The alpha4-containing form of protein phosphatase 2A in liver and hepatic cells

The Ser/Thr phosphatase PP2A is a set of multisubunit enzymes that regulate many cellular processes. In yeast, the PP2A regulatory subunit Tap42 forms part of the target of rapamycin (TOR) signaling pathway that links nutrient and energy availability to cell growth. The physiological intersection between the mammalian orthologs of Tap42 and TOR, alpha4 and mTOR, has not been fully characterized. We used two in vivo models of liver growth in the rat, late gestation fetal development and regeneration after partial hepatectomy, to explore the regulation of the alpha4-containing form of PP2A. The alpha4/PP2A catalytic subunit (alpha4/PP2A-C) complex was present in both fetal and adult liver extracts. There was a trend towards higher levels of alpha4 protein in fetal liver, but the complex was more abundant in adult liver. Fractionation of extracts by ion exchange chromatography and transient transfection of the AML12 mouse hepatic cell line indicated that alpha4 associates with PP2A-C but that these complexes have low catalytic activity with both peptide and protein substrates. alpha4 was able to associate with forms of PP2A-C that were both methylated and non-methylated at the carboxy-terminus. The mTOR inhibitor rapamycin did not block the formation of alpha4/PP2A-C in liver or hepatic cells, nor did it appear to modulate PP2A activity. Furthermore, sensitivity to the growth inhibitory effects of rapamycin among a panel of hepatic cell lines did not correlate with levels of alpha4 or alpha4/PP2A-C. Our results indicate that the yeast Tap42/TOR paradigm is not conserved in hepatic cells.