activation by hi FGF-2 occured upstream of mitochondrial engagement since it was not affected by the Bax inhibiting peptide. Inhibition of the ERK1/2 activating pathway either pharmacologically or by overexpression of dominant negative MEK1 (the upstream activating kinase) prevented chromatin compaction by hi-FGF-2 in cardiomyocytes and HEK293 cells. We propose that nuclear hi-FGF-2 triggers communication beween nucleus and mitochondria, resulting in ERK1/2-dependent chromatin compaction and cell death, but that ERK1/2 activation is, by itself, insufficient for promoting chromatin compaction.