ABSTRACT
Tyrosine phosphorylation of the insulin receptor beta-subunit is a crucial event for insulin signal transduction into the cell (1). In particular, phosphorylation of tyrosine 1146, 1150 and 1151 is necessary for complete activation of the insulin receptor which initiates further downstream signalling events (2). Serine phosphorylation of the insulin receptor beta-subunit occurs also in vivo (3). These serine phosphorylation sites are not characterized as well as the tyrosine phosphorylation sites and their role for receptor activation is still not completely clear. Based on studies with phorbol esters it was suggested that serine phosphorylation of the insulin receptor might have inhibitory function. First, Takayama et al., showed in rat hepatoma cells that phorbol ester stimulates serine and threonine phosphorylation of the insulin receptor beta-subunit (4). They suggested that PKC might mediate serine/threonine phosphorylation of the insulin receptor which inhibits its intrinsic tyrosine kinase (4). Subsequently, Bollag et al. showed that the isolated insulin receptor can serve as a direct substrate for PKC which leads to inhibition of the intrinsic receptor tyrosine kinase activity (5). Later on, this was also demonstrated in isolated fat and hepatoma cells (6, 7, 8). Although it was quite obvious from these studies that PKC is able to phosphorylate the insulin receptor, it took some years more to identify the first PKC dependent serine and threonine residues. Serine 1035 and 1037 near the ATP-binding site (9) and serine 1327 which is located at the Cterminus of the receptor (10) were identified as PKC dependent phosphorylation sites. Besides these serine residues, threonine 1348 presents also a PKC-dependent phosphorylation site (11, 12). Of major interest is also whether there is a specific PKC isoform involved in negative regulation of the insulin signal. Inhibition of the insulin receptor kinase have been demonstrated after activation of PKC alpha (12) and PKC beta (13) with phorbol ester. Recently we have shown that PKC delta and theta have also a strong inhibitory effect on insulin receptor kinase (14). These PKC isoforms are mediating their inhibitory effect on the insulin receptor probably by serine phosphorylation of IRS-1 (14). Serine phosphorylation of IRS-1 can also interfere with its own phosphorylation on tyrosine residues (15, 16). Until now PKC, MAPK, GSK-3 and PI 3-kinase have been shown to phosphorylate IRS-1 on serine residues (14, 17-21). Among many potential serine phosphorylation sites only serine 612 was identified as PKC-dependent phosphorylation site of IRS-1 which seems also to be a direct target of MAPK (18). It can be assumed that there are more relevant serine phosphorylation sites on IRS-1 which are able to impair insulin signalling. This is evidenced by studies with
different serine mutants of IRS-1 showing that point mutations on serine 612, 632, 662, and 731 can still be phosphorylated on other serine residues after ocadaic treatment (22) or after PKC theta activation (own observation). These serine residues have to be identified to examine their potential role on the development of insulin resistance.
