The gene encoding rat liver glycogen phosphorylase contains multiple polyadenylation signal sequences

Glycogen phosphorylase is activated in response to glucose deprivation but is not responsible for enhanced glucose transport activity in 3T3-L1 adipocytes

We have previously shown that glucose deprivation activates glucose transport in a time- and protein synthesis-dependent fashion in 3T3-L1 adipocytes, a mouse cell line. Coincident with this is loss of glycogen. Because glycogen phosphorylase (GP) is responsible for glycogen degradation, we have examined its regulation to determine the relationship between transport activation and glycogen turnover. We first cloned the adipose GP cDNA and found sequence similarity to rat and human liver GP. Because the mouse liver GP cDNA sequence was unavailable, we cloned this cDNA as well and showed 100% identity between mouse adipose and liver sequences. A 3.1 kb transcript was readily observed in total RNA isolated from mouse liver or adipose by Northern blot analysis but, surprisingly, not in either total or poly(A) selected RNA from 3T3-L1 adipocytes. To evaluate regulation in 3T3-L1 adipocytes, we amplified GP mRNA from total RNA using multiplex, semi-quantitative PCR but found that expression did not change in response to deprivation. GP protein levels did not change either. However, endogenous GP activity from glucose-deprived cells was significantly elevated relative to controls, due to an increase in the phosphorylated form of GP (GPa). Finally, we overexpressed GP to determine its direct influence on the glucose transport system. These results were negative, which suggests that the nutrient control of glucose transport and GP occurs independently despite kinetic similarities in transport activation and glycogen turnover.

Regulation of the rat muscle glycogen phosphorylase-encoding gene during muscle cell development

The muscle isozyme of glycogen phosphorylase (MGP) catalyzes the hydrolysis hydrolysis of intracellular glycogen in mammalian tissues and is produced in skeletal muscle, brain and heart. The MGP gene is developmentally and neutrally regulated in skeletal muscle, but little is known about the gene's transcriptional regulation. We have isolated and characterized the 5' flanking region of rat MGP. Truncated portions of the MGP 5' flanking region were coupled to the bacterial cat reporter gene and used in transient transfection assays in the mouse muscle C2C12 cell line. The region between -211 and +62 contained the smallest regulatory domain capable of demonstrating developmentally regulated myogenic expression in C2C12 cells. This was in contrast with findings from another investigation that transfected this cell line with human MGP [Lockyer and McCracken, J. Biol. Chem. 266 (1991) 20262-20269]. A 172-nucleotide (nt) region between -839 and -666 functioned as a potent enhancer in C2C12 cells when coupled to its cognate promoter, but not when coupled to a simian virus 40 promoter. This rat MGP enhancer region is 78% identical to a comparable region of the human MGP 5' flanking region, but contains only one putative regulatory element that has been previously identified in other muscle genes. These data suggest that rat MGP transcription in C2C12 muscle cells is modulated by a potent enhancer that utilizes novel regulatory elements.

Characterization of the 5' flanking region of the gene encoding rat liver glycogen phosphorylase

A genomic region encompassing 800 bp of the promoter-regulatory region and exon 1 of the gene (LGP) encoding rat liver glycogen phosphorylase has been isolated and characterized. Transcripts of the LGP gene initiate predominantly within an 8-bp region 48-bp upstream from the start codon. Additional transcripts were detected that initiate as far as 95 bp upstream from the start codon. To identify cis-acting sequences involved in regulating transcription, HepG2 cells were transfected with vectors containing serial deletions of the promoter-regulatory region of LGP ligated to the cat reporter gene. Two upstream regions were found to enhance transcription. One of these regions contains an alternating purine-pyrimidine sequence. LGP, which lacks a consensus TATA sequence, is like TATA-less and CAAT-less housekeeping genes in that it contains G + C-rich domains upstream from multiple transcription start points. Nuclear proteins from adult rat tissues bound in a tissue-specific fashion to one of these G + C-rich regions.