Expression of the high molecular weight fibroblast growth factor-2 isoformof 210 amino acids is associated with modulation of protein kinases C delta and epsilon and ERK activation

The high molecular weight (HMW) fibroblast growth factor (FGF)-2 isoform of 210 amino acids initiated at a CUG start codon possesses a nuclear localiz ation sequence and is not secreted, In contrast, the low molecular weight ( LMW) isoform of 155 amino acids initiated at the AUG start codon can be sec reted and activates the cell surface FGF receptors, The two isoforms posses s different biological properties; however, little is known about the intra crine regulatory mechanisms involved in the biological effects of the HMW F GF-2 isoform, Using pancreatic cells stably transfected with cDNAs leading to the expression of either the HMW FGF-2 (A3 cells) or the LMW form (A5 ce lls), we provide evidence that the two FGF-2 isoforms differentially modula te PKC levels. The LMW FGF-2 up-regulated the PKC epsilon levels by 1.6-fol d; by contrast the HMW isoform down-regulated the level of this PKC isotype by about 3-fold and increased the amount of PKC delta by 1.7-fold. PKC mRN As were also modified, suggesting that PKC expression was regulated at a pr etranslational level. Additionally, expression of different levels of the H MW FGF-2 with an inducible expression system confirmed the role of this iso form on PKC delta and epsilon expressions. Increased activation of ERK-1 an d -2 was also observed in cells expressing the HMW FGF-2, By using differen t PKC inhibitors and a dominant negative PKC delta, it was found that ERK a ctivation was PKC delta -dependent. These data indicate that expression of HMW FGF-2 can modify PKC levels by acting at the intracellular level and th at the overexpression of PKC delta induces ERK-1/2 activation. The expressi on of a dominant negative FGFR1 did not reduce ERK-1/2 activation by the HM W FGF-2, suggesting that ERK activation does not require FGFR activity. The signaling cascade downstream of ERK might be involved in the known mitogen ic effect exerted by this FGF-2 isoform.