Survey of the Human Pancreatic β-Cell G1/S Proteome Reveals a Potential Therapeutic Role for Cdk-6 and Cyclin D1 in Enhancing Human β-Cell Replication and Function In Vivo

Abstract

OBJECTIVESTo comprehensively inventory the proteins that control the G1/S cell cycle checkpoint in the human islet and compare them with those in the murine islet, to determine whether these might therapeutically enhance human β-cell replication, to determine whether human β-cell replication can be demonstrated in an in vivo model, and to enhance human β-cell function in vivo.RESEARCH DESIGN AND METHODSThirty-four G1/S regulatory proteins were examined in human islets. Effects of adenoviruses expressing cdk-6, cdk-4, and cyclin D1 on proliferation in human β-cells were studied in both invitro and in vivo models.RESULTSMultiple differences between murine and human islets occur, most strikingly the presence of cdk-6 in human β-cells versus its low abundance in the murine islet. Cdk-6 and cyclin D1 in vitro led to marked activation of retinoblastoma protein phosphorylation and cell cycle progression with no induction of cell death. Human islets transduced with cdk-6 and cyclin D1 were transplanted into diabetic NOD-SCID mice and markedly outperformed native human islets in vivo, maintaining glucose control for the entire 6 weeks of the study.CONCLUSIONSThe human G1/S proteome is described for the first time. Human islets are unlike their rodent counterparts in that they contain easily measurable cdk-6. Cdk-6 overexpression, alone or in combination with cyclin D1, strikingly stimulates human β-cell replication, both in vitro as well as in vivo, without inducing cell death or loss of function. Using this model, human β-cell replication can be induced and studied in vivo.

Id3 upregulates BrdU incorporation associated with a DNA damage response, not replication, in human pancreatic β-cells

“…35 Several of these proteins are also differentially expressed between human and rodent β-cells. 35,75 Murine β-cells express abundant cdk-4 and cyclin D2, and genetic mouse models have shown them to be critical for β-cell proliferation and diabetes development.…”

“…35 Several of these proteins are also differentially expressed between human and rodent β-cells. 35,75 Murine β-cells express abundant cdk-4 and cyclin D2, and genetic mouse models have shown them to be critical for β-cell proliferation and diabetes development. [76][77][78][79] In contrast, human β-cells contain high levels of the analogous enzyme cdk-6, and cyclins D1 and D3 are thought to play a role.…”

“…Human β-cells express all four Id proteins. 35 Studies with mouse and human cell lines have shown that Id2 influences expansion of pancreatic progenitors 36 and expression of Ids in human islets is induced by glucose uptake. In turn, Ids may play a role in regulating insulin transcription and secretion 37,38 but little is known regarding the relevance of Id proteins to adult human β-cell replication.…”

“…Recently, it was reported that overexpression of cdk-6 and cyclin D1 in isolated human β-cells in vitro induced BrdU incorporation and Ki67 expression. 35,80 It will be important to determine whether there is concomitant cell cycle completion and an increase in β-cell number as suggested by transplantation studies.…”

Of rodents and men: Species-specific glucose regulation and type 2 diabetes research

“…In the murine β-cell G1/S proteome, the E2F2 protein is abundantly expressed, and loss of E2F2 expression by gene knockout leads to severe pancreatic dysfunction (Iglesias et al, 2004). However, human islets lack E2F2, but contain E2F3 and E2F7 that are absent in murine islets (Fiaschi-Taesch et al, 2009). The cyclin family members involved in cell cycle progression also differ: humans primarily express cyclin D3 with variable amounts of cyclin D1, and little or no cyclin D2, but rodents express and utilize cyclin D2 to an extent that rodents lacking cyclin D2 develop islet hypoplasia, hypoinsulinemia, and diabetes (Fiaschi-Taesch et al, 2010).…”