Mice deficient in C/EBP alpha have defective development of adipose tissue, but the precise role of C/EBP alpha has not been defined. Fibroblasts from C/EBP alpha(-/-) mice undergo adipose differentiation through expression and activation of PPAR gamma, though several clear defects are apparent. C/EBP alpha-deficient adipocytes accumulates less lipid, and they do not induce endogenous PPAR gamma, indicating that cross-regulation between C/EBP alpha and PPAR gamma is important in maintaining the differentiated state. The cells also show a complete absence of insulin-stimulated glucose transport, secondary to reduced gene expression and tyrosine phosphorylation for the insulin receptor and IRS-1. These results define multiple roles for C/EBP alpha in adipogenesis and show that cross-regulation between PPAR gamma and C/EBP alpha is a key component of the transcriptional control of this cell lineage.

Insulin receptor complementary DNA has been cloned from an insulin-resistant patient with leprechaunism whose receptors exhibited multiple abnormalities in insulin binding. The patient is a compound heterozygote, having inherited two different mutant alleles of the insulin receptor gene. One allele contains a missense mutation encoding the substitution of glutamic acid for lysine at position 460 in the alpha subunit of the receptor. The second allele has a nonsense mutation causing premature chain termination after amino acid 671 in the alpha subunit, thereby deleting both the transmembrane and tyrosine kinase domains of the receptor. Interestingly, the father is heterozygous for this nonsense mutation and exhibits a moderate degree of insulin resistance. This raises the possibility that mutations in the insulin receptor gene may account for the insulin resistance in some patients with non-insulin-dependent diabetes mellitus.

Defects in insulin-receptor function have been associated with insulin-resistant states such as obesity and non-insulin-dependent diabetes mellitus (NIDDM). Several types of mutations in the insulin-receptor gene have been identified in patients with genetic syndromes of extreme insulin resistance. In some patients, insulin resistance results from a decrease in the number of insulin receptors on the cell surface. In one patient with leprechaunism (leprechaun/Minn-1), there is greater than 90% decrease in the levels of insulin-receptor mRNA. This patient is a compound heterozygote for two mutations in the insulin-receptor gene, both of which act in a cis-dominant fashion to decrease levels of mRNA transcribed from that allele. In one allele, there is a nonsense mutation at codon 897. All 22 exons of the other allele have a normal sequence, so that the mutation in this allele appears to map outside the coding sequence of the gene. Impaired insertion in the plasma membrane also causes insulin resistance. In two sisters (patients A-5 and A-8) with type A extreme insulin resistance, there is an 80-90% decrease in the number of insulin receptors expressed on the surface of their cells. Both sisters, whose parents are first cousins, are homozygous for a point mutation in which valine is substituted for phenylalanine at position 382 in the alpha-subunit of the insulin receptor. This mutation retards the posttranslational processing of the receptor and impairs the transport of receptors to the cell surface. Another patient with leprechaunism (leprechaun/Ark-1) is a compound heterozygote with two different mutant alleles of the insulin-receptor gene. In the allele derived from the father, there is a nonsense mutation at codon 672 that truncates the insulin receptor by deleting the COOH-terminal of the alpha-subunit and the entire beta-subunit. This truncated receptor, lacking a transmembrane domain, appears not to be expressed at the plasma membrane. In leprechaun/Ark-1, there is a missense mutation in the allele of the insulin-receptor gene derived from the mother. This point mutation results in substitution of glutamic acid for lysine at position 460 in the COOH-terminal half of the alpha-subunit. This mutation increases receptor affinity and impairs the ability of acid pH to dissociate insulin from the receptor within the endosome. There is a defect in recycling the receptor back to the plasma membrane associated with this defect. This results in an accelerated rate of receptor degradation and a consequent decrease in the number of receptors on the cell surface in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

The incidence of type 2 diabetes (T2D) and its costs to the health care system continue to rise. Despite the availability of at least 10 drug classes for the treatment of T2D, metformin remains the most widely used first-line pharmacotherapy for its treatment; however, marked interindividual variability in response and few clinical or biomarker predictors of response reduce its optimal use. As clinical care moves toward precision medicine, a variety of broad discovery-based “omics” approaches will be required. Technical innovation, decreasing sequencing cost, and routine sample storage and processing has made pharmacogenomics the most widely applied discovery-based approach to date. This opens up the opportunity to understand the genetics underlying the interindividual variation in metformin responses in order for clinicians to prescribe specific treatments to given individuals for better efficacy and safety: metformin for those predicted to respond and alternative therapies for those predicted to be nonresponders or who are at increased risk for adverse side effects. Furthermore, understanding of the genetic determinants of metformin response may lead to the identification of novel targets and development of more effective agents for diabetes treatment. The goals of this workshop sponsored by the National Institute of Diabetes and Digestive and Kidney Diseases were to review the state of research on metformin pharmacogenomics, discuss the scientific and clinical hurdles to furthering our knowledge of the variability in patient responses to metformin, and consider how to effectively use this increased understanding to improve patient outcomes.