Transferrin receptor recycling in rat hepatocytes is regulated by protein phosphatase 2A, possibly through effects on microtubule-dependent transport.

To understand the regulation of receptor-mediated endocytosis in hepatocytes, we have used two specific inhibitors of serine-threonine protein phosphatases (PP), microcystin (MCYST) and okadaic acid (OKA) as probes to alter protein phosphorylation in hepatocytes. We have then examined the impact of these changes on the specific binding and uptake of transferrin (Tf) in hepatocytes. The measurement of PP activity in hepatocyte lysates showed that OKA and MCYST shared a common inhibition of protein phosphatase 2A (PP2A). Our results showed that both OKA (250 nmol/L) and MCYST (500 nmol/L) significantly reduced Tf uptake at steady state (P < or = .05). The measurement of Tf internalization after 15 minutes in protein phosphatase inhibitor-pretreated cells revealed that the initial uptake was also significantly reduced. Binding studies showed that pretreatment with either of the phosphatase inhibitors did not result in significant changes in the K(d) for Tf binding to transferrin receptor (TfR). Additionally, no significant changes in the number of TfR in the plasma membrane were observed in phosphatase inhibitor-pretreated cells. The treatment of hepatocytes with nocodazole (NOC), which results in microtubule disassembly and inhibition of microtubule-based vesicle transport, caused comparable reductions in initial and steady state levels of transferrin accumulation. The changes in transferrin accumulation by both phosphatase inhibitors and nocodazole were accompanied by redistribution of the microtubule-anchored Golgi apparatus and lysosomal network from the perinuclear region to the cell periphery. Our data show that the regulation of Tf uptake by receptor-mediated endocytosis is mediated by PP2A and additionally may occur through regulation of microtubule-based vesicle transport.