Localization of a nuclear serine/threonine protein phosphatase in insulin-secreting INS-1 cells: potential regulation by IL-1beta.

Emerging evidence suggests critical roles for protein phosphatase 2A (PP2A) in islet beta cell function, including survival and demise (Kowluru A: Biochemical Pharmacol 69:1681-1691, 2005). Herein, we identified an okadaic acid (OKA)-sensitive PP2A-like phosphatase in the nuclear fraction from insulin-secreting INS-1 cells. Western blot analysis indicated relatively higher abundance of the catalytic subunit of protein phosphatase 4 (PP4c) compared to PP2Ac in this fraction. Autoradiographic and vapor-phase equilibration analyses suggested that the nuclear PP4c undergoes OKA-sensitive carboxylmethylation (CML) when S-adenosyl-L-((3)H-methyl) methionine (SAM) was used as the methyl donor. Exposure of INS cells to interleukin-1beta (IL-1beta; 600 pM; 48 h) resulted in a marked increase in nitric oxide (NO) release with concomitant reduction in the degree of expression, the CML and the catalytic activity of only PP4, but not PP2A, in the nuclear fraction. Immunoprecipitation studies suggested potential complexation of PP4c with nuclear lamin-B, a key regulatory protein involved in the nuclear envelope assembly. Based on these findings, we propose that IL-1beta-mediated inhibition of PP4 activity might result in the retention of lamin-B in its phosphorylated state, which is a requisite for its degradation by caspases leading to the apoptotic demise of the beta cell (Veluthakal et al.: Am J Physiol Cell Physiol 287:C1152-C1162, 2004).